Module 1: The Atom Flashcards
14.1 Discovering the Invisible Atom
Atoms are so small that the number of them in a baseball is roughly equal to the number of Ping-Pong balls that could fit inside a hollow sphere as big as Earth. Atoms are so small that we could stack microscope on top of micro-scope and never “see” an atom. This is because atoms are smaller than the wavelengths of visible light. This is why, atoms cannot be seen with visible light, which passes by the atom with no reflection. In contrast, we can see a bacterium through the microscope because the bacterium is much larger than the wavelengths of visible light, so it reflects visible light back toward the eye.
The origin of most atoms goes back to the birth of the universe. Hydrogen, H, the lightest atom, was probably the original atom, and hydrogen atoms make up more than 90% of the atoms in the known universe. Heavier atoms are produced in stars. There, enormous temperatures and pressures force hydrogen atoms to join, which makes heavier atoms. Anything you look at on Earth today is made of atoms manufactured in stars that exploded billions of years, before our solar system came into being. You are made of stardust, as is everything that surrounds you. In this way, you don’t “own” the atoms that make up your body—you’re simply the present caretaker. There will be many caretakers to follow.
started 12/08/24
VIDEO 1: Air Out
Kai and Maile do an experiment based on Lavoisier’s principles by measuring a change that is a result of a chemical reaction.
VIDEO 2: Nanotechnology
Atoms are smaller than the wavelengths of visible light. This makes them impossible to see with an optical microscope. A scanning probe microscope works by a different principle.
Atoms are smaller than wavelengths of visible light. Shine visible light on an atom, and it will pass right on by. This is to say that atoms are impossible to see in a conventional sense, even with the most powerful optical microscope. But atoms can be “felt” using a device called a scanning probe microscope.
Atom itself is made of even smaller particles, such as electrons, protons and neutrons. The study of the subatomic behaviour is what we call CHEMISTRY.
INTEGRATED SCIENCE - BIOLOGY, EARTH SCIENCE
A Breath of Air
What do your rungs and Earth’s atmosphere have in common? There are about as many atoms of air in your lungs at any moment as there are breathfuls of air in Earth’s atmosphere. About 10^22 atoms make up a liter of air, and there are about 10^22 liters of air in the atmosphere. This is a very, VERY large number. That many golf balls would fill the North and South Atlantic Oceans. Yet, because atoms are so small, that many fit within your lungs.
You exhale many, many breaths, and so other people breathe in many, many atoms that were once in your lungs—that were once a part of you. And of course, vice versa: with each breath you take in, you recycle atoms that were once a part of everyone who ever lived. It can be truly said that we are literally breathing one another. Science tells us we’re all one family.
14.2 Evidence for Atoms
Because atoms are so tiny, scientists didn’t hypothesize their existence until the early 1800s. They realized that if atoms are real, it would make sense that some materials transform into other materials. Iron, for example, transforms into rust when iron atoms combine with oxygen atoms. Thinking in terms of atoms, scientists were soon able to create new materials, such as dyes and medicines, not found in nature.
The first direct evidence for atoms was discovered in 1827 by a Scottish botanist, Robert Brown. He was studying grains of pollen in a drop of water under a microscope. He noticed that the grains were continually moving about. These little grains seemed to jitter around randomly. At first he wondered if they were alive, as they had come from a living plant. But later he found that dust particles and grains of soot moved in the same way. This perpetual jiggling of particles was named the Brownian motion.
Only in the early 1900s, Einstein explained that the Brownian motion results from collisions between invisible atoms and visible particles. Brown’s pollen grains were moving because they were constantly being jostled by groups of atoms that make up the water.
Today, we still cannot see atoms directly, but we can generate images of them indirectly. In the mid-1980s, researchers developed the scanning probe microscope, which produces images by dragging an ultrathin needle back and forth over the surface of a sample. The atomic images generated by scanning probe microscopes are not photographs taken by a camera. Rather, they are computer-generated images produced by the movements of an ultrathin needle. A scanning probe microscope can also move individual atoms into desired positions. This ability opened up the field of nanotechnology.
14.2 Evidence for Atoms
During the early 1900s, scientists came to realize through their experiments that atoms are not the smallest particles of matter. Instead, atoms themselves are made of even smaller particles, which we call subatomic particles, 3 examples are protons, neutrons, and electrons.
Protons and neutrons are bound together at the atom’s center to form the atomic nucleus, which makes up most of an atom’s mass. Surrounding the nucleus are the tiny electrons.
All atoms and all things made of atoms, including ourselves, are mostly empty space, because the bulk of an atom’s mass is concentrated within its nucleus. Surrounding that nucleus is empty space through which electrons are buzzing around. The size of an atom, therefore, is determined by how far away electrons move around the nucleus. Because electrons are even smaller than the nucleus, and because they are widely separated from each other (as well as from the nucleus), atoms are indeed mostly empty space—much as our solar system is mostly empty space.
If a typical atom were expanded to a diameter of 3 km, about as big as a
medium-sized airport, the nucleus would be about the size of a basketball. Atoms are mostly empty space.
So if atoms are mostly empty space, why don’t they simply pass
through one another? This is because electrons repel the electrons
of neighboring atoms. Therefore, two atoms can get only so close to each other before they start repelling. This explains why gravity doesn’t pull you through the floor as you stand. While the force of gravity pulls you down, the electric force of repulsion between the atoms of the floor and your feet pushes you up. As you stand on the floor, these two forces are balanced and you find yourself neither falling nor rising.
When the atoms of your hand push against the atoms of a wall, repul-sions between electrons in your hand and electrons in the wall prevent your hand from passing through the wall. You sense this repulsion as a pressure that pushes back. Also, our sense of touch comes from these electrical repulsions. Interestingly, when you touch someone, your atoms and those of the other person do not meet. Instead, atoms from the two of you get close enough so that you sense an electrical repulsion. There is still a tiny, though imperceptible, gap between the two of you.
The number of different kinds of atoms is surprisingly small. Just as only three colors red, green, and blue can be combined to form any color on a television screen, and the 26 letters of the alphabet make up all the words in a dictionary, only a few kinds of atoms combine in different ways to produce all substances. To date, we know of slightly more than 100 distinct atoms. Of these, about 90 are found in nature. The remaining atoms have been created in the laboratory.
INTEGRATED SCIENCE - CHEMISTRY
A First Look at the Periodic Table
Any material made of only one type of atom is classified as an element. Pure gold, for example, is an element—it contains only gold atoms. Nitrogen gas is an element because it contains only nitrogen atoms. Likewise, the graphite in your pencil is an element—carbon. Graphite is made up solely of carbon atoms. All of the elements are listed in a chart called the Periodic Table ,which has 18 groups (vertical columns) and 7 periods (horizontal rows). Not all periods contain the same number of elements. The 6th and 7th periods include a subset of elements, which are listed separately.
Each element is designated by its atomic symbol, which comes from the letters of the element’s name. For example, the atomic symbol for carbon is C, and that for chlorine is Cl. Elements with symbols derived from Latin names are usually those that were discovered earliest. Gold has the atomic symbol Au after its Latin name, aurum. Lead has the atomic symbol Pb after its Latin name, plumbum.
Only the first letter of an atomic symbol is capitalized. The symbol for the element cobalt, for instance, is Co, while CO is a combination of two elements: carbon, C, and oxygen, O. Most materials are made from more than one kind of atom. These materials are called compounds. Water, H2O, for example, is made from the combination of hydrogen and oxygen atoms.
Each VERTICAL column of the periodic table is called a GROUP of elements (or sometimes a family of elements). Elements within the same group (vertical column) have similar properties. For example, that gold (Au), silver (Ag), and copper (Cu), are all within the same group (number 13 LK: ??). Note how these elements are all “precious metals” used for both jewelry and money.
Each HORIZONTAL row is called a PERIOD of elements. Across any period (horizontal row), the properties of elements gradually change. This gradual change is called a periodic trend. One periodic trend is that the size of atoms becomes smaller, when moving from left to right. However, the mass becomes greater when moving from left to right. A horizontal row is called a period because it corresponds to one full cycle of a trend. For example, atomic size and atomic mass are periodic (repeating) properties: a potassium atom K is larger but less massive than a nickel atom Ni. (Plumbers got their name from lead = plumbum (Pb), as they once worked with lead pipes.)
CHECK YOUR THINKING
How can you tell that the element potassium, K, was identified by humans before the element nickel, Ni?
Answer: The atomic symbol for potassium K does not match its modern name.
14.3 Protons, Neutrons, and Electrons
A proton is a subatomic particle in the atomic nucleus that carries a positive electric charge. The atomic nucleus also includes another subatomic particle—a neutron. The neutron is a nuclear particle with about the same mass as the proton, but no electric charge. Any object with no net electric charge is said to be electrically neutral (which is how the neutron got its name). Protons and neutrons are both nucleons, which is the general term given to a subatomic particle found within the nucleus.
An electron is a subatomic particle with a negative 1 electric charge. Electrons orbit around the nucleus of an atom. A proton carries a positive electric charge. . The proton and electron have the same quantity of charge, but the charges are opposite. The number of protons in the nucleus of any atom is equal to the number of electrons orbiting the nucleus. The proton’s positive charge and the electron’s negative charge cancel each other. This guarantees that the atom has an overall net electric charge of zero, so it is electrically neutral. For example, an electrically balanced oxygen atom has 8 electrons and 8 protons. However, the proton is heavier, it is nearly 2000 times as massive as the electron.
Each element is identified by its atomic number. This is the number of protons contained in each atomic nucleus. Hydrogen, with 1 proton per atom, has atomic number 1. Helium, with 2 protons in its nucleus, has atomic number 2. Iron has the chemical symbol Fe and the atomic number 26, which means that there are 26 protons in an iron atom. Conversely, all atoms that contain 26 protons are, by definition, iron atoms.
Nanotechnology is the technology of tiny things. It began with microtechnology, which was ushered in some 60 years ago. Engineers learned to build electronic circuits within the size of a micron, which is 10^(-6) meters, thus the term microtechnology. Such circuits have had a major impact on society—in technologies such as personal computers, cell phones, and the Internet.
Today we are moving to the realm of the nanometer which is 10^(-9) meters and is the realm of individual atoms. Technology at this scale is called nanotechnology. Nanotechnology deals with incredibly tiny objects from 1 to 100 nanometers (nm) in scale. For perspective, a water molecule is only about 0.2 nm. Most experts agree that the first big benefits of nanotechnology will arise in computer science and medicine.
14.3 Protons, Neutrons, and Electrons
VIDEO 1: Subatomic Particles Organized
Atomic nucleus = protons (positively charged, +1 charge) + neutrons (neutral, zero charge), both are called nucleons, and they whizz around within the nucleus
Electron “cloud” = cloud of negatively charged (-1 charge) electrons that whizz around the nucleus. Electrons and protons balance each other out, as the number of electrons = number of protons.
Relative mass: if the mass of an electron is 1, then the relative mass of a proton is 1836 and of a neutron is 1841. So both protons and neutrons are about 2000x more massive than electrons. Electrons do not contribute much to the mass of an atom. As mentioned before, howvever, most of the atom is empty space.
Atomic number is the number of PROTONS that each atom of a given element contains; it is used to define an atom. These are the numbers we see in the Periodic Table, just above the symbol of an element.
Isotopes are atoms of the same element that contain different numbers of neutrons. The number of protons defines an element so it does not vary, but the number of neutrons can vary. For example, hydrogen isotopes can have a different number of neutrons: none, 1 or 2. A hydrogen isotope with no neutrons is called protium, with 1 - deuterium, and with 2 - tritium. These atoms are isotopes of one another, as if all 3 are part of the same family with the same last name “hydrogen.”
Mass number is used to distinguish one isotope from another, it is the total number of NUCLEONS that an atom contains. For hydrogen isotopes, we can write: H-1, H-2, H-3. For iron, we can have: Fe-55 or Fe-56.
How can we figure out the number of neutrons?
Number of neutrons = Mass number - Atomic number
For Fe-55: 55 nucleons - 2 protons = 29 neutrons
For Fe-56: 55 nucleons - 2 protons = 30 neutrons
We say that Fe-55 and Fe-56 are isotopes of one another, which means that they have a different number of neutrons. Isotopes have the same atomic number but a different number of neutrons.
14.3 Protons, Neutrons, and Electrons
VIDEO 2: Atomic Mass
Atomic mass is the average mass of all the isotopes of an element. Carbon-12 has 12 nucleons, Carbon-13 has 13 nucleons, Carbon-14 has 14 nucleons; they are isotopes of one another. If you got a sample of graphite, which isotope have you got?
Percent abundance is 99% for Carbon-12, which means that 99 out of 100 carbon atoms are Carbon-12 isotopes. Carbon-13 has a percent abundance of 1%, and there is very little of Carbon-14, there is not much of it in our environment.
Carbon-14 has a special property, it is radioactive.
So if we have 1 atom out of 100 atoms, which is Carbon-13, so it has an extra nucleon vs Carbon-12, so it will be a little heavier, which means it will increase the average mass of all the atoms. We say: the average “atomic mass” of any macroscopic sample of carbon is slightly more than 12, because of Carbon-13. The atomic mass of carbon = 12.011.
The atomic mass of an element is the number given below the symbol in the Periodic Table, and they are not whole numbers. Most oxygen atoms have 16 nucleons, why then the atomic mass of oxygen is 15.999? This is because there are some oxygen-15 isotopes.
DO NOT CONFUSE THE MASS NUMBER, THE ATOMIC NUMBER (number of protons), and THE ATOMIC MASS NUMBER (number of nucleons) - focus your attention on the second word of each term.
The mass number (the number of nucleons) is always a whole number, but the atomic mass (the average mass of all isotopes) is not a whole number. The atomic mass is measured in atomic mass units (amu). There is a relationship between amu and grams, both are units of mass, see the conversion formula below.
14.4 Isotopes and Atomic Mass
An element has a definite number of protons, but its number of neutrons may vary. Atoms of the same element that contain different numbers of neutrons are isotopes of one another. We identify isotopes by their mass number, which is the total number of protons and neutrons they contain. In other words, mass number is the number of nucleons, which means that isotopes have different mass numbers.
Most water molecules, H20, consist of hydrogen atoms with no neutrons. The few that do, however, are heavier, and because of this difference they can be isolated. Such water is appropriately called “heavy water.”
Isotopes of an element differ only by mass, not by electric charge. That means they cannot be distinguished from one another. For example, about 1% of the carbon we eat is the carbon-13 isotope, with seven neutrons per nucleus. The remaining 99% of the carbon in our diet is the common carbon-12 isotope, with six neutrons per nucleus. Our bodies can’t tell the difference between isotopes.
The total mass of an atom is called its atomic mass. This is the sum of the masses of all the atom’s electrons, protons, and neutrons. Because the mass of electrons is so small compared with the mass of protons and neutrons, electrons contribute practically nothing to atomic mass.
The atomic mass of an element is defined as the average atomic mass of its various isotopes. This is why the atomic masses shown in the periodic table are not whole numbers.
14.5 Electron Shells
The identity of a song is determined by how its musical notes are arranged. In a similar way, the identity of an atom is determined by how its electrons are arranged. Within the atom, electrons behave as though they are arranged in shells. There are at least 7 shells, and each shell can hold only a limited number of electrons. The innermost shell (the first energy level) can hold a max of 2 electrons; the 2nd and 3rd shells can hold a max of 8 electrons each; the 4th and 5th shells can hold a max of 18 electrons each; and the 6th and 7th shells can hold a max of 32 electrons (LK: explained in the video, why 2, 8, 18 and 32). The higher the shell number, the greater the energy of electrons in that shell.
The electrons of the outermost shell in any atom are the first to interact with other atoms, so they are most easily transferred or shared with other atoms. This means that the outermost electrons are the ones that participate in chemical bonding and are therefore very important. They are called VALENCE electrons The term valence comes from the Latin valentia, “strength.” Atoms combine to form MOLECULES, which are tightly held groups of atoms, by way of valence electrons, so valence electrons in an atom are responsible for the formation of molecules. Any electrons in an atom which are not in the valence shell are called CORE electrons.
The Bohr’s Shell Model of an atom was developed in 1913 by the Danish scientist Niels Bohr:
1. The 7 shells = 7 periods (horizontal rows) in the Periodic Table.
2. The number of elements in each period (horizontal row) is equal to the shell’s capacity for electrons. The first shell (LK: first from the nucleus) can hold max 2 electrons. That’s why, we find only 2 elements, hydrogen and helium, in the 1st period. The second and third shells - each - can hold up to 8 electrons, so 8 elements are found in both the 2nd and 3rd periods, and so on.
3. Elements within the same group (vertical columns) are organised according to the number of valence electrons, hence the group number is a good predictor of how chemically reactive an element is. For example, atoms of the first group, which include hydrogen, lithium, and sodium, each have a single valence electron. The atoms of the second group, which includes beryllium and magnesium, have two valence electrons. Similarly, atoms of the last group, which include helium, neon, and argon, have their outmost shells filled to capacity with valence electrons—2 for helium, and 8 for neon and argon.
Electron shells are not what atoms look like, but they help us understand and predict the properties and behaviors of atoms.
14.5 Electron Shells
Khan Academy
The periodic table, electron shells, and orbitals (#1)
An early model of the atom was developed in 1913 by the Danish scientist Niels Bohr (1885–1962). The Bohr model shows the atom as a central nucleus containing protons and neutrons, with the electrons in circular electron shells at specific distances from the nucleus, similar to planets orbiting around the sun. Each electron shell has a different energy level, with those shells closest to the nucleus being lower in energy than those farther from the nucleus. By convention, each shell is assigned a number and the symbol n—for example, the electron shell closest to the nucleus is called 1n.
In order to move between shells, an electron must absorb or release an amount of energy corresponding exactly to the difference in energy between the shells. For instance, if an electron absorbs energy from a photon, it may become excited and move to a higher-energy shell; conversely, when an excited electron drops back down to a lower-energy shell, it will release energy, often in the form of heat.
Atoms, like other things governed by the laws of physics, tend to take on the lowest-energy, most stable configuration they can. Thus, the electron shells of an atom are populated from the inside out, with electrons filling up the low-energy shells closer to the nucleus before they move into the higher-energy shells. The shell closest to the nucleus, 1n, can hold 2 electrons, while the next shell, 2n, can hold 8 electrons, the third shell, 3n, can hold up to 18 electrons, and so on.
If we consider just the first three rows of the table, which include the major elements important to life, each row corresponds to the filling of a different electron shell: helium and hydrogen place their electrons in the 1n shell, while second-row elements like Li start filling the 2n shell, and third-row elements like Na continue with the 3n shell.
The number of electrons in the outermost shell of a particular atom determines its reactivity, or tendency to form chemical bonds with other atoms. The outermost shell is known as the valence shell, and the electrons found in it are called valence electrons. In general, atoms are most stable, least reactive, when their outermost electron shell is full. Most of the elements important in biology need a minimum of 8 electrons in their outermost shell in order to be stable, and this rule of thumb is known as the OCTET RULE. For example, argon does not have a full outer shell, since the 3n shell can hold up to 18 electrons, while argon has only 8, yet it is stable like neon and helium because it has 8 electrons in the 3n shell and thus satisfies the octet rule.
14.5 Electron Shells
Khan Academy
The periodic table, electron shells, and orbitals (#2)
The Bohr model is useful to explain the reactivity and chemical bonding of many elements, but it actually doesn’t give a very accurate description of how electrons are distributed in space around the nucleus. Specifically, electrons do not circle the nucleus, but rather spend most of their time in sometimes-complex-shaped regions of space around the nucleus, known as electron orbitals. We cannot know where an electron is at any given moment in time, but we can mathematically determine the volume of space in which it is most likely to be found—say, the volume of space in which it will spend 90% of its time. This high-probability region makes up an orbital, and each orbital can hold max 2 electrons (one has the opposite spin of the other).
We can break each electron shell down into one or more subshells, which are simply sets of one or more orbitals. Subshells are designated by the letters - s/d/p/f - and each letter indicates a different shape. For instance, s-subshells have a single, spherical orbital, while p-subshells contain 3 dumbbell-shaped orbitals at the right angle to each other. Most of organic chemistry—the chemistry of carbon-containing compounds, which are central to biology—involves interactions between electrons in s- and p subshells. However, atoms with many electrons may place some of their electrons in d- and f-subshells, which have more complex shapes and contain 5 and 7 orbitals, respectively.
The first electron shell, 1n, corresponds to a single 1s orbital. The 1s orbital is the closest orbital to the nucleus, and it fills with electrons first, before any other orbital. Hydrogen has just one electron, so it has a single spot in the 1s orbital occupied. This can be written as 1s^1. Helium has 2 electrons, so it can completely fill the 1s orbital with its 2 electrons. This is written out as 1s^2. In the periodic table, hydrogen and helium are the only two elements in the first row,which reflects that they have electrons only in their first shell.
The second electron shell, 2n, has two subschells: one spherical s orbital plus three dumbbell-shaped p orbitals, each orbital can hold max 2 electrons. After the 1s orbital is filled, the second electron shell begins to fill, with electrons going first into the 2s orbital and then into the 3
p orbitals. lements in the second row of the periodic table place their electrons in the 2n shell as well as the 1n shell. For instance, lithium (Li) has 2 electrons: 2 fill the s orbital, and the third is placed in the 2s orbital, giving an electron configuration of 1s^2-2s^1.
14.5 Electron Shells
Khan Academy
The periodic table, electron shells, and orbitals (#3)
The third electron shell, 3n, also contains an s orbital and 3 p orbitals, and the third-row elements of the periodic table place their electrons in these orbitals, similar to the elements in 2n shell. The 3n shell also contains a
d orbital, but this orbital is considerably higher in energy than the
3s and 3p orbitals and does not begin to fill until the 4th row of the periodic table. This is why third-row elements, such as argon, can be stable with just 8 valence electrons: their s and p subshells are filled, even though the entire 3n shell is not.
While electron shells and orbitals are closely related, orbitals provide a more accurate picture of the electron configuration of an atom. That’s because orbitals actually specify the shape and position of the regions of space that electrons occupy.
How did scientsis work out all of this? It can’t be figured out using microscopes. Most of it is based on theory worked out using a lot of maths. Experimental observations, such as the energy released or absorbed when electrons move from one state to another, corroborate the theory. Experimental observations, using techniques such as X-ray crystallography, provide information on the shape of molecules which, in turn, corroborates the theoretical shapes of the orbitals.
To sum up:
1.All atoms are made up of energy levels (called shells) that hold 1 or more subshells.
2.Every subshell has a slightly varying energy from its “shell” energy level, depending on the distance from the nucleus. Every subshell holds a number of orbitals s/p/d/f that can hold max 2 electrons (one has the opposite spin of the other).
3.The first shell of all atoms. which is the closest to the nucleus, has 1 subshell containing 1 s orbital. This means that the first shell can hold max 2 electrons. The second shell has 2 subshells: 1 s-orbital and 3 p-orbitals. This means that the second shell can hold a total of 8 electrons.
4.Every orbital has a certain shape, this shape tells us where electrons are likely to spend most of their time.
The Elements song by Tom Lehrer
https://www.youtube.com/watch?v=U2cfju6GTNs
14.5 Electron Shells
VIDEO: The Noble Gas Shell Model
LK: Helium (He), neon (Ne) and argon (Ar) are called inert or noble gases, because their outermost shells either are filled to capacity with electrons or satisfy the octet rule, which makes them highly stable as single atoms, so these elements are non-reactive.
Orbitals of similar energies can be grouped together in what we call atomic shells. In the shell diagram, we have the orbitals in the order of increasing potential energy.
We will group the orbitals according to the energy levels. The 2s and 2p have similar energy levels so we put them together in the same group. The 3s and 3p are similar in their energy level so they are in the same group (LK: mistake in the video). So we make separations between these groups of orbitals.
How many electrons can reside in the first row of orbitals?
We know that 2 electrons can go into one orbital, so only 2 electrons can be found in the first row of orbitals, which contains only one 1s orbital.
The 2nd row can contain one 2s and three 2p orbitals, a total of 4 different orbitals. A total of 8 electrons can reside within the 2nd row of orbitals: 4 orbitals x 2 electrons per orbital = 8 electrons
The 3rd row can have one 3s and three 3p orbitals, again a total of 4 different orbitals, so a total of 8 electrons can reside within the 3rd row of orbitals: 4 orbitals x 2 electrons per orbital = 8 electrons
The 4th row can have one 4s, three 4p, and five 3d electrons, so 9 different orbitals. With 2 electrons per orbital, this gives: 9 x 2 = 18 electrons. The same for the 5th row: 18 electrons. The 6th row has four f-orbitals, in addition to s/p/d orbitals, this adds up to 32 electrons. The 7th row can also have 32 electrons.
Let’s focus on the 2nd row: we have a 2s orbital and three 2p orbitals. An orbital is a REGION of SPACE where electrons of this energy level are found 90% of the time (it turns out that electrons are not in well-defined circular or elliptical orbits, this is why scientists introduced the idea of an ORBITAL). Each orbital can be depicted as a shell, so we get a series of concentric shells, which means that they share the same centre, the atom’s nucleus. The first shell consists merely of a 1s orbital and has a capacity for 2 electrons. The 2nd shell has 4 orbitals and hence 8 electrons, and so on. Thus, the capacity for electrons of each shell depends on how many orbitals the shell is comprised of. It is a conceptual model that describes how electrons are arranged, it is not to be taken literally. Atomic orbitals are grouped together, based on their energy levels.
Let’s relate this to the Periodic Table. In the first group (vertically), hydrogen has 1 electron in its outermost shell, lithium also has 1 electron in its outermost shell, the same for sodium. This is why, elements above and below one another (that is, within the same group) in the Periodic Table have similar physical and chemical properties, because they have similar electron configurations. For example, in the noble gases - helium, neon and argon - each shell is filled to its capacity, this is why they have similar properties.
