Atomic structure Flashcards
History of an atom - 1 - John Dalton
1800’s - John Dalton suggests that each of the elements are made from just one type of atom: tiny spheres that could not be divided.
History of an atom - 2 - JJ Thomson
1897 - J Thomson discovers the electron and proposes the plum pudding model: the atom is a ball of positive charge and the negative charge are embedded in it (like blueberries in a blueberry muffin).
History of an atom - 3 - Ernest Rutherford
1911 - Ernest Rutherford fired alpha particles at a piece of very thin gold foil(about 10,000 atoms thick).
Thomson’s plum pudding model predicted that…
All alpha particles… passed straight through
However what was observed was:
Most alpha particles… passed straight through
A very few alpha particles… were deflected by more than 90 degrees
Most of the atom is empty space
All the positive charge and most of the mass is concentrated in a small volume (the nucleus)
History of an atom - 4 - Niels Bohr
1913 - Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances The theoretical calculations of Bohr agreed with experimental observations.
History of an atom - 5 - James Chadwick and others
1920-1932 - Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name protons was given to these particles.
About 20 years after scientists had accepted that atoms have nuclei, in 1932 James Chadwick carried out an experiment which provided evidence for neutral particles in the nucleus. These became known as neutrons. The discovery of neutrons resulted in a model of the atom which was pretty close to the modern-day version.
Structure of the atom
Elements are made of tiny particles of matter called atoms.
Each atom is made of subatomic particles called protons, neutrons and electrons.
Their size is so tiny that we can’t really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass is used.
The mass of an atom is concentrated in the nucleus, because the nucleus contains the heaviest subatomic particles (the neutrons and protons).
The nucleus is also positively charged due to the protons.
Electrons orbit the nucleus of the atom, contributing very little to its overall mass, but creating a ‘cloud’ of negative charge.
The electrostatic attraction between the positive nucleus and negatively charged electrons orbiting around it is what holds an atom together.
Sub atomic particles
The protons, neutrons and electrons that an atom is made up of are called subatomic particles.
These subatomic particles are so small that it is not practical to measure their masses and charges using conventional units (such as grams or coulombs).
Instead, their masses and charges are compared to each other, and so are called ‘relative atomic masses’ and ‘relative atomic charges’.
These are not actual charges and masses, but rather charges and masses of particles relative to each other.
Protons and neutrons have a very similar mass, so each is assigned a relative mass of 1.
Electrons are 1840 times smaller than a proton and neutron, and so their mass is often described as being negligible.
Nucleus
Atoms are extremely small with a radius of about 1 x 10-10 metres.
The central nucleus contains protons and neutrons only which are packed close together in a small region of space.
The radius of the nucleus is about 10 000 times smaller than that of the atom, so it is an extremely small region of space compared to the overall size of the atom.
This means that rather than being evenly spread out throughout the atom, virtually all of the atom’s mass is concentrated inside the nucleus.
Electrons have a much smaller mass than protons and neutrons (1 proton has the same mass of around 1840 electrons) and move in the space outside the nucleus in orbits.
Isotopes
Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number.
Isotopes display the same chemical characteristics.
This is because they have the same number of electrons in their outer shells, and this is what determines their chemistry.
The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only.
Relative atomic mass
The relative atomic mass of each element is calculated from the mass number and relative abundances of all the isotopes of a particular element.
Ar = (% of isotope A x mass of isotope A) + (% of isotope B x mass of isotope B) + (% of isotope C x mass of isotope C)… / 100
Mendeleev’s periodic table
In 1869 the Russian chemist Dmitri Mendeleev created his first draft of the periodic table.
He organised the elements into vertical columns based on their properties and the properties of their compounds.
He then started to arrange them horizontally in order of increasing atomic mass and as he worked, he found that a pattern began to appear in which chemically similar elements fell naturally into the same columns.
There were exceptions though as some elements didn’t fit the pattern when arranged by atomic mass.
Mendeleev worked to include all the elements, but he didn’t force an element to fit the pattern, rather he left gaps in the table that he thought would best be filled by elements that had not yet been discovered.
He also switched the order of the elements to maintain consistency down the columns.
Mendeleev’s predictions
Mendeleev quickly realised that elements with the same properties should be placed in the same column.
He realised that gaps in the table must correspond to elements that had not yet been discovered or isolated.
He used the properties and trends of other elements in the group with the gap to predict the properties of these undiscovered elements.
Mendeleev left a gap between silicon and tin and used his knowledge of the properties of those two elements to make predictions about the physical and chemical properties of the undiscovered element.
He called this element ‘eka-silicon’ which comes from the Greek ‘like silicon’ and when the element germanium was discovered in 1887 it was found to almost exactly match the properties Mendeleev had predicted.
No one doubted that Mendeleev had got the right idea about ordering the elements.
Strangely enough, Mendeleev always denied the existence of an eighth group of elements, even after the discovery of the noble gases in Mendeleev’s final years.
Problems with Mendeleev’s table
Once he was finished, Mendeleev thought he had organised the elements systematically but there were still some elements which didn’t quite fit in as neatly as he wanted.
This is because isotopes were not known in Mendeleev’s time, and he made no provisions for them in his table.
This meant that there was always going to be some level of inaccuracy in Mendeleev ́s work even though he did also consider the elements chemical properties as well as their atomic mass when sorting them.
Mendeleev switched the order of tellurium and iodine is his table, because even though tellurium was heavier than iodine, the chemistry of iodine fitted better with the other halogen elements; it was a nagging problem that was not solved in his lifetime.
The discovery of the proton lead to the determination of atomic number for each element.
This number is used to arrange the elements in the modern-day periodic table which fits with Mendeleev ‘s patterns.
Modern periodic table
There are over 100 chemical elements which have been isolated and identified.
Elements are arranged on the periodic table in order of increasing atomic number.
Each element has one proton more than the element preceding it.
This is done so that elements end up in columns with other elements which have similar properties.
The table is arranged in vertical columns called groups and in rows called periods.
Electronic configurations
We can represent the electronic structure of atoms using electron shell diagrams.
Electrons orbit the nucleus in shells and each shell has a different amount of energy associated with it.
The further away from the nucleus, the more energy a shell has.
Electrons first occupy the shell closest to the nucleus which can hold a maximum of 2 electrons.
When a shell becomes full of electrons, additional electrons have to be added to the next shell.
The second shell and third shell can hold 8 electrons each.
The outermost shell of an atom is called the valence shell and an atom is much more stable if it can manage to completely fill this shell with electrons.
In most atoms, the outermost shell is not full and therefore these atoms react with other atoms in order to achieve a full outer shell of electrons (which would make them more stable).
Ions
An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons.
Negative ions are called anions and form when atoms gain electrons, meaning they have more electrons than protons.
Positive ions are called cations and form when atoms lose electrons, meaning they have more protons than electrons.
All metals lose electrons to other atoms to become positively charged ions.
All non-metals gain electrons from other atoms to become negatively charged ions.
Deducing subatomic particles in ions
An atom is neutral and has no overall charge.
Ions on the other hand have either gained or lost electrons causing them to become charged.
The number of subatomic particles in atoms and ions can be determined given their atomic (proton) number, mass (nucleon) number and charge
Ions have a different number of electrons to the number of protons, depending on their charge.
A positively charged ion has lost electrons and therefore has fewer electrons than protons.
A negatively charged ion has gained electrons and therefore has more electrons than protons.
Ionic bonding
Ionic compounds consist of a metal bonded to a non-metal via electron transfer.
The metal atom loose electrons to become a positively charged ion (cation) and obtain a full outer shell of electrons.
The non-metal atom gain electrons to become a negatively charged ion (anion) and obtain a full outer shell of electrons.
They form regular shaped giant ionic lattices in which strong electrostatic forces act in all directions.
Properties of ionic bonds
High melting and boiling point - Strong electrostatic forces of attraction between opposite charged ions throughout the giant structure.
They are hard but brittle - Strong level of attraction, when layers slide ions of the same charge align and the structure breaks.
They are generally soluble in water - Ions are attracted to the water molecules and the attraction breaks the lattice apart.
They never conduct electricity in a solid state - The ions are held in position and are not free to move.
They often conduct electricity as a liquid (when molten or dissolved in water) - the ions are free to move.
Covalent bonding
Covalent compounds are formed between 2 or more non metal elements.
These elements generally want to gain electrons to gain a full outer shell.
A covalent bond consists of a shared pair of electrons.
Sometimes covalent bonds are called molecular bonds.
There are two types of covalent substances.
Each atom donates 1 electron to the shared pair of electrons, which make up the covalent bond.
By doing this each atom has the same electron structure as a noble gas.
Simple covalent
A simple covalent bond is formed between two or more non metals.
These are composed of tiny separate particles called molecules.
These contain several atoms bonded strongly together by covalent bonds however, the forces between molecules are weak intermolecular forces.
Properties of simple covalent
Melting and boiling points are low. many are either liquid or gaseous at room temperature - the covalent bonds within these molecules are strong but the intermolecular force between molecules are weak. the intermolecular forces break when a substance melts or boils.
Electrical conductivity is poor - there are no ions or electrons that are free to move.
Insoluble in water - they are not charged, as oxygen is negative and hydrogen is positive, (opposite attracts).
