Unit 1: Topic 7 - Periodic Trends

Question 1

How is the periodic table organized?

Answer 1

• The position of the elements on the periodic table is not at all random; it is based on their atomic structures.
• The modern periodic table is organized in numerical order based on the number of protons that atoms for a given element have. This is known as the element’s atomic number.
• Increasing atomic numbers on the elements also generally corresponds with an increasing atomic weight, which is measured in amu(atomic mass units).
• Organizing the periodic table in this way reveals various trends in the atomic/ionic radius, ionization energy, electron affinity, and electronegativity of elements.
• Sometimes, this also leaves gaps on the table for any undiscovered elements. The way that the periodic table is set up can even help us predict the properties of these unknown elements!

Question 2

What are periods and groups on the periodic table? How can you predict the properties of elements belonging to the same period or group?

Answer 2

• Each horizontal row on the periodic table is called a period. There are 7 periods(rows) on the periodic table. Elements in the same period all have similar electron configurations in that they all have the same electron ground state energy level(same amount of atomic orbitals). The period an element is in, starting from the top to the bottom of the table, will tell you the number of electron orbitals it has. For example, elements in the second period will have 2 electron orbitals. In addition, as you go from the left to the right, elements in a period transition from displaying metallic characteristics to nonmetallic characteristics.

Question 3

What is the octet rule?

Answer 3

• The octet rule in chemistry essentially states that an atom is most stable when surrounded by 8 electrons in its valence shell.
• An electron configuration with 8 valence electrons is also known as the noble-gas configuration. This is because noble gases(group 18 elements) are the only elements that already have 8 electrons in their valence shell.
• For example, halogens(group 17 elements) are one electron away from obtaining a full octet in their valence shell. Therefore, these elements will all generally readily bond with other elements so that they can have a full shell.
• Since noble gases already have a full octet, they are already in their most stable state. Therefore, they tend not to bond with other atoms or molecules, making them not very reactive.

Question 4

Why do all elements in group 17 have the same ionic charge?

Answer 4

The periodic table is based on both trends and placement of elements in ascending order of proton count (atomic number). Group 17 is defined as the halogens, which are elements that have 7 valence electrons and are in need of one more. Placing all elements in this group that have a (1-) charge assists with figuring out what elements will easily bond with others.

Question 5

Which electron configuration would be more reactive element

1s2 2s2 2p6 3p2 3p4

1s2 2s2 2p4

Answer 5

1s2 2s2 2p4

This is the electron configuration for Oxygen (O), while the other one is the configuration for Sulfur (S). As you move down the column, you will generally be met with less reactivity in the elements with negative ion charges. This is because the electronegativity decreases down a group as there is a larger distance for the attraction of the nucleus and valence electrons.

Question 6

What is Coulomb’s law?

Answer 6

• Coulomb’s Law can be summarized with the following equation: F = k(Q1 * Q2)/r^2, where F is the force, Q1 and Q2 are the different charges, and r is the difference between the charges. The constant k has the value 8.988 * 10^9 N m^2 * C ^(-2).

Question 7

What are shielding and effective nuclear charge?

Answer 7

• The effective nuclear charge refers to the amount of attraction exerted on valence electrons by the nucleus.
• The shielding effect occurs when core electrons, or electrons that are closer to the nucleus, end up “shielding” or blocking the valence electrons on the outermost shell. This is because the full attractive force from the nucleus is being reduced due to repulsive forces between electrons(like charges repel each other). This has the effect of reducing the effective nuclear charge that is being exerted on those valence electrons by the attractive forces from the nucleus.
• The equation for calculating the effective nuclear charge of an electron is as follows:
  Zeff(effective nuclear charge) = Z - S
  where Z refers to the number of protons the atom has, and S refers to the number of core electrons, or simply the number of electrons between the electron in question and the nucleus

Question 8

What is ionization energy?

Answer 8

• Ionization energy is the minimum amount of energy required to remove an electron from an atom or ion. There is different ionization energy for each successive removal of electrons, but the removal of the first(and most loosely held) electron is most commonly talked about.
• You may think of the ionization energy as a measure of the difficulty of removing an electron or the strength by which the electron is bound. Therefore, the higher the ionization energy, the harder it is to remove the electron.

Question 9

What are atomic and ionic radii?

Answer 9

• An atom’s atomic radius is defined as the total distance from the nucleus of an atom to the outermost orbital of its electron. It is usually used to describe the size of the atom. However, it is difficult to accurately determine this because of the uncertainty in the position of the outermost electron(electrons are in constant motion).
• The atomic radius can be measured using the distance between the nuclei of two atoms(of the same element) that are barely touching each other, meaning the outermost electron orbitals of each atom are just barely touching each other. Taking this “diameter” and dividing it by 2 will give us the radius.
• The ionic radius may be bigger or smaller than the atomic radius, depending on the charge of the ion. The ionic radius and atomic radius are the same for atoms of neutral elements(like noble gases) that don’t tend to gain or lose electrons.
• An anion has an additional electron, which will increase the size of the electron cloud. Therefore, anions typically have a larger ionic radius than the atomic radius of their parent atom.

Question 10

What are the trends in ionization energy across the periodic table?

Answer 10

• Ionization energy generally increases when moving from left to right along a period. This is because, with each extra proton added as you go from left to right, there will be an increase in the effective nuclear charge in the atom since the number of protons will increase while the number of core electrons stays the same. Because the amount of electrostatic attraction between the nucleus and the outer electrons is directly proportional to the nuclear charge(Q), the amount of electrostatic attraction will also get increasingly larger as you go from left to right. If the forces of attraction are larger, it will get increasingly difficult to remove the valence electrons so that the ionization energy will increase.
• Ionization energy generally decreases when moving from top to bottom along a group. This is because as you go from top to bottom, the atomic radius will increase, therefore increasing the distance between the nucleus and the valence electrons. Since the amount of electrostatic attraction is inversely proportional to the distance between two objects, as you go from top to bottom, the electrostatic attraction will get increasingly smaller, making it easier to remove a valence electron from an atom. Therefore, the ionization energy will get increasingly smaller.

Question 11

How is ionization energy affected as you move from left to right and top to bottom across the periodic table?

Answer 11

As you move farther to the right, your ionization energy is going to increase. This is because as you move towards the right, there are more protons added to the nucleus and no additional sublevel for electrons. This causes the atomic radii to decrease as the strength of the nucleus increases.

As you progress from top to bottom, the ionization energy is going to decrease. As you add more shells of electrons, the forces between the outermost ones and the nucleus start to lessen, which makes it easier to strip an electron off of the outer shell.

Question 12

How are the atomic radii affected as you move from left to right and top to bottom across the periodic table?

Answer 12

The atomic radii as you move from left to right decreases. As you add more protons, the additional electrons will stay in the same shell, and there will be a stronger attraction to those electrons since there is increased nucleus size.

As you move from top to bottom, there will be an increase in atomic radii, as there is an additional shell every time you move down a row on the periodic table.

Question 13

How does the addition or removal of an electron affect the atomic radii?

Answer 13

When an electron is removed or added, the new radius is called the ionic radius. When there is an additional electron added to the atom, the radius will increase. This is due to the fact there are more electrons to pull into the nucleus, which causes a slight increase in the radius. The exact opposite happens once there is a loss of electrons. Once the electrons are lost, there is an additional pull on the electrons that still are present, which causes the radius to get smaller.

Question 14

How is electron affinity affected as you move left to right and top to bottom on the periodic table?

Answer 14

As you move to the right, your electron affinity will increase. This is because as you move along a period, your nuclear attraction increases with each element. As you move down a group, your affinity decreases as the shells get farther and farther away, causing a weaker pull toward the nucleus.

Question 15

What would be the differences in electronegativity, atomic radii, and ionization energy between Chlorine and Magnesium?

Answer 15

Using the trends we learned earlier since Chlorine is farther to the right than Magnesium, we can deduce that it will have a stronger electronegativity than Magnesium. And then, by using the fact that Magnesium is farther to the left than Chlorine, we can make the assumption that it has larger atomic radii as well. Finally, since Chlorine is farther right than Magnesium, we can use the trends to see that it will have higher ionization energy as well!