Periodic Properties

Purpose

• In this lesson, students will learn how to predict properties of atoms based on their position on the periodic table. This is important in understanding chemical and physical behavior of the elements. While it will not be covered here, the properties are a result of the electron configurations of the atoms that you learned about in the previous lessons.

Atomic Radius

• The relative size of an atom can be predicted based on its position on the periodic table.

– Atoms on the left hand side of the table and closer to the bottom are larger than atoms on the right and closer to the top.

• For example:

– Carbon is a smaller atom than Boron. We predict this because boron is farther to the left.

– Silicon is a larger atom than Carbon. We predict this because silicon is farther down the column.

Pause and Practice

• Choose the larger of the atoms from the pair:

– Mg or Sr

– Ca or C

– Cl or P

Pause and Practice Answers

• Choose the larger of the atoms from the pair:

– Mg or Sr

• Sr (farther down)

– Ca or C

• Ca (farther down and left)

– Cl or P

• P (farther left)

First Ionization Energy

• First ionization energy is defined as the energy required to remove an electron from an atom.

– It takes 496 kJ of energy to remove an electron from a mole of sodium atoms.

Na Na+ + e-

• First ionization energy gets larger as one goes toward the top of a column and toward the right hand side of the periodic table:

– For example:

• Nitrogen has a larger first ionization energy than Phosphorus. We predict this because nitrogen is farther toward the top of the column.

• Nitrogen has a larger first ionization energy than carbon. We predict this because nitrogen is farther to the right.

Pause and Practice

• Choose the atom with the larger first ionization energy from the pair:

– Mg or Sr

– Ca or C

– Cl or P

Pause and Practice Answers

• Choose the atom with the larger first ionization energy from the pair:

– Mg or Sr

• Mg (farther toward the top)

– Ca or C

• C (farther toward the top and to the right)

– Cl or P

• Cl (farther to the right)

Electron Affinity

• Electron affinity is the energy absorbed or released when an atom gains an electron.

– For example:

• When one mole of fluorine atom gains an electron to make the F-

ion, it releases 328 kJ of energy.

F + e- F-

• Electron affinity tends to increase up and to the right.

– For example:

• Fluorine has a larger electron affinity than oxygen (farther right).

• Fluorine has a larger electron affinity than chlorine (farther up).

Pause and Practice

• Choose the atom with the larger electron affinity from the pair:

– Li or K

– Ca or C

– Cl or P

Pause and Practice

• Choose the atom with the larger electron affinity from the pair:

– Li or K

• Li (farther toward the top)

– Ca or C

• C (farther right and toward the top)

– Cl or P

• Cl (farther right)

Trends and overall categorization

• One can see that the trends tend to go toward the upper right or lower left corners on the periodic table. Looking at the periodic table, you will notice that there is a dividing line – the stair step – that separates the periodic table into two regions.

– Non-metals are on the right and above the stair step

– Metals are to the left and below the stair step.

Metals and Non-Metals are separated by the stair-step line

• Metals

– Tend to lose electrons and make positive ions

– Tend to be solids at room temperature

– Conduct electricity

– Have small first ionization energies

– Have small electron affinities

• Non-metals

– Tend to gain electrons and form negative ions

– Solids, liquids or gases at room temperature

– Tend to not conduct electricity

– Have large first ionization energies

– Have large electron affinities

Misconception alert!

• If you are asked why Sr is larger than Mg, you should not say that it is larger, because it is farther down the column on the periodic table. It is farther down on the table, because that is where we humans put it. Srwas larger than Mg before the periodic table existed. While we can use the periodic table to predict properties, the periodic table is not the reason for the properties. The reason that it is larger is because it has electrons in higher energy levels (5s) compared to Mg (3s). Higher energy levels tend to be larger. While this is not something you need to know now, it is something that you will cover in your chemistry studies.

Try the exercises