Section 4.5—Periodicity

Objectives:•Define periodic trend•Use periodic trends in atomic radius, ionization energy, and electron affinity to organize elements

•Periodicity is the presence of a predictable pattern in the properties of elements.•If the properties of an element change in a predictable way, we call it a “trend”. •In the periodic table, there are trends observed within a group (from top to bottom) and across a period (from left to right).• Note that there are some exceptions to these periodic trends!!

Periodic Trends

Trend 1: Atomic Radii

Periodic Trends

Atomic radius is defined as half the distance between the nuclei of identical atoms that are chemically bonded together.

Atomic radius is a measure of atomic size.

Distance between nuclei

Atomic radius of atom

Atomic Radii Trends

Period Trend:

Decreases

Group Trend:

Increases

Moving left to right, the number of protons, neutrons and electrons all increase.

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Lithium atom Beryllium atom

As the # of protons/electrons increase, the attraction between the positive nucleus and negative electron cloud increases. This attraction “pulls” in on the electrons.

Radius decreases

Why do atomic radii decrease across a period?

Protons, neutrons and electrons are also added as you move down a group.

Move down the periodic table

Lithium atom

Sodium atom

Radius increases+ ee

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Why do atomic radii increase down a group?

However, the electrons are added in newenergy levels. The inner electrons act like a “shield” for the outer electrons, so they do not feel the pull of the nucleus so strongly.

Trend 2: Ionization Energy

What is Ionization Energy?

•An ion is a particle that has a positive or negative charge. •It got this charge when the particle lost electrons (it becomes positively-charged +) or gained electrons (it becomes negatively-charged -)•The energy that is needed to remove ONE of the outermost electrons is called the ionization energy.

Ionization Energy Trends

Period Trend: INCREASES

Group Trend: DECREASES

Recall that, as you move left to right across a period, the radius of the atom decreases.

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Lithium atom Beryllium atom

When an atom is smaller, the electrons are closer to the nucleus, and therefore feel the pull more strongly. It is harder to pull electrons away from the atom.

Radius decreases

IE increases

Why does Ionization Energy increase across a period?

Why does ionization energy decrease down a group?

Recall that, as you move down a group, the radius increases.

Move down the periodic table

Lithium atom

As the outer electrons are farther from the nucleus, they will feel the “pull” of the nucleus less. It is easier to remove them from the atom.

Radius increases + ee

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IE decreases

Sodium atom

Trend #3: Electron Affinity

What is Electron Affinity?

•Recall that to form negative ions, an atom gains electrons.•The energy released when an electron is added to an atom is called the electron affinity.

Electron Affinity Trends

Period Trend: Increases

Group Trend: Decreases

Why does Electron Affinity increase across a period?

Again, recall that atomic radius decreases as you move across a period.

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Lithium atom Beryllium atom

Recall that in smaller atoms, the electrons feel the pull of the protons more strongly. Such an atom can handle an extra electron more easily as it can be more “controlled” by the closer nucleus.

Radius decreases

EA increases

Why does electron affinity decrease down a group?

As you move down a group, the radius increases.

Move down the periodic table

Lithium atom

Radius increases+ ee

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EA decreases

Sodium atom

As the size of the atom increases, the outer electrons feel the “pull” of the nucleus less and less. A larger atom is less able to “control” a new electron added.

Ionic Charge & Radii

Review

There are 2 kinds of charged particles.

A cation is positively charged ion. It results from an atom losing electrons.

An anion is a negatively charged ion. It results from an atom gaining electrons.

Ionic Radii—Cations

Since atoms lose electrons to create cations, the atom ends up having more protons than electrons. Each proton, therefore, has a greater pull on each electron. A cation is smaller than its parent atom!

Creating a cation, losing electrons

Lithium atom Li+ ion

Radius decreases

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Ionic Radii—Anions

Since atoms gain electrons to create negative ions, the atom ends up having more electrons than protons. In this case, each proton has a weaker pull on each electron. An anion is bigger than it parent atom.

Creating an anion, gaining electrons

Oxygen atom O2- ion

Radius increases

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Let’s Practice

Example:List Li, Cs and K in order of

increasing

Atomic radii

Ionization Energy

Electron Affinity

Example:List Li, N and C in order of

increasing

Atomic radii

Ionization Energy

Electron Affinity

Let’s Practice

Example:List Li, Cs and K in order of

increasing

Atomic radii

Ionization Energy

Electron Affinity

Example:List Li, N and C in order of

increasing

Atomic radii

Ionization Energy

Electron Affinity

Li, K, Cs

Cs, K, Li

Cs, K, Li

N, C, Li

Li, C, N

Li, C, N