Section 4.5—Periodicity Objectives: Define periodic trend Use periodic trends in atomic radius, ionization energy, and electron affinity to organize elements

Periodic Trends 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!!

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

Why do atomic radii decrease across a period? Moving left to right, the number of protons, neutrons and electrons all increase. e e Move across the periodic table e e n p n e p p p n p e p n Radius decreases n n p n e Beryllium atom Lithium 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.

Why do atomic radii increase down a group? Protons, neutrons and electrons are also added as you move down a group. Sodium atom + e e + Move down the periodic table Radius increases Lithium atom However, the electrons are added in new energy 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

Why does Ionization Energy increase across a period? Recall that, as you move left to right across a period, the radius of the atom decreases. Move across the periodic table e e e e n p n e e p Radius decreases p p n p p n n n p n e IE increases 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.

Why does ionization energy decrease down a group? Recall that, as you move down a group, the radius increases. Sodium atom + e Move down the periodic table e + Radius increases IE decreases 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.

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. Move across the periodic table e e e e n Radius decreases p n e e p p p n p p n n n p EA increases n e Beryllium atom Lithium 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.

Why does electron affinity decrease down a group? As you move down a group, the radius increases. Sodium atom + e Move down the periodic table e + Radius increases EA decreases Lithium 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! e + Creating a cation, losing electrons e + Radius decreases Li+ ion Lithium atom

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. e + e e + Creating an anion, gaining electrons Radius increases e Oxygen atom O2- ion

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 Li, K, Cs Cs, K, Li Example: List Li, N and C in order of increasing Atomic radii Ionization Energy Electron Affinity N, C, Li Li, C, N