Understanding Periodic Trends: Atomic Radii, Ionization Energy, and Electron Affinity

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

2. 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!!

3. Trend 1: Atomic Radii

4. 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

5. Atomic Radii Trends Period Trend: Decreases Group Trend: Increases

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

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

8. Trend 2: Ionization Energy

9. 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.

10. Ionization Energy Trends Period Trend: INCREASES Group Trend: DECREASES

11. Move across the periodic table Radius decreases IE increases 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. e e e e n p n e e p p p n p p n n n p n e 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.

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

13. Trend #3: Electron Affinity

14. 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.

15. Electron Affinity Trends Period Trend: Increases Group Trend: Decreases

16. Why does Electron Affinity increase across a period? Move across the periodic table Radius decreases EA increases Again, recall that atomic radius decreases as you move across a period. e e e e n p n e e p p p n p p n n n p 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.

17. Why does electron affinity decrease down a group? e e e e + e e e e e + e e e e e Move down the periodic table Radius increases EA decreases As you move down a group, the radius increases. Sodium atom 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.

18. Ionic Charge & Radii

19. Review There are 2 kinds of charged particles. A cationis positively charged ion. It results from an atom losing electrons. An anionis a negatively charged ion. It results from an atom gaining electrons.

20. e e e e e + + Creating a cation, losing electrons Radius decreases 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! Li+ ion Lithium atom

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

22. 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

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