1 PERIODIC TRENDS

2 PERIODICITYPERIODICITY Period Law- - physical and chemical properties of elements are a periodic function of their atomic numbers

3 General Periodic Trends Atomic and ionic size Ionization energy Electron affinity, electronegativity

4 Effective Nuclear Charge Z*

5 Effective Nuclear Charge, Z* AtomZ* Experienced by Electrons in Valence Orbitals Li+1.28 Be B+2.58 C+3.22 N+3.85 O+4.49 F+5.13 Increase in Z* across a period

6 General Periodic Trends Higher effective nuclear charge Electrons held more tightly Larger orbitals. Electrons held less tightly.

7 Periodic Trend in the Reactivity of Metals Reactivity Periodic Trend in the Reactivity of Metals ReactivityLithium Sodium Potassium MOST

2. Reactivity for MetalsReactivity As you go down a group for metals the number of energy levels increase. Because of this, reactivity increases because the atom is more willing to give away its electron (react).

3.Nonmetalic Trends: Gain electrons Nonmetals on right side, form anions Going right elements are more nonmetallic (better gainers of electrons) Going UP elements become more nonmetallic (want to gain)

8. Reactivity nonmetals: Gain e The reason Across = fill the energy level Going UP a group, nonmetals have same valence but fewer total electrons Flourine is the most reactive nonmetal.

11 Atomic Radii Figure 8.9

12 Atomic Size Size increases, down a group.Size increases, down a group. Because electrons are added into additional energy levels, there is less attraction.Because electrons are added into additional energy levels, there is less attraction. Size decreases across a period.Size decreases across a period. Because, increased effective nuclear charge.Because, increased effective nuclear charge. Size increases, down a group.Size increases, down a group. Because electrons are added into additional energy levels, there is less attraction.Because electrons are added into additional energy levels, there is less attraction. Size decreases across a period.Size decreases across a period. Because, increased effective nuclear charge.Because, increased effective nuclear charge.

13 Atomic Size Size decreases across a period owing to increase in Z*. Each added electron feels a greater and greater + charge. Large Small

14 Trends in Atomic Size See Figures 8.9 & 8.10

15 Ion Sizes CATIONS are SMALLER than the atoms from which they come.CATIONS are SMALLER than the atoms from which they come. The electron/proton attraction has gone UP and so size DECREASES.The electron/proton attraction has gone UP and so size DECREASES. Li,152 pm 3e and 3p Li +, 78 pm 2e and 3 p + Forming a cation.

16 Ion Sizes ANIONS are LARGER than the atoms from which they come.ANIONS are LARGER than the atoms from which they come. The electron/proton attraction has gone DOWN and so size INCREASES.The electron/proton attraction has gone DOWN and so size INCREASES. Trends in ion sizes are the same as atom sizes.Trends in ion sizes are the same as atom sizes. Forming an anion. F, 71 pm 9e and 9p F -, 133 pm 10 e and 9 p -

17 Trends in Ion Sizes Figure 8.13

18 Ionization Energy IE = energy required to remove an electron from an atom in the gas phase. Mg (g) kJ ---> Mg + (g) + e-

19 Mg (g) kJ ---> Mg + (g) + e- Mg + (g) kJ ---> Mg 2+ (g) + e- Mg + has 12 protons and only 11 electrons. Therefore, IE for Mg + > Mg. IE = energy required to remove an electron from an atom in the gas phase. Ionization Energy

20 1 st IE: Mg (g) kJ ---> Mg + (g) + e- 2 nd IE: Mg + (g) kJ ---> Mg 2+ (g) + e- 3 rd IE: Mg 2+ (g) kJ ---> Mg 3+ (g) + e- Energy cost is very high to dip into a shell of lower n (core electrons). This is why ox. no. = Group no. Ionization Energy

21 Trends in Ionization Energy

22 Trends in Ionization Energy IE decreases down a group Because size increases. IE increases across a period Because effective nuclear charge increases IE increases across a period Because effective nuclear charge increases

23 Electron Affinity A few elements GAIN electrons to form anions. Electron affinity is the energy involved when an atom gains an electron to form an anion. X(g) + e- ---> X - (g) E.A. = ∆E

24 Trends in Electron Affinity

25 Affinity for electron increases across a period (EA becomes more negative). Affinity decreases down a group (EA becomes less negative). Atom EA F-328 kJ Cl-349 kJ Br-325 kJ I-295 kJ Atom EA F-328 kJ Cl-349 kJ Br-325 kJ I-295 kJ Trends in Electron Affinity

26 Electron Affinity of Oxygen ∆E is EXOthermic because O has an affinity for an e-. [He]      O atom EA = kJ + electron O [He]       - ion

27 Electron Affinity of Nitrogen ∆E is zero for N - due to electron- electron repulsions. EA = 0 kJ [He]     Natom  [He]    N - ion  + electron

28 Electronegativity So how is this different from electron affinity? Electron Affinity – is rating of how well an atom wants to gain an electron Electronegativity – is rating of how well an atom keeps the electron once it is bonded to another atom

29 Electronegativity

30 Electron Configurations and the Periodic Trends

31 “Your Best Friend” Periodic table