Understanding the Periodic Nature of the Elements

DETERMINING THE RADIUS OF AN ATOM Br 2 = A (1.14 A)  C-C = 1.54 A (0.77 A)  C-Br = 1.91 A   

TRENDS IN ATOMIC SIZE Atomic size correlates to the number of energy levels (n) in an atom and the effective nuclear charge (Z eff ) of that atom

Li:1s 2 2s 1 Be:1s 2 2s 2 B:1s 2 2s 2 2p 1 n = 2, Z eff = 1 n = 2, Z eff = 2 n = 2, Z eff = 3 Na:1s 2 2s 2 2p 6 3s 1 n = 3, Z eff = 1 Mg:1s 2 2s 2 2p 6 3s 2 n = 3, Z eff = 2 O:1s 2 2s 2 2p 6 3s 2 3p 1 n = 3, Z eff = 3

1s 2 2s 2 2p 6 3s 2 3p 2 Si: IONIZATION ENERGY (I 1,2,…n ) The energy required to remove an electron from a gaseous atom: M(g)  M(g) + + e -

Ionization energy increases from left to right across a period Ionization energy decreases going down a group

Be:1s 2 2s 2 B:1s 2 2s 2 2p 1 n = 2, Z eff = 2 n = 2, Z eff = 3 1s 2 2s 2 1s 2 2s 2 2p 1 These 2e - do a better job of shielding the 2p 1 electron than they do of shielding each other  decrease in I 1

N:1s 2 2s 2 2p 3 O:1s 2 2s 2 2p 4 n = 2, Z eff = 5 n = 2, Z eff = 3 1s 2 2s 2 1s 2 2s 2 2p 1 These 2e - repel one another  decrease in I 1

ELECTRON AFFINITY (E a ) The energy change that occurs when an electron is added to a gaseous atom: M(g) + e -  M -

Adding one or more electrons is an exothermic process for most neutral atoms and for all positively charged ions. Adding one or more electrons is an endothermic for process for some neutral atoms and for all negatively charged anions ions. The greater the attraction between the species and the added electron, the more negative the electron affinity.

Electron affinity generally increases from left to right across the periodic table Electron affinity undergoes very little change going down a group Note that VA elements are less negative (don’t attract electrons as well as) IVA elements N:1s 2 2s 2 2p 3 n = 2, Z eff = 5 C:1s 2 2s 2 2p 2 n = 2, Z eff = 4 Adding this electron would increase the electron-electron repulsion and reduce the electron-nucleus attraction for the added electron  less negative E a In general, atoms with filled or half-filled subshells have more positive electron affinities than do elements on either side of them in the periodic table

Metallic Elements Distinguishing luster Good thermal and electrical conductivity Most metallic oxides are basic Exist in solution as cations. Nonmetallic Elements Nonlusterous Solids are brittle Most nonmetal oxides are acidic Exist in solution as cations.

Group Trends: The Active Metals Active metals are soft, possess low densities and low melting points Alkali metals possess the lowest ionization energies of the elements Alkali metals are prepared by passing an electric current (electrolysis) through molten salt 2M(s) + H 2  2MH(s) Alkali metals readily combine with most nonmetals 2M(s) + S  M 2 S(s) 2M(s) +Cl 2  2MCl(s) Alkali metals react vigorously with water 2M(s) + H 2 O  2MOH + H 2 (g) Alkali metals react with oxygen to form oxides, peroxides, and superoxides 4Li(s) + O 2  2Li 2 O 2Na(s) + O 2  2Li 2 O 2 K + O 2  KO 2

Alkaline Earth metals are very reactive but less so than alkali metals Alkaline metals harder, more dense than, and melt at higher temperatures that alkali metals. Alkaline earth metals possess higher ionization energies than alkali metals Some Alkaline metals react directly with water Beryllium and magnesium do not react with water, however, Mg will react with steam. Mg(s) + H 2 O(g)  2MgO(s) + H 2 (g) Group Trends: The Alkaline Earth Metals Calcium, Strontium, and barium react directly with water Ca(s) + H 2 O(g)  2CaOH(s) + H 2 (g)

Transitional Elements: Comparing 1A and 1B Elements K:1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 Cu:1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 10 Because the ‘d electrons’ partially shield the 4s electron in Cu and because there is a greater nuclear charge, the 4s electron experiences a larger effective nuclear charge than its counterpart in potassium Note that both elements have 4s 1 valence electrons

Group Trends: Selected Nonmetals Hydrogen is a nonmetal and is diatomic in nature Hydrogen possess no electron shield and therefore possess a higher ionization energy than other iA elements (I 1 = 1312 kJ/mol ) Hydrogen reacts with active metals to from solid metal hydrides Mg(s) + H 2 (g)  2NaH Ca(s) + H 2 (g)  CaH 2 Hydrogen generally reacts with nonmetals to produce molecular compounds

Group Trends: Selected Nonmetals Oxygen is a colorless diatomic gas. All the other VIA elements are solid Both oxygen and sulfur exist as allotropes (different forms of the same element) O 2 (g)  O 3 (g)  H = kJ Oxygen and Sulfur to a lesser degree, possess the ability to attract electrons. Tends to react with metals to produce a metal oxide. Also forms nonmetal oxides

Halogens are generally known as salt formers Halogens exist as diatomic molecules Melting points of Halogens increase with increasing atomic number Group Trends: Selected Nonmetals