Quiz #7

November 10, 2000

Name: Largo, Key

1. Atomic size decreases across a period, even though the number of electrons increases. Explain why this trend is observed.

As you go across a period, the atomic number (Z) increases indicating similar increases in the number of protons and electrons. The added electrons, however, experience an increased electrostatic attraction due to the increased positive charge, as the degree of shielding remains constant. Thus, Z_eff increases as you go across a period, resulting in a greater attractive force on the valence electrons, pulling them in closer to the nucleus. Thus, atomic radii decrease as you go across a period.

2. Titanium (Z = 22) is a transition metal. What is the ground state electron configuration for the Ti²⁺ ion?

For the neutral Ti atom:

1s²2s²2p⁶3s²3p⁶4s²3d²

We remove electrons, however, from the 4s orbital first, giving the following for Ti²⁺:

1s²2s²2p⁶3s²3p⁶3d²

3. O^2- and Na⁺ and Al³⁺ are all isoelectronic with Ne. Arrange these four isoelectronic species in order of increasing radius. Give a very brief justification for your answer.

Atomic number increases as we go from O^2- to Ne to Na⁺ to Al³⁺, so Z_eff increases (number of electrons are same for all, so shielding is the same). Thus, species radius will decrease with increasing Z_eff.

In order of increasing radius: Al³⁺ < Na⁺ < Ne < O^2-