ODD (i.e.top) PROBLEMS:

1. Predict the empirical formula of the ionic compound containing: Ca and S.
   1. Ca₂S
   2. CaS₂
   3. CaS
   4. Ca₂S₃
   5. Ca₃S₂
   ANSWER: Ca wants to be Ca²⁺, and S wants to be S^2-; therefore the formula is CaS, C.

2. What is the formal charge on the "starred" oxygen?
   1. -2
   2. -1
   3. 0
   4. +1
   5. +2
   ANSWER: The oxygen has two non-bonding electrons and one-half of six bonding electrons for a total of five. Oxygen wants six, therefore its formal charge is +1, D.

3. What is the molecular geometry of SeF₂?
   1. Linear
   2. Bent: 90^o
   3. Bent: 109.5^o
   4. Bent: 107^o
   5. Bent: 104.5^o
   ANSWER: Its reasonably difficult for me to put Lewis dot structures on the web so I will describe it for you instead: SeF₂ is isoelectronic with water so it too has two single bonds, and two pairs of non-bonding electrons remaining on the selenium. The Octet rule on all three atoms is thereby satisfied. The molecular geometry should also be expected to be similar to H₂O with a bond angle of approximately 104.5, E.^o

4. Ca²⁺ is isoelectronic with which of the following ions?
   
   1. Mg²⁺
   2. Rb⁺
   3. Na⁺
   4. K⁺
   5. Br^-
   ANSWER: Ca²⁺ has an [Ar] electron configuration. Only K⁺ possesses the same electron configuration, D.

EVEN (i.e. bottom) PROBLEMS:

1. Predict the empirical formula of the ionic compound containing: Al and Cl.
   1. AlCl₂
   2. AlCl₃
   3. AlCl
   4. Al₂Cl₃
   5. Al₂Cl
   ANSWER: Al wants to be Al³⁺, and Cl wants to be Cl^-; therefore the formula is AlCl₃, B.

2. What is the formal charge on the "starred" nitrogen?
   1. -2
   2. -1
   3. 0
   4. +1
   5. +2
   ANSWER: The nitrogen has four non-bonding electrons and one-half of four bonding electrons for a total of six. Nitrogen wants five, therefore its formal charge is -1, B.

3. What is the molecular geometry of HCN?
   1. Linear
   2. Bent: 90^o
   3. Bent: 109.5^o
   4. Bent: 107^o
   5. Bent: 104.5^o
   ANSWER: Its reasonably difficult for me to put Lewis dot structures on the web so I will describe it for you instead: A single bond exists between the H and the C, and a triple bond between the C and the N. The total electron count is, of course, 10. The Octet and Duet rules are fully satisified, and with two electron "blobs" and no non-bonding electrons on the carbon, HCN must have a linear geometry, A.

4. Cl^- is isoelectronic with which of the following ions?
   
   1. Br^-
   2. I^-
   3. S^2-
   4. Mg²⁺
   5. Na⁺
   ANSWER: Cl^- has an [Ar] electron configuration. Only S^2- possesses the same electron configuration, C.