Concept Check 2.1 Question: Why are the atomic weights of the elements generally not integers? Cite two

reasons.

Answer: The atomic weights of the elements ordinarily are not integers because: (1) the

atomic masses of the atoms normally are not integers (except for 12C), and (2) the atomic weight

is taken as the weighted average of the atomic masses of an atom's naturally occurring isotopes.

Concept Check 2.2

Question: Give electron configurations for the Fe3+and S2- ions.

Answer: The Fe3+ ion is an iron atom that has lost three electrons. Since the electron

configuration of the Fe atom is 1s22s22p63s23p63d64s2 (Table 2.2), the configuration for Fe3+ is

1s22s22p63s23p63d5.

The S2- ion a sulfur atom that has gained two electrons. Since the electron configuration

of the S atom is 1s22s22p63s23p4 (Table 2.2), the configuration for S2- is 1s22s22p63s23p6.

Concept Check 2.3

Question: Offer an explanation as to why covalently bonded materials are generally less

dense than ionically or metallically bonded ones.

Answer: Covalently bonded materials are less dense than metallic or ionically bonded

ones because covalent bonds are directional in nature whereas metallic and ionic are not; when

bonds are directional, the atoms cannot pack together in as dense a manner, yielding a lower mass

density.