6. When an oxygen atom forms its most common ion, it
a) gains one electron.
b) gains two electrons.
c) loses one electron.
d) loses two electrons.

gains two electron!

Explanation:

When a barium atom forms its most common ion, it gains two electron!

substituting a hydrogen atom with a halogen in a hydrocarbon:

- the single bond remains single. a is wrong.

- the bond with hydrogen is as strong as with a halogen. c is wrong

- a hydrocarbon is saturated if there is no double bond. we do not know that so d is wrong.

- carbon will always have four valence electrons. e is wrong.

by elimination, the correct answer is b. the boiling point of the new compound increases.