Fluorine has the highest electronegativity of any element, and therefore always has a −1
oxidation number in its compounds.6 . E
At, astatine, is a radioactive element. Elements in the periodic table with atomic masses
shown in parentheses have no stable isotopes.7 . C
Atoms, molecules, or ions that are isoelectronic have identical electron configurations. This
may apply to monoatomic species (for example, F−, Ne, and Na+) or polyatomic species (like
NH 3 and H 3 O+).8 . D
Isomers have identical molecular formulae but different arrangements of atoms. For example,
there are two compounds with the formula C 2 H 6 O: one with the oxygen between the two
carbon atoms (CH 3 OCH 3 ), and one with the two carbon atoms bonded to each other and an OH
group bonded to one of the two carbon atoms (CH 3 CH 2 OH). These two isomers have different
chemical and physical properties and different names.9 . E
Isotopes have the same number of protons, but different numbers of neutrons, and therefore
different mass numbers. Since they have the same number of protons, they have the same
chemical identity.10 . A
Allotropes are different elemental forms of the same element. Carbon appears as graphite, as
diamond, and as fullerenes. Phosphorus appears as white phosphorus or red phosphorus.
Oxygen appears as oxygen gas (also called dioxygen), the common form in the element, and as
O 3 , ozone, a less common form that is a strong oxidizing agent, a component of smog, and a
component of the “ozone layer,” which absorbs much of the ultraviolet radiation falling on
the Earth from the sun.
11 . B
CO 2 is a major greenhouse gas, which absorbs sunlight and converts it to heat, causing the
temperature of the atmosphere to warm. (Methane is also a greenhouse gas, but to a lesser
extent.)