- Restatement: How are ionization energy and atomic radius affected by the heat of reaction for
M(s) + Br 2 (g) →MBr 2 (s)
I. Ionization energy
A.Definition — the amount of energy that a gaseous atom must absorb so that the
outermost electron can be completely separated from the atom.
B.The lower the ionization energy, the more metallic the element.
C.With all other factors held constant, energy is required to form the M2+ion
(endothermic)
D.With larger ionization energies, the heat of the reaction becomes more positive or
more endothermic.
II. Atomic radius
A.Definition — half the distance of closest approach between two nuclei in the
ordinary form of an element.
B.Positive ions are smaller than the metal atoms from which they are formed.
C.Lattice energy — energy released when an ionic (metal M +; nonmetal Br) solid
forms from its ions.
D.The change that occurs from M(s) →M2+, which exists as the cation in the ionic
solid MBr 2 , results in a decrease in the radius.
E.In the calculation of lattice energy,
.. r
QQ
LE=^12
the lattice energy is inversely proportional to the radius, r.
F. Because atomic distances (r) are decreasing in the reaction, lattice energy is in-
creasing (−∆H), which has the effect of making the heat of reaction more negative,
or more exothermic.
G.Q 1 and Q 2 being opposite in sign further confirms the fact that bringing cations and
anions together is an exothermic process.Part II: Specific Topics