Chemistry - A Molecular Science

4.2

NAMING IONS AND PREDIC

TING THEIR CHARGE

Anions

:^ Filled sublevels are more stable than partially filled sublevels, so atoms react

with one another to reduce the number of partia

lly filled sublevels. Nonmetals tend to gain

electrons, so they add electrons to fill their valence shell, which requires 8 electrons (ns

2

np

6 ) for a nonmetal. The number of valence electrons in a nonmetal atom equals its group

number, so the number of electrons require

d to fill the valence shell is (8 - Group

Number). Each added electron carries a charge of

-1, so the charge on an anion equals the

number of electrons added times -1. The resulting anion is named by replacing the ending of the atom’s name with -

ide

. Oxygen is a Group 6A nonmetal and therefore gains (8 - 6)

= 2 electrons to acquire a -2 charge, so the ox

ide

ion is O

2-. Nitrogen, a Group 5A

nonmetal, gains three electrons to form the nitr

ide

ion, N

3-. Monatomic anions

• are

isoelectronic

† with the next noble gas, so their electron configurations are all [noble gas].

* Monatomic ions are ions that consist of only one atom. For example,

2-O

is a monatomic ion but O

2- and NO 2

1- are polyatomic ions. 3

Example 4.2

What is the charge on the anions formed by the halogens? What is the name of the ion formed from Cl, and what

is its electron configuration?

The halogens are the elements in Group 7A, so they gain (8 - 7) = 1 electron to form -1 ions. Cl

1- is the chloride ion. It is isoelectronic wi

th argon, so its configuration is [Ar].

Cations

: Metals tend to lose electrons, so they eliminate partially filled sublevels by

emptying them to produce cations. The charge

on a cation equals the number of electrons

lost, and the name of the cation is the same as the name of the atom. There are two rules that determine which electrons are lost.

1. The valence electrons with the highest

n quantum number are lost first.

2. When the valence electrons are in more than one sublevel within the same level, the

electrons in the highest

l sublevel are lost first.

The first rule means that the transition meta

ls lose the outermost s electrons before

they lose any d electrons

, so the order in which the electrons are removed is not simply the

reverse of the filling order. This is due to the f

act that the energies of the 4s and 3d orbitals

in an atom are very similar, and the chang

es in the environment of the valence electrons

that result when a metal atom becomes an ion

are sufficient to reverse the order of their

energy. Thus,

most

transition elements readily form a +2 ion by losing their

ns

2 electrons.

The second rule applies to the heavy elements

in the p block that lose the np electrons

before the ns electrons. Tl, 6s

2 6p

1 , forms a +1 ion by losing its 6p electron first and a +3

† Isoelectronic means having the same electron configuration. Two

species with the same number of electrons are isoelectronic.

Chapter 4 The Ionic Bond

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