Electron Configurations and the Properties of Atoms

3p 3p
3s
3s 1
2p
2p 2s

2s

1s 1s
S S2–

[Ne]3s23p4 1 2 e– [Ne]3s23p6

The total number of electrons gained when an element forms an anio
electron configuration of the element. In the case of Cl, only one electron
a second electron would occupy the higher-energy 4s orbital. An electron
farther from the nucleus and therefore feels less attractive “pull” from the
same reason, sulfur is often found as a –2 anion and not a –3 anion.

Finally, not all main-group elements commonly form ions. The Grou
metals lose one, two, and three electrons, respectively, to form cations wh
the cation is equal to the group number. The Group 5A, 6A, and 7A nonm
two, and one electrons, respectively, to form anions where the charge on t
to (8 − the group number). The Group 4A elements, however, are not co
+4 or −4 ions because the gain or loss of four electrons requires a great

Example Problem 7.5.2 Write electron configurations for anions.

Write electron configurations for the following ions in spdf and orbital box not
a. O2− (Do not use noble gas notation.)
b. I− (Use noble gas notation.)

Solution:
You are asked to write an electron configuration for an anion using spdf and
notation.
You are given the identity of the anion.
a. Oxygen is element 8. The element gains two electrons in its highest-energy

the O2− ion.

Unit 7 Electron Configurations and the Properties of Atoms

on is related to the
is gained because
n in this orbital is
e nucleus. For the

up 1A, 2A, and 3A
here the charge on
metals gain three,
the anion is equal
ommonly found as

deal of energy.

tation.

d orbital box

y orbitals to form

c

218

b

Example Problem 7.5.2 (continued)

O: 1s22s22p4 2p
1s 2s

O22: 1s22s22p6 2p
1s 2s

b. Iodine is element 53. The element gains one electron in its highest-energy o
the I− ion.

I: [Kr]5s24d105p5 [Kr] 4d 5p
5s

I2: [Kr]5s24d105p6 [Kr] 4d 5p
5s

7.5c Ion Size

The size of an ion is related both to the size of the atom from which it is fo
charge. In Interactive Figure 7.5.1, notice that

● cations are smaller than the atoms from which they are formed;

● anions are larger than the atoms from which they are formed; and

● anions are generally larger than cations.

Cations are smaller than the atoms from which they are formed prima
have fewer electrons. For example, magnesium has 12 electrons and loses
the Mg2+ cation.

3s 3s
2p
2p 1
2s
2s

1s 1s
Mg Mg2+

Unit 7 Electron Configurations and the Properties of Atoms

orbital to form Video Solution
ormed and the ion
Tutored Practice
Problem 7.5.2

arily because they
2 of them to form

219

Interactive Figure 7.5.1

Explore relative sizes of neutral atoms, cations, and anions.

1A 2A 3A 5A 6A

Li1, 78 Be21, 34 N32, 146 O22, 140
Li, 152 Be, 113 N, 71 O, 66

© 2013 Cengage Learning Na1, 98 Mg21, 79 Al31, 57 S22, 184
Na, 186 Mg, 160 Al, 143 S, 104

K1, 133 Ca21, 106 Ga31, 62 Se22, 191
K, 227 Ca, 197 Ga, 122 Se, 117

Rb1, 149 Sr21, 127 In31, 92 Te22, 211
Rb, 248 Sr, 215 In, 163 Te, 143

Cs1, 165 Ba21, 143 Tl31, 105
Cs, 265 Ba, 217 Tl, 170

Ionic radii (pm)

A Mg2+ ion is smaller than a Mg atom because the highest-energy occ
changed from 3s to 2p. The 2p orbital is smaller than the 3s orbital, so the ra
is smaller than the radius of the Mg atom. In addition, the fact that Mg2+ ha
than Mg minimizes electron repulsion forces, also decreasing the size of the

Anions are larger than the atoms from which they are formed primaril
added electrons. Consider formation of the sulfide ion, S2−, from a sulfur
ion has 18 electrons, 2 more than a sulfur atom. The additional electron
subshell, which was already partially occupied in a sulfur atom.

Unit 7 Electron Configurations and the Properties of Atoms

7A
0 F2, 133

F, 71

Cl2, 181
Cl, 99

1 Br2, 196
Br, 114

1 I2, 220
I, 133

cupied orbital has
adius of the cation
as fewer electrons
e cation.
ly because of their
atom. The sulfide
ns occupy the 3p

220

3p 3p
3s
3s 1
2p
2p 2s

2s

1s 1s
S S2–

The added electrons increase the existing repulsive forces between e
subshell, causing the electrons to move away from one another. This in
expansion of the electron cloud and an anion that is larger than the neutr

Consider the following isoelectronic ions, species with the same nu
but different numbers of protons.

Electron
Protons Electrons Configuration Ion Radi

O22 8 10 1s22s22p6 140 pm
F2 9 10 1s22s22p6 133 pm
Na1 11 10 1s22s22p6 102 pm
Mg21 12 10 1s22s22p6
66 pm

The ion with the largest radius in this isoelectronic series is O2−, and th
Mg2+. The oxide ion has only 8 protons to attract its 10 electrons, wherea
ion has 12 protons and thus more strongly attracts its 10 electrons.

Unit 7 Electron Configurations and the Properties of Atoms

electrons in the 3p
n turn leads to an
ral atom.
umber of electrons

ius

he smallest ion is Section 7.5 Mastery
as the magnesium

221

Unit Recap

Key Concepts

7.1 Electron Spin and Magnetism
● Electrons exert a magnetic field due to electron spin (7.1a).
● An electron has two possible spin states, indicated by the spin q

(ms = +½ or −½) (7.1a).
● Materials with unpaired electrons are magnetic (7.1b).
● The magnetic nature of a material can be categorized as diamagnetic,

ferromagnetic (7.1b).

7.2 Orbital Energy
● In a single-electron atom, the energy of an orbital is directly related t

quantum number (7.2a).
● In multielectron atoms, orbital energy depends on both n and ℓ (7.2a

7.3 Electron Configuration of Elements
● The electron configuration of an element shows how electrons are dis

als (7.3a).
● An atom’s ground state is the lowest-energy electron configuration for
● The Pauli exclusion principle states that no two electrons within an a

same set of four quantum numbers (7.3a).
● Electron configurations are written using spdf and orbital box notatio
● Hund’s rule of maximum multiplicity states that the lowest-energy el

tion is the one where the maximum number of electrons is unpaired
● Noble gas notation is a shortcut used when writing electron configu

noble gas symbol is used to represent core electrons (7.3b).
● Valence electrons are the electrons beyond the core electrons (7.3d)

7.4 Properties of Atoms
● Orbital energy in elements decreases as you move left to right across

and increases as you move down a periodic group (7.4a).

Unit 7 Electron Configurations and the Properties of Atoms

quantum number

, paramagnetic, or

to n, the principal
a).

stributed in orbit-
r that atom (7.3a).
atom can have the
on (7.3b).
lectron configura-
(7.3b).
urations, where a
).

the periodic table

222

● The effective nuclear charge for the highest-energy electrons in an at
charge felt by those electrons, taking into account the attractive forc
trons and the nucleus and the repulsive forces between electrons (7.

● Effective nuclear change generally increases in elements as you m
across the periodic table (7.4a).

● The covalent radius of an element is the distance between the nucle
that element when they are held together by a single bond (7.4b).

● Metallic radius is the distance between two atoms in a metallic crysta
● Atom size generally decreases as you move left to right across the p

increases as you move down within a periodic group (7.4b).
● Ionization energy is the amount of energy required to remove an ele

eous atom (7.4c).
● Ionization energy generally increases as you move left to right across

and decreases as you move down within a periodic group (7.4c).
● Electron affinity is the energy change when an electron is added to a gas
● Electron affinity values generally become more negative as you m

across the periodic table and become less negative (more positive) a
within a periodic group (7.4d).
7.5 Formation and Electron Configuration of Ions
● Metals generally form cations by losing their highest-energy valence
● Nonmetals generally form anions by gaining electrons in their highes
(7.5b).
● Cations are generally smaller than the atom from which they are for
are generally larger than the atoms from which they are formed (7.5c
● Isoelectronic ions have the same number of electrons but different nu
(7.5c).

Unit 7 Electron Configurations and the Properties of Atoms

tom is the nuclear
ces between elec-
.4a).
move left to right

ei of two atoms of

al (7.4b).
periodic table and

ectron from a gas-

the periodic table

seous atom (7.4d).
move left to right
as you move down

electrons (7.5a).
st-energy orbitals

rmed, and anions
c).
umbers of protons

223

Key Terms 7.3 Electron Config
electron configuratio
7.1 Electron Spin and Magnetism ground state
electron spin Pauli exclusion princ
spin state spdf notation
spin quantum number, ms orbital box notation
diamagnetic Hund’s rule of maxim
paramagnetic noble gas notation
ferromagnetic core electrons
valence electrons
Unit 7 Review and Challenge Problems

Unit 7 Electron Configurations and the Properties of Atoms

guration of Elements 7.4 Properties of Atoms
on effective nuclear charge
ciple covalent radius
metallic radius
mum multiplicity ionization energy
electron affinity

7.5 Formation and Electron Configuration
of Ions
isoelectronic ions

224