Valence and Common Polyatomic Groups · Intro Chemistry

Why do we need valence?

Water is $\ce{H2O}$ , not $\ce{HO}$ or $\ce{H3O}$ . Sodium chloride is $\ce{NaCl}$ , yet calcium chloride is $\ce{CaCl2}$ . When atoms of different elements combine, they do so in fixed ratios. Valence化合价 is a number that describes how many atoms of other elements an atom combines with.

Valence is the standard term in Chinese textbooks. English-language textbooks usually express the same idea with ion charges and oxidation numbers, which you’ll meet later. For simple compounds, all three give the same numbers.

Rules of valence

Valences can be positive or negative. In a compound, the valences add up to zero.
In an element on its own, the valence is 0.
Hydrogen is usually +1 and oxygen is usually −2.
When a metal combines with a nonmetal, the metal is usually positive and the nonmetal negative.
Some elements have more than one valence. Iron can be +2 or +3, and carbon can be +2 or +4.

How to write a valence

Write the valence directly above the symbol, sign first, then the number, and always include the 1: $\overset{+2}{\ce{Mg}}$ , $\overset{-2}{\ce{O}}$ , $\overset{+1}{\ce{Na}}$ .

Don’t confuse this with an ion’s charge, which goes at the upper right, number first, then the sign, with the 1 left out: $\ce{Mg^2+}$ , $\ce{Na+}$ .

Valences of common elements

Valence	Common elements
+1	K, Na, Ag, H
+2	Ca, Mg, Ba, Zn, Cu, Fe (iron(II))
+3	Al, Fe
−1	Cl (with metals or hydrogen)
−2	O, S (with metals or hydrogen)
More than one	Fe: +2, +3; Cu: +1, +2; C: +2, +4; S: −2, +4, +6; N: −3, +2, +4, +5

Common polyatomic groups

Some groups of atoms always stay together during reactions. These are polyatomic groups原子团, which form polyatomic ions. The group as a whole has one valence, equal to the sum of the valences inside it. For $\ce{SO4}$ : (+6) + (−2) × 4 = −2.

Name	Formula	Valence
Hydroxide	$\ce{OH}$	−1
Nitrate	$\ce{NO3}$	−1
Sulfate	$\ce{SO4}$	−2
Carbonate	$\ce{CO3}$	−2
Phosphate	$\ce{PO4}$	−3
Ammonium	$\ce{NH4}$	+1

Finding a valence: the total is zero

If you know the valences of the other elements, “the total is zero” lets you find the unknown one. Work it out yourself first, then click “Show answer” to check:

Try it: find a valence using "the total is zero"

Known: K is +1, O is −2
Let the valence of Mn be x
Set up: $(+1) \times 1 + x \times 1 + (-2) \times 4 = 0$
Solve: $x = +7$

Example 1: The chlorine in bleach

Household bleach contains sodium hypochlorite, $\ce{NaClO}$ . What is the valence of Cl?

Step-by-step solution

Known. Na is +1 and O is −2.
Define the unknown. Let the valence of Cl be x.
Set up. $(+1) \times 1 + x \times 1 + (-2) \times 1 = 0$
Solve. x = +1. Here chlorine is +1, not its usual −1. The same element can have different valences in different compounds.

Example 2: Two iron oxides

What is the valence of iron in $\ce{FeO}$ and in $\ce{Fe2O3}$ ?

Step-by-step solution

$\ce{FeO}$ . x + (−2) = 0, so x = +2.
$\ce{Fe2O3}$ . 2x + (−2) × 3 = 0, so x = +3.
Names. In English, the valence goes in the name as a Roman numeral: FeO is iron(II) oxide and $\ce{Fe2O3}$ is iron(III) oxide. Lesson 1.5.4 covers naming.

Common mistakes

Mistake 1: Mixing up valence and ion charge. Magnesium with a valence of +2 is written $\overset{+2}{\ce{Mg}}$ (directly above, sign first). The magnesium ion is written $\ce{Mg^2+}$ (upper right, number first).

Mistake 2: Using the usual valence for an element on its own. The oxygen in $\ce{O2}$ has a valence of 0, not −2. Oxygen is usually −2 only in compounds.

Mistake 3: Forgetting to multiply by the number of atoms. For iron in $\ce{Fe2O3}$ , write 2x + (−2) × 3 = 0, not x + (−2) = 0.

Quick check

$\ce{ClO2}$ is a common disinfectant. What is the valence of Cl in it?

Which one shows magnesium with a valence of +2?

Summary

In a compound, the valences add up to zero. In an element on its own, the valence is 0.
Write a valence directly above the symbol, sign first. An ion’s charge goes at the upper right, number first.
Common polyatomic groups: hydroxide −1, nitrate −1, sulfate −2, carbonate −2, phosphate −3, ammonium +1.
“The total is zero” lets you find the valence of an unknown element in a compound.

On to AP (for later)

AP Chemistry describes this idea with oxidation numbers (topic 4.9, redox) and predicts ion charges from an element’s position in the periodic table (topic 1.8). You’ll need to know the common polyatomic ions by heart.

Valence and Common Polyatomic Groups化合价与常见原子团

Learning goals

Before you start

Why do we need valence?

Rules of valence

How to write a valence

Valences of common elements

Common polyatomic groups

Finding a valence: the total is zero

Example 1: The chlorine in bleach

Example 2: Two iron oxides

Common mistakes

Quick check

Summary

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