Valencies of Common Elements

Introduction

There are more than a hundred elements, but you do not need to learn a hundred separate valencies. The periodic table does most of the work for you. Elements in the same group usually share the same valency, and a simple rule links valency to group number. With this pattern and a short list of exceptions, you can find the combining power of almost any common element in seconds.

Core explanation

The pattern for main groups.

Group Example elements Usual valency Typical ion --- --- --- --- 1 H, Li, Na, K 1 Na⁺ 2 Mg, Ca, Ba 2 Mg²⁺ 3 B, Al 3 Al³⁺ 4 C, Si 4 (forms covalent bonds) 5 N, P 3 N³⁻ 6 O, S 2 O²⁻ 7 F, Cl, Br, I 1 Cl⁻ 0 He, Ne, Ar 0 none

Rising, then falling. For Groups 1 to 4, valency equals the group number: 1, 2, 3, 4. For Groups 4 to 7, valency equals 8 minus the group number: 4, 3, 2, 1. So valency climbs to a peak of 4 in the middle and then falls again. Group 0 has valency 0 because noble gases hardly ever combine.

Why the pattern exists. The group number of a main-group element equals the number of electrons in its outer shell. Metals on the left have few outer electrons and lose them, so sodium (1 outer electron) loses 1 and has valency 1, and aluminium (3) loses 3. Non-metals on the right have nearly full outer shells and gain or share electrons: chlorine (7) needs 1 more, oxygen (6) needs 2. Carbon, in the middle with 4, shares all four.

Hydrogen. Hydrogen is usually placed at the top of Group 1 and has valency 1, although it is a non-metal. It can lose its electron to form H⁺ or share it in a covalent bond.

Transition metals. Elements in the central block, such as iron, copper and zinc, do not follow the group rule. Many have more than one valency — iron has 2 or 3, copper 1 or 2 — which is shown in names with Roman numerals. A few have one common valency to learn: zinc is 2 and silver is 1.

A short list to memorise. For most formula writing at this level you need: H, Na, K, Ag, Cl, Br, I = 1; Mg, Ca, Zn, Ba, O, S = 2; Al, N = 3; C, Si = 4.

Step-by-step reasoning

To find the valency of a main-group element:

1. Find the element's group number in the periodic table. 2. If the group is 1 to 4, the valency equals the group number. 3. If the group is 4 to 7, the valency equals 8 minus the group number. 4. If it is Group 0, the valency is 0.

Visual explanation

Sketch the main groups as eight columns and write the usual valency above each: 1, 2, 3, 4, 3, 2, 1, 0. The numbers form a hill that rises to 4 at carbon's group and slopes down again to the noble gases, where the hill reaches flat ground.

Real-world analogy

Think of outer electrons as money needed to reach a target of eight. Someone with one coin finds it easiest to give it away; someone with seven only needs to borrow one more. People in the middle, with four, share. Valency is how many coins change hands.

Real-world example

Calcium's valency of 2 explains why the calcium compound in bones and teeth, calcium phosphate, and the one in limestone, calcium carbonate, both contain calcium in fixed ratios with other groups. Knowing that calcium always has valency 2 lets chemists predict the formulae of calcium compounds used in building, medicine and farming.

Why?

Why do elements in the same group have the same valency? Because they have the same number of outer-shell electrons. Lithium, sodium and potassium each have one outer electron, so each loses one electron and each has valency 1. The inner shells differ, but the outer shell controls combining power.

Common misconception

"The valency of an element always equals its group number." This works only for Groups 1 to 4. Oxygen is in Group 6 but has valency 2, and chlorine is in Group 7 but has valency 1.

Worked example

Question: Using the periodic table, state the valencies of potassium (Group 1), barium (Group 2), phosphorus (Group 5) and iodine (Group 7).

Reasoning: Potassium and barium are in Groups 1 and 2, so their valencies equal the group numbers: 1 and 2. Phosphorus and iodine are in Groups 5 and 7, so their valencies are 8 − 5 = 3 and 8 − 7 = 1.

Answer: K = 1, Ba = 2, P = 3, I = 1.

Quick check

1. What is the usual valency of aluminium, in Group 3? Answer: 3.

Exam focus

Learn the hill pattern 1, 2, 3, 4, 3, 2, 1, 0 and be ready to explain it using outer-shell electrons. Examiners often ask you to deduce a valency from group position and then use it to write a formula, so accuracy here carries marks forward into later questions.

Advanced insight

Some non-metals in period 3 and below can show higher valencies than the simple rule predicts. Phosphorus forms both PCl₃ (valency 3) and PCl₅ (valency 5), and sulfur forms SF₆ (valency 6). These larger atoms can use more of their outer electrons in bonding, so the "8 minus group" rule gives only the lowest common valency.

Summary

Valency follows the periodic table. In Groups 1 to 4 it equals the group number; in Groups 4 to 7 it equals 8 minus the group number; Group 0 has valency 0. The pattern comes from the number of outer-shell electrons. Transition metals such as iron and copper can have more than one valency, while zinc (2) and silver (1) are fixed.

Practice questions

1. State the valency of sulfur and explain it using its group. Answer: 2; sulfur is in Group 6, so its valency is 8 − 6 = 2. 2. Why do sodium and potassium have the same valency? Answer: Both are in Group 1 and have one outer-shell electron, which they lose. 3. What is the valency of neon, and why? Answer: 0, because it is a noble gas with a full outer shell and does not normally combine. 4. Give the valencies of magnesium, nitrogen and bromine. Answer: Magnesium 2, nitrogen 3, bromine 1. 5. Explain why iron cannot be given a single valency from its position in the periodic table. Answer: Iron is a transition metal, which does not follow the group rule and can have valency 2 or 3.