Finding Valency from Hydrogen Compounds

Introduction

Four simple molecules — hydrogen chloride, water, ammonia and methane — form a neat pattern. Each contains one atom of another element joined to one, two, three or four hydrogen atoms. Laid out in order, they make a valency ladder : every step up adds one hydrogen and one unit of combining power. This ladder is one of the quickest ways to remember the valencies of important non-metals and to predict formulae for elements you have not met before.

Core explanation

Hydrogen as the measuring stick. Hydrogen has valency 1: each hydrogen atom forms exactly one bond. So if you count how many hydrogen atoms are attached to one atom of another element, you have measured that element's valency directly.

The ladder.

Step Compound Formula Hydrogen atoms Valency of the other element --- --- --- --- --- 1 Hydrogen chloride HCl 1 Cl = 1 2 Water H₂O 2 O = 2 3 Ammonia NH₃ 3 N = 3 4 Methane CH₄ 4 C = 4

Shapes to go with the formulae. Each molecule has a characteristic shape. HCl is a simple straight pair. Water is bent, with an angle of about 104.5° between its two O–H bonds. Ammonia is a low pyramid with the nitrogen at the top. Methane is a tetrahedron, with bond angles of about 109.5°. You do not need to explain these shapes yet, but they show that the four bonds of carbon point in different directions in space.

Reading the ladder backwards across the periodic table. Chlorine is in Group 7, oxygen in Group 6, nitrogen in Group 5 and carbon in Group 4. For these non-metals, the valency equals 8 minus the group number: 8 − 7 = 1, 8 − 6 = 2, 8 − 5 = 3, 8 − 4 = 4. The ladder therefore runs right to left across period 2 (with chlorine from period 3 standing in for fluorine, which also forms HF).

Predicting new formulae. Elements in the same group have the same valency, so they form similar hydrides. Sulfur is below oxygen, so its hydride is H₂S. Phosphorus is below nitrogen: PH₃. Silicon is below carbon: SiH₄. Bromine is below chlorine: HBr. One ladder lets you predict a whole set of formulae.

What the ladder does not include. Metals mostly do not form simple molecular hydrides, and the noble gases form none. For metals, valency is usually found from chlorides or oxides instead, as in NaCl or MgO.

Step-by-step reasoning

To predict the hydride of an unfamiliar non-metal, such as selenium:

1. Find its group: selenium is in Group 6. 2. Identify the ladder member in that group: oxygen, in H₂O. 3. Use the same valency: 2. 4. Write the formula with two hydrogen atoms: H₂Se.

Visual explanation

Imagine four ball-and-stick models on a shelf in order: a chlorine ball with one white hydrogen ball, a red oxygen with two, a blue nitrogen with three and a black carbon with four. The number of white balls climbs by one along the shelf, like the rungs of a ladder.

Real-world analogy

The ladder is like a set of adaptors with one, two, three and four ports. Plug hydrogen "devices" into each, and each adaptor fills up with exactly its number of ports. Once you know which adaptor a new element resembles, you know how many devices it takes.

Real-world example

The ladder contains some of the most important substances on Earth. Water covers most of the planet; ammonia is made in vast amounts for fertilisers; methane is the main component of natural gas; and hydrogen chloride dissolves in water to form hydrochloric acid, which your stomach uses to help digest food.

Why?

Why does valency rise from chlorine to carbon? Chlorine atoms have seven outer electrons and need one more to reach eight, so they share one pair with hydrogen. Oxygen has six and needs two; nitrogen has five and needs three; carbon has four and needs four. Each shared pair is one bond.

Common misconception

"Water is H₂O because oxygen likes hydrogen more than chlorine does." Liking has nothing to do with it. Oxygen simply has a valency of 2, so it forms two bonds; chlorine has valency 1 and forms one.

Worked example

Question: Arsenic is in Group 5, below phosphorus. Predict the formula of its hydride and give its valency.

Reasoning: Group 5 matches nitrogen on the ladder, which forms NH₃ with valency 8 − 5 = 3. Arsenic should also have valency 3 and combine with three hydrogen atoms.

Answer: AsH₃; valency 3.

Quick check

1. What is the valency of oxygen, as shown by the formula of water? Answer: 2, because one oxygen atom combines with two hydrogen atoms.

Exam focus

Learn the four ladder formulae and the rule valency = 8 − group number for Groups 4 to 7. Examiners like asking you to predict the hydride of an element below one on the ladder, such as H₂S or PH₃, and to explain it using group position.

Advanced insight

Water and ammonia contain unused pairs of outer electrons called lone pairs: oxygen has two and nitrogen one. These lone pairs push the bonds together, which is why water is bent and ammonia is pyramidal rather than flat. Methane has no lone pairs, so its four bonds spread out evenly in a tetrahedron.

Summary

Counting hydrogen atoms in a hydride measures valency directly. The ladder HCl, H₂O, NH₃, CH₄ shows valencies 1, 2, 3 and 4 for chlorine, oxygen, nitrogen and carbon. For Groups 4 to 7, valency is 8 minus the group number, and elements in the same group form hydrides with matching formulae, such as H₂S, PH₃ and SiH₄.

Practice questions

1. Give the formulae of the four hydrides on the valency ladder in order of increasing valency. Answer: HCl, H₂O, NH₃, CH₄. 2. Predict the formula of the hydride of phosphorus. Answer: PH₃, because phosphorus is in Group 5 like nitrogen and has valency 3. 3. Bromine is in Group 7. What is its valency and the formula of its hydride? Answer: Valency 1; HBr. 4. Explain why the hydride of sulfur is H₂S. Answer: Sulfur is in Group 6, like oxygen, so its valency is 8 − 6 = 2 and each sulfur atom combines with two hydrogen atoms.