Valency: turning connection capacity into a chemical formula
A chemical formula is a compact statement about which elements are present and the smallest whole-number ratio in which their atoms or formula units combine.
Big idea and learning goals
Valency is an introductory way to describe the combining capacity of an atom of an element: how many connections it can make in a simple compound. In this lesson valency is written with a Roman numeral, such as oxygen(II). It helps us predict formulas of simple binary compounds, compounds made from two elements.
By the end, you should be able to read subscripts correctly; use common valencies to write a binary formula; reduce subscripts to the lowest whole-number ratio; check that the total combining capacities match; distinguish a subscript from a coefficient; and explain where the valency model is useful and where it is limited.
1. An operational connection model
Imagine that each atom has a certain number of connection points. Hydrogen(I) has one, oxygen(II) has two, nitrogen(III) often has three in introductory examples, and carbon(IV) has four. A stable model is built when the connection points supplied by one element match those required by the other.
Valency has no plus or minus sign in this operational model. It is not the number of atoms in a formula, not an electric charge, and not a photograph of how atoms look. Later chemistry introduces oxidation states, ions, bonding theories, and molecular geometry in more detail.