Atoms and Molecules - Write chemical formulae and calculate molecular and formula unit mass
Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
The chemical formula of a compound is a symbolic representation of its composition.
The combining capacity (or binding capacity) of an element is known as its Valency. For example, the valency of Hydrogen is and Oxygen is .
An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an anion (e.g., ) and a positively charged ion is called a cation (e.g., ).
Polyatomic ions are a group of atoms carrying a charge, such as the Ammonium ion () or the Hydroxide ion ().
When writing a chemical formula, the valencies or charges on the ions must balance.
When a compound consists of a metal and a non-metal, the symbol of the metal is written first. For example: Iron Sulphide is written as and not .
In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio, for example, . Brackets are not needed if the number of ions is .
The criss-cross method is used to write formulae: write the symbols of the combining elements, write their valencies below them, and then cross over the valencies of the combining atoms.
📐Formulae
💡Examples
Problem 1:
Write the chemical formula for Calcium Hydroxide.
Solution:
Explanation:
The symbol for Calcium is with a valency of . The symbol for the Hydroxide ion is with a valency of . Crossing the valencies, we get . We omit the subscript and use brackets for the polyatomic ion to show that there are two hydroxide groups for every calcium atom.
Problem 2:
Determine the formula for Carbon Tetrachloride.
Solution:
Explanation:
The symbol for Carbon is (valency ) and Chlorine is (valency ). By criss-crossing the valencies, the from Carbon goes to Chlorine and the from Chlorine goes to Carbon, resulting in .
Problem 3:
Write the formula for Aluminium Sulphate.
Solution:
Explanation:
Aluminium () has a valency of . The Sulphate ion () has a charge of . When we criss-cross these values, the becomes the subscript for and the becomes the subscript for the entire group, requiring brackets: .