Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
A Chemical Formula is a symbolic representation of one molecule of an element or a compound. It uses symbols of elements and numerical subscripts to show the proportion of atoms.
A Compound is a substance formed when two or more elements combine chemically in a fixed ratio by mass. For example, always contains two atoms of Hydrogen for every one atom of Oxygen.
The Subscript in a formula (the small number written at the bottom right of a symbol) indicates the number of atoms of that specific element present in the molecule. If no number is written, it is assumed to be .
Atomicity refers to the total number of atoms present in one molecule of an element or compound. For example, the atomicity of Ozone () is .
In a chemical formula, the metal (or more electropositive element) is usually written first, followed by the non-metal. For example, in Sodium Chloride (), Sodium () is the metal.
📐Formulae
💡Examples
Problem 1:
How many atoms of each element are present in a molecule of Sodium Carbonate ()?
Solution:
Sodium () = atoms, Carbon () = atom, Oxygen () = atoms.
Explanation:
The subscripts tell us the quantity: has a subscript of , has no subscript (which means ), and has a subscript of .
Problem 2:
Calculate the total atomicity of a Glucose molecule, represented by the formula .
Solution:
Atomicity =
Explanation:
To find the atomicity, we sum the number of atoms of all elements: Carbon atoms + Hydrogen atoms + Oxygen atoms = atoms.
Problem 3:
Write the chemical formula for a compound containing one atom of Magnesium and two atoms of Chlorine.
Solution:
Explanation:
We write the symbol for Magnesium () first and Chlorine () second. Since there are two Chlorine atoms, we add the subscript to .