Concise Selina Solutions for Class 7 Chemistry Chapter 3 Elements, Compounds and Mixtures

Compounds are substances made from atoms of different elements joined by chemical bonds. A chemical reaction can only separate them. An element is a material that consists of a single type of atom. Each atom type contains the same number of protons. Chemical bonds link elements together to form more complex molecules called compounds. A compound consists of two or more types of elements held together by covalent or ionic bonds.

Any substance that contains only one kind of an atom is known as an element. Because atoms cannot be created or destroyed in a chemical reaction, elements such as phosphorus (P4) or sulfur (S8) cannot be broken down into simpler substances by these reactions.

Elements combine to form chemical compounds that are often divided into two categories. Metals often react with non metals to form ionic compounds. These compounds are composed of positive and negative ions formed by adding or subtracting electrons from neutral atoms and molecules.

A mixture is made by merely mixing elements and compounds. No new chemical bonds are formed.