1. the smallest complete unit of a compound or diatomic gas shell
2. a new substance formed by the combination of two or more different atoms element
3. path of electrons around an atomic nucleus molecule
4. a substance consisting of only one type of atom compound

Explanation:

1. The smallest complete unit of a compound or diatomic gas

ATOM

Atoms are small tiny particles that makes up a molecule. It is regarded as the smallest indivisible particle that parts in a chemical reaction.

2. A new substance formed by the combination of two or more different atoms

COMPOUNDS

Compounds are substances composed of two or more kinds of atoms. Their properties differs from those of the constituent elements that combines to make them up.

3. Path of electrons around an atomic nucleus

SHELL

The shell describes the path of an electron round a nucleus

4. A substance consisting of only one type of atom

MONOATOMIC MOLECULE

It is an aggregate of one type of atom.