The smallest unit of matter is the atom.

Explanation:

• Atom : The basic building block of matter, consisting of a nucleus (protons and neutrons) surrounded by electrons.
• Atoms combine to form molecules, which make up all substances around us.

If you go even smaller, atoms themselves are made of subatomic particles:

• Protons
• Neutrons
• Electrons

And protons and neutrons are made of even smaller particles called quarks. But in terms of matter as we usually understand it, the atom is the smallest unit that retains the properties of an element.