Plastic, rocks, and steel are all matter. They are made of tiny particles that can't be broken down into smaller pieces. These particles are called:

These particles are all called atoms.

Explanation:

Atoms are referred to as the building blocks of all matter. This is because atoms are the smallest pieces that all matter can be broken down into, whether the matter is in a solid, liquid or gaseous state. Atoms combine to form bigger particles called molecules. These molecules are what forms the objects in our surrounding environment. Atoms are comprised of three particles: electrons which have a negative charge, protons which have a positive charge and neutrons which have no charge.

The tiny unbreakable particles which are the building blocks of plastic, rocks and steel are termed as atom.

Explanation:

It is known that every matter is built by atoms bonded together in different structure. The atoms are present in solids, liquids and gases, the only difference is their binding strength and bond length.

The solids experience the highest binding strength while gases experience the lowest binding strength. The solids are also classified in different categories based on the arrangement of atoms to build the specified solid.

In this case, plastics are made of atoms arranged in polymer chain or network like structure, stone has different bond type between the atoms used for building of it, the same case goes for steel, the bond type.

And the arrangement of atoms are having different crystal structure. But the main building blocks or the tiny particles which cannot be broken down into smaller pieces are termed as atoms.