Ответить на вопросы:
1 What is called a molecule?
2 Is a molecule visible to the human eye?
3 What is the atom composed by?
4What do we call the smallest particles of the atom?

Ответ:

A molecule is an electrically neutral group of two or more atoms held together by chemical bonds.

No, we can't see them with the naked eye.

The atom is consists of the nucleus and electrons. The nucleus consists of protons and neutrons.

Most often when we say the smallest particles, we mean electrons, protons and neutrons.