Like this: The atom is the basic building block of the Universe.
Unless the sentence requires the plural form, you wouldn't use it.
"There are many types of atoms."
At the atomic scale things look very different.
One atom is too small to see. A helium atom is the smallest type of atom.
Compounds are comprised from [and composed of] Atoms.
atoms are apart of everything in the world!
Monoacid has one replaceable hydrogen atom.
The atomic mass gives the mass of the atom. It is one of the physical property of the atom.
The particles in an atom are the protons, the neutrons, and the electrons.
Every hydrogen atom contains a single proton.
Electron in an atom is represented by electron cloud around the nucleus
"The electron cloud is the space in the atom where the electrons orbit the nucleus."
Sometimes, an atom cannot be split.
It was as small as an atom.
The atom is a basic unit of matter; the atom is the building block for us.