Atom

An atom is the smallest particle that can be described as a chemical.

• At the centre of an atom is a nucleus containing protons which are positively charged sub-atomic particles. The number of protons determines what chemical the atom is. A hydrogen atom has a single proton.
• The nucleus also contains a number of neutrons, sub-atomic particles with no charge.
• Surrounding the nucleus are very small negatively charged particles called electrons, held in place by their attraction to the positively charged nucleus.
• Quantum mechanics is used to describe the orbital motion of electrons around a nucleus.
• In an atom, the number of electrons always matches the number of protons.
• If an electron is removed from an atom the result is a charged particle called an ion.
• For a single atom, the number of protons and neutrons combined determines its atomic mass.
• Atomic mass is a measure of the total mass of protons and neutrons in an atom and is used to order elements in the periodic table.

https://en.wikipedia.org/wiki/Atom