An atom is the smallest unit of an element, of which there are 94 naturally-occurring ones and (at time of writing) 24 synthesised ones.

An atom is comprised of a nucleus, containing protons and neutrons, surrounded by electrons. The number of protons defines the element, and is called its atomic number. For example, all hydrogen atoms have one proton (atomic number = 1), all carbon atoms have six protons (atomic number = 6), all nitrogen atoms have seven protons (atomic number = 7), and all oxygen atoms have eight protons (atomic number = 8).

This means that any atom gaining or losing a proton is automatically a different element. (Radioactive elements do this.)

Protons are positively-charged, neutrons have no charge, and electrons are negatively-charged.

Atoms overall have no charge, and the number of protons is equal to the number of electrons. The number of neutrons may or may not equal the number of protons, and depends on both the element and even the type of element (known as an isotope).

An atom which loses or gains an electron(s) becomes positively- or negatively-charged respectively, and is called an ion.