An atom is the smallest particle of an element that retains the chemical properties of that element, and an element is defined as a substance that can’t be broken down or separated into simpler substances through a chemical reaction. Elements contain just one type of atom, and each different element contains a different type of atom. Take the element sulfur (S). A pile of sulfur (a yellow, powdery or crystallized substance) sitting on a table represents a single element—sulfur—and this pile of sulfur is made up of only one type of atom—sulfur atoms.
Each atom, regardless of its identity, is made up of three types of subatomic particles. Protons, which are positively charged and situated at the center of the atom (also known as the atomic nucleus); neutrons, which are electrically neutral (meaning that they have no charge) and are also in the nucleus of the atom; and electrons, which are negatively charged and are situated outside the nucleus.
The number of protons an atom possesses is what gives the atom its identity—all atoms of a particular element have the same number of protons in their nuclei. For example, all of the sulfur atoms in the pile of sulfur we looked at above have 16 protons in their nucleus. If they had one more proton in their nucleus, they would have a different identity—they’d be chlorine (Cl) atoms, and with one less, they’d be phosphorus (P) atoms.
https://socratic.org/questions/how-do-protons-determine-the-properties-of-an-atom
