What makes one element different from another is the number of protons in the nucleus of its atoms. An atom is the smallest particle of an element that retains the properties of that element. Atoms contain protons and neutrons in the nucleus and electrons outside the nucleus. Each element has a unique number of protons, known as its atomic number.
The chemical elements are arranged in the Periodic Table in order of atomic number, or number of protons. The number of neutrons for a given element can vary without affecting its chemical properties, and the number of electrons changes when ions are formed. As an example, a potassium atom or ion always has 19 protons. That's what makes it potassium and distinguishes it from other elements. Most potassium atoms have 20 neutrons, but some have 21 neutrons and some have 22 neutrons. They all behave the same chemically and physically. Neutral potassium atoms have 19 electrons, but they tend to lose one electron, forming a potassium ion.