Atoms are the smallest bit of an element that have its chemical proprieties. They are made of 3 smaller particles which are called protons, neutrons and electrons.
The protons have a positive charge and weigh 1 amu, while the electrons have negative charge and are approximately 2000 times lighter than a proton. Also the neutrons have no charge and also weigh 1 amu.
The protons and the neutrons bind forming the nucleus located in the center of the atom. At atom is about a 100,00 times bigger than its nucleus. Atoms are mostly empty space.
Each of the electrons orbit the nucleus. The number of electrons orbiting the nucleus is the atomic number. Every element has a different atomic number.
Number of protons and the number of electrons must be the same, or atoms would be charged particles.
Atoms may have many isotopes. Isotopes differ in the number of neutrons but have the same number of protons and electrons. That means the atomic number is the same for all isotopes. Stable isotopes are not radioactive.