Take a look around you. What do you see? A wall, a friend, a science book, money? All of these things are different in how they look, but the same in what they are made of. All of the above mentioned things are made of matter Matter is anything that has mass and takes up space. All matter is made of atoms. All atoms are made of 3 subatomic particles: protons, neutrons, and electrons. So really, all matter is made of protons, neutrons and electrons. No matter what object you hold in your hand, clothing you wear, or hair gel you use, they are all made out of the same subatomic particles.

If all matter is made of atoms, containing only those 3 subatomic particles, why doesn't all matter look basically the same? Simply because the atoms are different. Atoms have different numbers of subatomic particles, but the factor that distinguishes one atom from another is the number of protons an atom has in its nucleus. The number of protons is the atom's atomic number.

In order for atoms to be the same element, they must have the same number of protons. But atoms can be different in that they have different numbers of neutrons and electrons. An atom with different numbers of neutrons, but the same number of protons is called an isotope. An atom that loses or gains electrons is called an ion.

Atoms are put together in an orderly fashion. Most of an atoms mass is in its nucleus, the central most place in the atom. Protons and neutrons make up most of this mass, with each particle having a weight of 1 atomic mass unit (amu). The electrons are arranged around the nucleus in energy levels or rings. Each energy level can hold a certain amount of electrons. The first energy level can only hold 2 electrons. The 2nd energy level can hold up to 8, and the third energy level can hold up to 8 (for elements 11-18). An atoms outer most energy level can only hold up to 8 electrons. These outer most electrons are called valance electrons.

An easy was to remember proton number, neutron number, electron number and mass number is APE-MAN. No, not the hairy man that we all want to get a selfie with, but an acronym that helps us remember a little math trick.
Atomic# = Proton# = Electron#      Mass# = Atomic# + Neutron#

The atomic number is also called the proton number. The electron number (in a perfect atom) is always equal to the proton number. The mass number is the total number of protons and neutrons in an atom's nucleus