From the Matter module, you know that elements are made-up of protons ( $+$ ), neutrons ( $\pm$ ), and electrons (). Protons and neutrons have a atomic mass number of 1. Because electrons are so small, their atomic mass number is rounded to 0. By adding the number of protons and neutrons in the nucleus of an atom, you can calculate the mass number of the atom. Carbon-12 has 6 protons and 6 neutrons so its mass number is 12.

Most elements have different forms of atoms. Carbon-12 has 6 protons, 6 neutrons, and 6 electrons. Carbon-13 has 6 protons, 7 neutrons, and 6 electrons. Carbon-14 has 6 protons, 8 neutrons, and 6 electrons. These different forms are called isotopes. (see graphics below) Using a complex averaging of all the different mass numbers of an element, scientists have calculated the atomic mass of each element. The atomic mass of carbon (C) is 12.01. This shows that almost all carbon atoms have a mass number of 12 (6 protons and 6 neutrons).

What is common between all the different atoms of carbon? That's right! Each one has 6 protons and 6 electrons. We will forget about the electrons for right now. All the atoms of an element have the same number of protons. All atoms of carbon have 6 protons. All atoms of oxygen have 8 protons. All atoms of gold have 79 protons. If the number of protons changes, the type of element changes. The number of protons in the nucleus of an element is the atomic number of the element. So carbon has an atomic number of 6. The elements are arranged into a table, called the periodic table, by this atomic number

Now lets look at electrons. Electrons are organized into orbitals/energy levels around the nucleus of an atom. The first orbital can hold 2 electrons. If the atom has more than two electrons, the electrons go into the second, then third and so on orbitals. After the first orbital, each orbital can hold up to 8 electrons. The electrons in the outermost orbital are called valence electrons. An atom "wants" its last orbital to be filled. The number of electrons an element needs to fill its outermost orbital determines the elements reactivity. Elements in the periodic table are grouped by the number of electrons needed in the outermost orbital; thus they have similar reactivity.

Lets look at helium. Helium has 2 protons so it has 2 electrons. These two electrons fill helium's outermost orbital so helium does not need any more electrons; it is not reactive. Lithium has 3 electrons. Two electrons are in the first orbital and 1 in the outermost orbital. Lithium has 1 valence electron. Beryllium has 4 electrons, two in its innermost orbital and 2 in the outermost orbital, thus 2 valence electrons. (See the graphic below for visual example of valence electrons of these elements.)

Number of valence electrons for some common elements