Before we discuss the structure of atoms, we need to learn the terms mass and weight. While we are on earth, the terms mass and weight reflect the same thing. If we are on the moon, our weight will be less but our mass will be the same. It has been said that if you want to lose weight, go to the moon. If you want to lose mass, go on a diet and exercise. For our purposes in this article, the terms mass and weight will be used interchangeably, since we are on earth.
Hydrogen, helium, carbon, nitrogen are called elements. There are just over 100 known elements, and they are listed on the periodic table. All matter is made up of various combinations of these 100+ elements. For example, water is made up of two elements, hydrogen and oxygen. Each water molecule has two hydrogen atoms and one oxygen atom – H2O.
All atoms have a nucleus, with electrons moving around it. It can be compared to the planets (electrons) that revolve around the sun (nucleus).
The nucleus contains most of the mass, or weight, of the atom. Inside the nucleus, you will find protons and neutrons. The number of protons in each type of atom is always the same. For example, hydrogen atoms always have one proton. Helium atoms always have two protons. All carbon atoms have six protons. All nitrogen atoms have seven protons, etc.
The periodic table is arranged in numerical order, according to the number of protons that the atoms of the element have. This number is called the atomic number. The atomic number of hydrogen is 1. The atomic number of helium is 2. The atomic number of carbon is 6. The atomic number of nitrogen is 7, etc.
The number of neutrons in atoms changes. Helium atoms sometimes have 2 neutrons and sometimes have 3 neutrons. Carbon atoms sometimes have 6 neutrons and sometimes have 7 neutrons. Nitrogen atoms sometimes have 7 neutrons and sometimes have 8 neutrons, etc. Scientists calculate an average.
If you add the number of protons and the calculated average number of neutrons in an atom, you will get the atomic mass, atomic weight, or mass number. All three of these terms refer to the same thing. If you look at a periodic table, you’ll see that the mass number of helium is 4.003, the atomic mass of carbon is 12.01, the atomic weight of nitrogen is 14.01, and so on. The reason they are not integers is because of what we discussed in the previous paragraph: the number of neutrons is a calculated average.
Protons have a positive charge. Neutrons have no electrical charge, they are neutral. The electrons have negative carge. In an electrically neutral atom, the number of electrons and the number of protons are the same, balancing it out to make it neutral. Electrons have very little mass or weight. Electrons move out of the nucleus in orbitals and participate in chemical reactions. The protons and neutrons stay where they are, in the nucleus. They do not participate in chemical reactions.