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's 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 a little more than 100 known elements, and they are listed in 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 MOLECULE of water has two atoms of Hydrogen and one atom of Oxygen - H2O.
All atoms have a NUCLEUS, with ELECTRONS zooming around it. It can be compared to planets (electrons) revolving 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 the element's atoms have. This number is called the ATOMIC NUMBER. Hydrogen's ATOMIC NUMBER is 1. Helium's ATOMIC NUMBER is 2. Carbon's ATOMIC NUMBER is 6. Nitrogen's ATOMIC NUMBER is 7, etc.
The number of NEUTRONS in atoms changes. Helium atoms sometimes have 2 NEUTRONS and others have 3 NEUTRONS. Carbon's atoms sometimes have 6 NEUTRONS and others have 7 NEUTRONS. Nitrogen atoms sometimes have 7 NEUTRONS and others have 8 NEUTRONS, etc. Scientists calculate an average.
If you add up the number of PROTONS and the calculated average of NEUTRONS in an atom, you will get the ATOMIC MASS, or ATOMIC WEIGHT, or MASS NUMBER. All these three terms refer to the same thing. If you look on a Periodic Table, you will 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, etc. The reason they are not whole numbers 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. ELECTRONS have a negative charge. 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 zoom around outside the nucleus in orbitals and take part in chemical reactions. The PROTONS and the NEUTRONS stay where they are, in the nucleus. They do not take part in chemical reactions.
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