Скачать или смотреть Understanding Atomic Number and Atomic Mass | Chemistry | Best Learning Videos | Short Notes

When you look at the periodic table you will see that each element has its own box, and within that box, you will find two numbers. The atomic number, or proton number, and the mass number, but what do these numbers mean?

The atomic number has the symbol 'z', this number tells you how many protons are in one atom of an element. The number is always the same for all atoms of a particular element.

Atoms of different elements have different atomic numbers, meaning they have different numbers of protons. For example, an atom of Hydrogen has an atomic number of 1 because it has 1 proton, but an atom of Oxygen has and atomic number of 8 because it has 8 protons.

The next number we look at is the mass number. The mass number has the symbol A.

The mass number tells you how many protons and neutrons are in one atom of an element. We need to remember that Protons and Neutrons each have a relative mass of 1 and that Electrons are so small, that their mass does not need to be considered in the mass number of an atom.

So if we know the mass number of an element, and we know the atomic number, we can calculate the number of Neutrons in an atom of a particular element. So the Mass Number = The Atomic Number + the Number of Neutrons. The Atomic Number is just the number of Protons and Atoms, therefore, the Mass Number = protons + neutrons.

So if we take oxygen: oxygen has a mass number of 16 and 8 Protons, how many neutrons does it have?

Remember, Mass Number = protons + neutrons We can rearrange this to show that Neutrons = Mass Number - Atomic Number. Neutrons = 16 - 8 = 8 Oxygen therefore has 8 neutrons.

Let take another example. Lithium has a mass number of 7 and an atomic mass of 3, how many neutrons does it have?

Mass Number = protons + neutrons; we can rearrange this to show that Neutrons = Mass Number - Atomic Number. Neutrons = 7 - 3, = 4 Lithium therefore has 4 neutrons.

So the atomic number is the number of protons in an atom and the mass number is the number of protons and neutrons in an atom.