The Atomic Number is the top number of an element in the periodic table, for example in the element Lithium, the atomic number would be 3. The Mass number is the bottom number, so in Lithium it would be... 7.
The Atomic number is defined as: the number of protons.
The Mass number is defined as: the number of protons and neutrons.
So, for Lithium, we can conclude that it has 3 protons (atomic number) and also 3 electrons as the number of electrons equals the number of protons. The mass number is 7, and since 3 of these are protons, there must be 4 neutrons in the nucleus of Lithium.
Isotopes are elements with the same number of protons and electrons but a different number of neutrons.
Take Chlorine for example. Two common Isotopes include 17 17 Cl and Cl 35 37
They have the same number of protons and electrons (17) but a different amount of neutrons. One Isotope has 35, and the other 37.
Isotopes have the same chemical properties (their chemical reactions are the same) because they have the same amount of electrons. But they have different physical properties (such as…