How do isotopes of a given element differ? How am they similar?
See explanation below.
Isotopes are atoms of the same element (same atomic number, ie same number of protons in the nucleus) but different atomic masses (different mass numbers) due to different numbers of neutrons in the nucleus.
For example, we may get an isotope of chlorine with 17 protons and 18 neutrons, hence atomic mass 35.
And then a different chlorine atom with 17 protons and 19 neutrons, hence atomic mass of 36.
The value of atomic mass (molar mass) given on the periodic table is the average mass once all possible occurring isotopes of the particular element is taken into consideration and is the average mass of 1 mole of that element.
in the case of chlorine, 35.5, which means that on average the mass of 1 mole of chlorine atoms would weigh 35g.