Chemical properties of isotopes

1. Isotopes of the same element have the same number of protons, so their neutral atoms also have the same number of electrons.
2. The same number of electrons means the isotopes have the same electronic configuration, including the same number of outer-shell electrons.
3. Chemical reactions involve the electrons of atoms, especially the outer-shell electrons, being transferred or shared, while the nucleus takes no part.
4. Isotopes of the same element therefore have the same chemical properties, because they have the same number of electrons and the same electronic configuration.
5. Chlorine-35 and chlorine-37 both have the configuration $2,8,7$, so both gain one electron to form a ${Cl}^{-}$ ion and both react with sodium to form sodium chloride.
6. The extra neutrons change only the mass of the atom, so isotopes can differ slightly in physical properties that depend on mass, such as density.

Relative atomic mass

1. Most elements exist naturally as a mixture of isotopes, so a sample of an element contains atoms with different masses.
2. The relative atomic mass, $A_r$, gives the mass of an average atom of the element, taking every isotope into account.
3. Masses of atoms are compared with the carbon-12 atom, which is given a mass of exactly 12, so $A_r$ has no units.
4. The abundance of an isotope is the percentage of the atoms of the element that are that isotope.
5. The average is a weighted mean, so the more abundant an isotope is, the more it pulls the value of $A_r$ towards its own mass.
   1. Chlorine contains three times as many chlorine-35 atoms as chlorine-37 atoms, so its $A_r$ of $35.5$ is much closer to 35 than to 37.
6. Relative atomic masses are often not whole numbers, while the mass number of a single isotope is always a whole number.
7. A calculated $A_r$ must always lie between the masses of the lightest and the heaviest isotopes of the element.

Calculating relative atomic mass

1. The relative atomic mass is calculated from the mass and abundance of each isotope using $A_r=\frac{sum\; of\; (isotope\; mass\times abundance)}{total\; abundance}$.
2. Multiply the mass of each isotope by its abundance.
3. Add these products together.
4. Divide the total by the sum of the abundances, which is $100$ when the abundances are given as percentages.
5. Give the answer to the number of decimal places asked for, which is usually one decimal place.