Isotopes and relative atomic mass

Last time

Before we start: last time, in Ionic bonding and ionic formulae, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Two kinds of chlorine

Look at the mass of chlorine in the Periodic Table. It is thirty-five point five. But no single chlorine atom has half a nucleon. The answer is that chlorine is a mixture of two kinds of atom, called isotopes.

What is an isotope?

Isotopes are atoms of the same element, so they have the same number of protons. But they have different numbers of neutrons, so they have different nucleon numbers. Learn it as one sentence: atoms of the same element with the same number of protons but different numbers of neutrons.

Chlorine-35 and chlorine-37

Both isotopes of chlorine have seventeen protons, because they are both chlorine. Chlorine thirty-five has thirty-five minus seventeen, which is eighteen neutrons. Chlorine thirty-seven has thirty-seven minus seventeen, which is twenty neutrons. And both have seventeen electrons, so the electron arrangement is the same.

Isotopes of carbon

Carbon has three isotopes. All three have six protons and six electrons. Carbon twelve has six neutrons, carbon thirteen has seven, and carbon fourteen has eight.

Same electrons

Chemical reactions depend on the electrons, and in particular the outer electrons. Both isotopes of chlorine have the arrangement two, eight, seven, so they react in exactly the same way.

Physical properties and radioactivity

Isotopes have the same chemical properties, but slightly different physical properties, such as mass and density. Some isotopes have unstable nuclei, and these are radioactive. Carbon fourteen is used to find the age of ancient objects, and cobalt sixty is used to treat cancer.

Relative atomic mass

The relative atomic mass, Ar, is the average mass of the atoms of an element, compared with one twelfth of an atom of carbon twelve. It is an average, taking into account how much of each isotope there is. We call it a weighted mean. That is why it is often not a whole number. Chlorine is thirty-five point five.

The method

First, multiply each nucleon number by its percentage abundance. Second, add the answers together. Third, divide the total by one hundred. Fourth, check that the answer lies between the lightest and the heaviest isotope.