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The Relative Mass of an Atom or Ion

In Chemistry, we often use the relative masses of particles rather than their actual masses in grams. This is because we are mainly interested in how the masses compare to each other.

We have learnt that protons and neutrons both have a relative mass of 1, and that electrons have a relative mass of 0.0005, which is so small that we often just think of it as 0.

We can also work out the relative masses of larger particles like atoms and ions.

The relative mass of an atom or ion is roughly equal to its mass number

Because each proton or neutron has a relative mass of one, we can find the approximate relative mass of an atom or ion by simply counting how many protons and neutrons it has. In other words, the relative mass is roughly equal to the mass number.

For example, a lithium-7 atom has a mass number of 7 (it has 3 protons and 4 neutrons). Since each proton or neutron contributes a relative mass of 1, the relative masses of all of the protons and neutrons add up to 7, meaning that the atom's relative mass is roughly 7.

There are two reasons why the relative mass of an atom or ion is not exactly equal to its mass number.

Firstly, the electrons contribute a very small amount of mass. For example, a lithium-7 atom has 3 electrons (the same as its number of protons). Each electron contributes a tiny amount of mass, which makes the relative mass slightly different to what we get when we just count the protons and neutrons.

Secondly, when the nucleus of an atom or ion is formed, a certain amount of energy is released as the protons and neutrons bind together. For complicated reasons that involve Einstein's famous E = mc2 equation, this release of energy causes the mass of the nucleus to decrease very slightly. Therefore the mass of the nucleus is very slightly less than the sum of the masses of the protons and neutrons that make it up.

However, both of these factors only have a tiny effect on the mass of the atom or ion, and the true relative mass is always extremely close to the mass number. Therefore, for the purposes of this course we can think of the relative mass of an atom or ion as being essentially the same as its mass number.

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How do you work out the relative mass of an atom or ion?

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