Relative Charges of Nuclei and Atoms

Earlier in the course we learnt about charge, which is a property of objects that causes them to be affected by the electrostatic force. We learnt that we do not always need to know the actual charges of different objects. Instead, we can often use relative charge which tells us how the charges of different objects compare to each other.

We learnt that a proton has a relative charge of +1, a neutron has a relative charge of 0, and an electron has a relative charge of -1.

The relative charges of protons, neutrons and electrons.

In this section we are going to think about the relative charge of a nucleus and the relative charge of an atom.

The charge of a nucleus is called the nuclear charge. The word 'nuclear' simply means 'relating to the nucleus'.

All of the nuclear charge comes from the protons

A nucleus is made up of protons and neutrons. Because neutrons are neutral, they do not affect the charge of the nucleus. This means that all of the nuclear charge comes from the protons.

Because protons are positively charged, nuclei are also positively charged. The size of the nuclear charge depends on how many protons the nucleus contains.

The nuclear charge equals the atomic number

Since each proton has a relative charge of +1, the relative charge of a nucleus is simply equal to the number of protons it contains. In other words, the nuclear charge is equal to the atomic number.

For example:

Hydrogen has atomic number 1. A hydrogen nucleus contains one proton, which means it has a nuclear charge of +1.

Helium has atomic number 2. A helium nucleus contains two protons, giving it a nuclear charge of +2.

Lithium has atomic number 3. A lithium nucleus contains three protons, giving it a nuclear charge of +3.

The calculation of lithium's nuclear charge is shown below:

Lithium's nuclear charge is +3, the same as its atomic number.

Remember that relative charge does not have units.

Atoms are neutral

In an atom, the number of electrons is always equal to the number of protons. This means that the charges of the electrons and protons cancel each other out, making the atom neutral overall.

For example, an oxygen atom has 8 protons, 8 electrons and 8 neutrons (in the most common isotope, which is oxygen-16).

The protons each have a relative charge of +1, which means that together they contribute +8 to the relative charge of the atom. The electrons each have a relative charge of -1, which means that they contribute -8 to the relative charge of the atom. The neutrons are neutral and therefore do not affect the relative charge of the atom.

The relative charge of the atom is found by adding +8 and -8, which gives 0. Therefore, the atom is neutral. We would get the same result no matter which atom we look at, because the number of protons is always equal to the number of electrons.

Like all atoms, an oxygen atom is neutral. This is because the number of electrons is equal to the number of protons and therefore their charges cancel each other out. The neutrons are neutral and do not affect the charge.

Flashcards

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