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Atomic and Atomic Mass Number

Atomic Number and Atomic Mass Number

The chemical properties of an element are determined by the charge of its nucleus, i.e. by the number of protons. This number is called the atomic number and is denoted by the letter Z.

Definition: Atomic Number (Z)

The number of protons in an atom.

You can find the atomic number on the periodic table (see periodic table diagram below). The atomic number is an integer and ranges from 1 to about 118.


The mass of an atom depends on how many nucleons its nucleus contains. The number of nucleons, i.e. the total number of protons plus neutrons, is called the atomic mass number and is denoted by the letter A.

Did You Know?

Currently element 118 is the highest atomic number for an element. Elements of high atomic numbers (from about 93 to 118) do not exist for long as they break apart within seconds of being formed. Scientists believe that after element 118 there may be an “island of stability” in which elements of higher atomic number occur that do not break apart within seconds.

Did You Know?

A nuclide is a distinct kind of atom or nucleus characterised by the number of protons and neutrons in the atom. To be absolutely correct, when we represent atoms like we do here, then we should call them nuclides.

Definition: Atomic Mass Number (A)

The number of protons and neutrons in the nucleus of an atom.

The atomic number (Z) and the mass number (A) are indicated using a standard notation, for example carbon will look like this: \(_{6}^{12}\text{C}\)

Standard notation shows the chemical symbol, the atomic mass number and the atomic number of an element as follows:


For example, the iron nucleus which has 26 protons and 30 neutrons, is denoted as \(_{26}^{56}\text{Fe}\) where the atomic number is \(Z = 26\) and the mass number \(A = 56\). The number of neutrons is simply the difference \(N = A – Z = 30\).


Do not confuse the notation we have used here with the way this information appears on the periodic table. On the periodic table, the atomic number usually appears in the top left-hand corner of the block or immediately above the element’s symbol. The number below the element’s symbol is its relative atomic mass. This is not exactly the same as the atomic mass number. This will be explained in “Isotopes”. The example of iron is shown below.


For a neutral atom the number of electrons is the same as the number of protons, since the charge on the atom must balance. But what happens if an atom gains or loses electrons? Does it mean that the atom will still be part of the same element? A change in the number of electrons of an atom does not change the type of atom that it is. However, the charge of the atom will change. The neutrality of the atom has changed. If electrons are added, then the atom will become more negative. If electrons are taken away then the atom will become more positive. The atom that is formed in either of these two cases is called an ion.

An ion is a charged atom. For example: a neutral sodium atom can lose one electron to become a positively charged sodium atom (\(\text{Na}^{+}\)). A neutral chlorine atom can gain one electron to become a negatively charged chlorine ion (\(\text{Cl}^{-}\)). Another example is \(\text{Li}^{+}\) which has lost one electron and now has only 2 electrons, instead of 3. Or consider \(\text{F}^{-}\) which has gained one electron and now has 10 electrons instead of 9.



Use standard notation to represent sodium and give the number of protons, neutrons and electrons in the element.

Step 1: Give the element symbol


Step 2: Find the number of protons

Sodium has 11 protons, so we have: \(_{11}^{23}\text{Na}\)

Step 3: Find the number of electrons

Sodium is neutral, so it has the same number of electrons as protons. The number of electrons is \(\text{11}\).

Step 4: Find \(A\)

From the periodic table we see that \(A = 23\).

Step 5: Work out the number of neutrons

We know \(A\) and \(Z\) so we can find \(N\): \(N = A – Z = 23 – 11 = 12\).

Step 6: Write the answer

In standard notation sodium is given by: \(_{11}^{23}\text{Na}\). The number of protons is 11, the number of neutrons is 12 and the number of electrons is 11.

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