RLO: Structure of the Atom

Notation

As carbon is one of the most important components of the human body, let's look at its symbol in more detail to see how other information regarding the atom is recorded.

In this example, the number of protons found within each carbon atom is placed at the top-left of the symbol and this is also known as the atomic number. The atoms of each chemical element have a unique number of protons and hence a unique atomic number. The number of protons plus the number of neutrons defines how heavy the atom will be and is known as the atomic mass. In this example, it is placed at the bottom-left of the symbol. Therefore, you can calculate the number of neutrons in the atom by subtracting the atomic number, representing the number of protons, from the atomic mass, representing the overall weight of the atom. So for carbon, 12 minus 6 equals 6, so there are 6 neutrons in the atom.

As we have seen, the number of electrons in an atom is equal to the number of protons. Therefore the number of electrons is also represented by the atomic number.

Conventions as to where these two values are placed in relation to the chemical symbols vary, but remember the atomic mass will always be the greater of the two numbers*.

Note:
Apart from hydrogen where atomic mass and atomic number are equal.