0.2 The atom  (Page 5/14)

The neutron

The neutron is electrically neutral i.e. it carries no charge at all. Like the proton, it is much heavier than the electron and its mass is $1,6749\times {10}^{-27}\mathrm{kg}$ (slightly heavier than the proton).

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 .

Atomic number (Z)

The number of protons in an atom

You can find the atomic number on the periodic table. 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 .

Atomic mass number (A)

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

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:

where the atomic number is $Z=26$ and the mass number $A=56$ . The number of neutrons is simply the difference $N=A-Z$ .

Don't confuse the notation we have used above with the way this information appears on the Periodic Table. On the Periodic Table, the atomic number usually appears in the top lefthand 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.