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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 × 10 - 27 kg (slightly heavier than the proton).

Interesting fact

Rutherford predicted (in 1920) that another kind of particle must be present in the nucleus along with the proton. He predicted this becauseif there were only positively charged protons in the nucleus, then it should break into bits because of the repulsive forces between the like-charged protons! Also,if protons were the only particles in the nucleus, then a helium nucleus (atomic number 2) would havetwo protons and therefore only twice the mass of hydrogen. However, it is actually four times heavier than hydrogen. This suggested that there must be something else inside the nucleus as well as the protons.To make sure that the atom stays electrically neutral, this particle would have to be neutral itself. In 1932 James Chadwick discovered the neutron and measured its mass.

Summary of the particles inside the atom
proton neutron electron
Mass (kg) 1 , 6726 × 10 - 27 1 , 6749 × 10 - 27 9 , 11 × 10 - 31
Units of charge + 1 0 - 1
Charge (C) 1 , 6 × 10 - 19 0 - 1 , 6 × 10 - 19

Interesting fact

Unlike the electron which is thought to be a point particle and unable to be broken up into smaller pieces, the proton and neutron can be divided. Protons and neutrons are built up of smaller particles called quarks . The proton and neutron are made up of 3 quarks each.

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:

A nuclide is a distinct kind of atom or nucleus characterized 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.

For example, the iron nucleus which has 26 protons and 30 neutrons, is denoted as:

26 56 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 .

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.

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Source:  OpenStax, Chemistry grade 10 [caps]. OpenStax CNX. Jun 13, 2011 Download for free at http://cnx.org/content/col11303/1.4
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