Know your atoms
Francine Taylor-Campbell, Contributor

Robert Brown from St. Jago High School gives a speech at the floral tribute ceremony to commemorate the 120th anniversary of the birth of the Right Excellent Marcus Mosiah Garvey at the National Heroes Park, Kingston, recently. - Peta-Gaye Clachar/Staff Photographer

You should be able to:

• Describe the structure of the first 20 atoms
• State the properties of electrons, protons and neutrons.
• Define atomic number, mass number and relative atomic mass.

POINTS TO NOTE

• Matter consists of particles which may be atoms, molecules or ions.
• An atom is the smallest part into which an element can be divided.
• An atom consists of three main particles - protons, neutrons and electrons.

From last week's lesson we learnt that matter is all around us, whether in the form of gases, liquids or solids. This matter is made of particles with an atom being the smallest part. An atom has two main sections:

1. A nucleus which is the centre and holds the protons and electrons.
2. Shells which contain the electrons.

Please note that the overall charge on an atom is zero (-1 + 1 = 0) hence atoms are uncharged. The mass of an electron is 1/1840 of a proton. This mass is very small, hence we generally say the mass of an electron is negligible or it has no mass. This is why the mass of an atom is derived from the number of protons and neutrons. Mass number = Protons + Neutrons.

Remember the masses given are not the actual masses but the relative mass when all three are compared.

The relative atomic mass (Ar) is the average mass of one atom of an element compared to the mass of one atom of carbon-12.

As was stated earlier, the protons (P) and neutrons (N) are in the nucleus and this gives the nucleus a positive charge, while the electrons are moving in the shells with a negative charge.

In an atom, the number of protons in the nucleus is called the atomic number and this is equal to the number of electrons in the shells.

Number of protons = number of electrons This is why the charges cancel out.

The element oxygen, for example, is written like this:

Mass number = P + N = 16

Atomic number = number protons = 8

Therefore, the number neutrons = 16 - 8 = 8

In a neutral atom number P = number E = 8

How many electrons and neutrons are present in chlorine?

It is important now for you to look at the first 20 elements of the periodic table. These elements are arranged in order of increasing atomic number. The first element is hydrogen

1 1H

Note that hydrogen has one proton and, therefore, one electron, but since the mass number = 1 (P + N) this means that H has no neutrons.

All the other elements can be written in this form.

The protons and neutrons are in the nucleus and this gives the nucleus a positive charge, while the electrons are moving in the shells with a negative charge.

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In an atom, the number of protons in the nucleus is called the atomic number and this is equal to the number of electrons in the shells.

Francine Taylor-Campbell is an independent contributor.