What's the matter, atoms?
Francine Taylor-Campbell, Contributor

Atomic Structure

In the last lesson we examined the three states of matter and discovered that we can change from one state to another. This week we will look at the structure of the atoms which make up matter.

At the end of the topic you will be able to:

• Show the electronic arrangement of an element given its atomic number
• Define isotopy and give three uses of isotopes
• Explain the basis of arrangement of elements in the periodic table.
• Important points
• An atom has two main sections, nucleus and energy levels (or shells).
• The nucleus is at the centre of the atom and holds the protons and electrons.
• Electrons are arranged in shells around the nucleus.
• Each shell or energy level can hold a fixed number of electrons. When a shell is filled, electrons will enter the next shell with the lowest energy.
• Isotopes are atoms of the same element with the same atomic number but different mass numbers.
• Elements are arranged in the periodic table in order of increasing atomic (proton) number.

What are the main differences between the sub-atomic particles?

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 that 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.

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.

The protons (P) and neutrons (N) are in the nucleus and give 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 (of an uncharged atom).

# of protons = # of electrons: This is why the charges cancel out.

Electrons are not stationary but continue to move in different energy levels around the nucleus. The arrangement of electrons in the shells or energy levels is governed by certain rules or guidelines.

1. Electrons start to fill up the shells of lowest energy first, that is the closest one to the nucleus.

2. The first shell (K shell) can hold a maximum of 2 electrons

3. The next shell (L shell) can hold a maximum of 8 electrons

4. The third shell (M shell) holds 8 electrons for the 1st 20 elements but can hold up to 18 electrons.

How are isotopes formed?

There are some elements that exist as isotopes. For example 3517 Cl and 3717 Cl

Notice that both elements have the chlorine symbol and have the same atomic number, but their mass numbers are different.

Remember mass number = P + N

If #protons = 17 then for 35Cl the # neutrons = 35 -17 = 18 while for 37Cl the #neutrons = 37-17 = 20 neutrons.

These two chlorine atoms will have the same chemical properties as their protons/electrons will be the same. It is the electrons in atoms that react and influence the chemical properties.

Let's look at another example oxygen-16 and oxygen-18.

168O and 188O

Both atoms have the same atomic number (8 protons) but one atom has 8 neutrons (16-8) while the other has 10 (18-8).

Next week we will attempt some examination questions on the topic.

Francine Taylor-Campbell is an independent contributor. Send questions and comments to kerry-ann.hepburn@gleanerjm.com