Atomic Structure
Atoms are made up of protons, neutrons and electrons. Protons are positively charged. Electrons are negatively charged. Neutrons don’t have a charge.
An atom consists of a nucleus, which contains protons and neutrons; and some electron shells which surround the nucleus and contain electrons. The neutrons however, are different. The number of protons and the number of neutrons add up to make the mass number of an element.
Understanding the Lack of Reactivity in Noble Gases (Group 0)
Noble gases have eight electrons on their outer shell, therefore, there is no need for them to gain or lose electrons. Basically they have a full outer shell so they don’t need to react. This is what makes them so unreactive.
How to Read Each Square on The Periodic Table
You probably already know that the periodic table is made up of lots and lots of squares, each containing an element and information about it.
Anyways we already know what the atomic mass number is (the number of protons + neutrons). It says 12.011 here but this is probably because this picture came from some super complicated periodic table. In IGCSE level however, the atomic mass should read 12. Anyways, the atomic number is the number of protons (and electrons), so to find the number of neutrons, if asked to, simply subtract the atomic mass by the atomic number.
Example: Calculate the number of neutrons Carbon has.
The answer: 12 – 6 = 6 neutrons
The Arrangement of Electrons
Atoms are surrounded by electron shells which contain electrons. But the arrangement is the same for ALL the elements, not matter how different they are.
Each shell can only hold a certain number of electrons. The very first shell can hold only two electrons. The second shell can hold eight. The third sometimes appear full with eight but can expand to a total of eighteen. However, this is beyond GCSE level, and for now, the shells only hold eight.
So how do you find the electron configuration? Well let’s use potassium (K) as an example.
Look up the atomic number of potassium. It should say 19. This tells you the number of protons, which is equal to the number of electrons so we can use that.
Arrange the electrons in shells, always filling up the inner shell before you go to the outer one. Remember the first, innermost shell can only take 2 electrons, the second one can take 8, and the third one, 8. You will find that you have one electron left. That goes on the fourth shell.
Your electron configuration should look like this: 2, 8, 8, 1.
Example: Work out the electron configuration of chlorine.
Chlorine has an atomic number of 17 – so 17 electrons.
17 – 2 (as the innermost shell only holds two electrons) = 15
15 – 8 (as the second shell only holds eight electrons) = 7 (This number is the number of electrons Chlorine has on its outer shell).
7 electrons does not fill up the third shell so we are left with the configuration: 2, 8, 7.
Isotopes
The number of neutrons in an atom can vary slightly. For example, there are three kinds of carbon atom, called carbon-12, carbon-13 and carbon-14. They all have the same number of protons, but the number of neutrons vary. These different atoms of carbon are called isotopes. Isotopes are atoms that have the same atomic number, but different mass numbers. They have the same number of protons, but different numbers of neutrons. The fact that they have varying numbers of neutrons makes no difference whatsoever to their chemical reactions. The chemical properties are governed by the number and arrangement of the electrons.