Elementary, Dear Watson

In this article you will learn about:

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• What elements are

• The Periodic Table of elements

• Atomic number and atomic mass

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Okay, Sherlock Holmes never actually said this famous line, but we'll take some liberties and pretend that he did.

A good place to start in your quest to understand the universe is to get to grips with elements, the ingredients for everything. All the stuff that you can see, hear, smell and touch (apart from a few things like light and heat, which we'll get to in another article) are made up of one or more of these ingredients.

If you want to make a cake, what do you need to bring to the table? Just like putting together this delicious dessert of soft, sugary goodness requires certain ingredients, such as milk, flour, eggs and sugar (or substitutes), if you want to build yourself, say a rock or a tree, or even a human, you will need certain elements. And I'm guessing you're not a completely hopeless cake baker and know not to mix 20 litres of milk with a tablespoon of flour, and so it is that elements need to be in the right proportions if they are to create anything useful (or even something that can exist at all.)

So here is a simple recipe: water, also known as H2O, is made of two parts hydrogen to one part oxygen. This means you need two hydrogen atoms and one oxygen atom if you want to make water. It would help if you head over to my article on atoms, and read up to the end of the section on Mass, to get acquainted with these little guys, before going any further.

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Okay, welcome back!

Now, although scientists can make water by combining oxygen and hydrogen, it's not as if they keep making things from scratch like this. It's generally difficult and too costly. I use the analogy of making or building something to explain what elements are. However, they can and do naturally combine together to make the universe we know and love.

The Periodic Table of Elements

The Periodic Table is not just a list of all the elements' names, they're ordered according to the size of their atoms and the size will tell you how many electrons and protons they have and this affects their chemistry (i.e. how they behave when next to atoms of other elements, if they will react with them and how strongly, etc. - you can get a better understanding from reading the rest of the article o after finishing this one). Therefore, when you order them by atomic number (a number indicating how many electrons and protons they have) you get them grouped together with those that are similar in chemistry.

So here's the first element in the table:

Each element has a symbol, the big letter or pair of letters, which for hydrogen is H. They also have an atomic number and an atomic mass. The atomic mass is the total mass of the atom when you add together its subatomic particles, i.e. protons, neutrons and electrons. Electrons have only an extremely tiny fraction of the mass of a proton or neutron, so you can leave them out of the sum and just add together what is in the nucleus. In the case of hydrogen, there is only one proton in the nucleus, so the atomic mass is 1. I know the picture shows 1.00794, but we'll worry about the extra numbers after the decimal point later and just round down to the nearest whole number. All the other elements also have neutrons, which match the number of protons, so calculating the atomic mass for them is a simple case of doubling the atomic number.

That's the basics of elements. I suggest you now head back to the article on atoms and read through the rest of that.