Everything on this table, from the phone to the salt to the plant, is built from atoms. Even the light-catching water!
Look around you. Your phone, the air you breathe, the water in your glass and even you are all made of atoms. All substances are made of atoms. Atoms are far too small to see, even with an ordinary microscope, but they are the building blocks of everything.
An element is a substance made of only one type of atom. Gold is an element: a gold ring contains only gold atoms. Oxygen is an element too, and so is carbon. There are about 100 different elements, and every one of them is shown in the periodic table.
If you keep cutting a piece of gold into smaller and smaller bits, you eventually reach a single gold atom. You cannot go any smaller and still have gold. That is why the spec defines an atom like this: an atom is the smallest part of an element that can exist.
Key terms:
Chemists all over the world use the same short code for each element. Each element's atoms are represented by a chemical symbol. For example, O represents an atom of oxygen and Na represents an atom of sodium.
A symbol is one or two letters. The first letter is always a capital and the second letter is always small. So Co is cobalt, an element, but CO (two capitals) is carbon monoxide, a compound of carbon and oxygen. Writing a capital in the wrong place changes the meaning completely.
Some symbols don't match the English name because they come from old Latin names. Sodium is Na (from natrium), potassium is K (kalium), iron is Fe (ferrum), copper is Cu (cuprum), silver is Ag (argentum) and gold is Au (aurum). These are worth learning by heart.
You get a periodic table in the exam, but you must be able to use the names and symbols of the first 20 elements, the elements in Groups 1 and 7, and other elements in the specification. Here are the first 20:
Group 1 also includes rubidium (Rb), caesium (Cs) and francium (Fr). Group 7 also includes bromine (Br), iodine (I) and astatine (At). Notice the UK spellings the exam uses: aluminium, sulfur and caesium.
A compound contains two or more elements chemically combined in fixed proportions. Water is a compound of hydrogen and oxygen. Table salt (sodium chloride) is a compound of sodium and chlorine.
Compounds are formed from elements by chemical reactions. A chemical reaction always makes one or more new substances, and it often involves a detectable energy change, such as the mixture getting hot, giving out light or getting cold.
A compound can be totally different from its elements. Sodium is a soft, very reactive metal and chlorine is a poisonous green gas, yet sodium chloride is the salt you sprinkle on chips.
Water is always H2O: two hydrogen atoms for every one oxygen atom. Water from a tap, a river or a cloud always has this exact ratio.
Compounds can only be separated into their elements by chemical reactions. Boiling water just gives steam, which is still water.
A compound is shown by a formula made from the symbols of the atoms it was formed from. The small number after a symbol tells you how many of that atom there are. If there is no number, there is one.
Key terms:
Some substances are just elements or compounds mixed together without being chemically joined. You will meet these in a later lesson, Mixtures and Separation Techniques.
You need to be able to name a compound when you are given its formula. A few simple rules do most of the work.
Some compounds just have names you need to know. H2O is water, CO2 is carbon dioxide ("di" means two) and NH3 is ammonia.
KBr: K is potassium, a metal, so it goes first. Br is bromine, and there are only two elements, so it becomes bromide. Answer: potassium bromide.
Li2O: Li is lithium. O is oxygen, the only other element, so it becomes oxide. Answer: lithium oxide. (The 2 tells you there are two lithium atoms, but it doesn't change the name.)
MgSO4: Mg is magnesium. SO4 is sulfate. Answer: magnesium sulfate.
Those fizzing bubbles are hydrogen gas: magnesium + hydrochloric acid → magnesium chloride + hydrogen
Chemical reactions can be shown by word equations or by equations using symbols and formulae. A word equation is the simplest kind. It uses only names.
When iron is heated with sulfur, the two elements react to make a new compound:
iron + sulfur → iron sulfide
You can also go the other way: given a symbol equation, you should be able to write it in words. For example:
2H2 + O2 → 2H2O becomes hydrogen + oxygen → water
The big numbers in front are for balancing, which you will learn in the next lesson, Writing Formulae and Balanced Equations. They never appear in a word equation.
Using "=" instead of an arrow. A reaction is a change, not a sum. Always use →.
Calling a compound "chlorine" or "oxygen". Once an element is combined, its name changes, so it is sodium chloride, not sodium chlorine.
Wrong capitals. CO and Co are different substances. So are NaCL and NaCl, and only NaCl is correct.
Mixing up -ide and -ate. Copper sulfide (CuS) contains no oxygen. Copper sulfate (CuSO4) does.
Calcium reacts with chlorine to make calcium chloride, CaCl2.
(a) Give the symbol for calcium. [1 mark]
(b) Write a word equation for this reaction. [1 mark]
(c) Calcium chloride is a compound. Explain what is meant by a compound. [2 marks]
(a) Ca
(b) calcium + chlorine → calcium chloride
(c) A substance containing two or more elements (1) chemically combined in fixed proportions (1).
When you write a symbol, check the capital letters before you move on. Ca is calcium, but CA or ca would lose the mark.
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