The two main ways in which atoms can be combined to form molecules are by electrovalent bonds and covalent bonds. Some molecules contain both electrovalent and covalent bonds, but many have just one or the other type. When you study enzymes, you will also need to know something about much weaker attractions between atoms in molecules of proteins: these are hydrogen bonds.

Electrovalent chemicals are held together by an electrical attraction between positively charged cations and negatively charged anions. You can read more about ions to refresh your memory if you need to. Cations are formed when an atom of a metallic element loses one or more of its electrons. Hydrogen is the only non-metal to form a cation. Anions are formed when an atom or group of atoms gains one or more electrons. This means that cations have a positive charge and anions have a negative charge.

When an electrovalent chemical is dissolved in water it will dissociate. That means that the cations and anions separate. These chemicals are called electrolytes because they will conduct electricity. You should have a look at my animations of atoms to see how they are composed of protons, neutrons and electrons. Group I metals have a single outer electron which they lose to form cations with a single positive charge. Group II metals have two outer electrons and form ions with a double positive charge. The Halogens have seven outer electrons, Group VII, and can gain one electron to form anions with a single negative charge.

Crystals of electrovalent compounds consist of a lattice of ions. Sodium chloride (ordinary table salt) consists of sodium and chloride ions. Each sodium ion is surrounded by six chloride ions, and each chloride ion is surrounded by six sodium ions. Have a think about that: you should realise that a crystal will contain exactly the same number of each kind of ion.

Covalent bonds are formed when atoms share electrons. These substances do not conduct electricity and do not dissociate into ions if they dissolve in water. The simplest covalent molecule is that formed between two atoms of hydrogen. A molecule of hydrogen consists of two atoms bonded together by a pair of electrons. This pair of electrons orbits the nuclei of both atoms so holding them together. This is the most common way in which non-metallic atoms are combined into molecules.

A catalyst is a substance which alters the rate of a chemical reaction but is chemically unchanged at the end of the reaction.

Why not memorise this definition?

“….. but is chemically unchanged at the end of the reaction.

” This means that there is just as much catalyst at the end of a reaction as there was at the beginning. The catalyst is used over and over again. Because catalysts work so rapidly and are used again and again, it is only necessary to have very small quantities of catalyst present to make a chemical reaction go faster.

You might have a “catalytic converter” in the exhaust pipe of your car. If the catalyst was Platinum you might expect silly people to start stealing car exhaust pipes; but there is so little catalyst there that it would not be worthwhile for them. A little bit of catalyst goes a long way! What does the catalytic converter do? Well without it the fumes from your car would cause too much pollution and the car might fail its MOT.

Perhaps you don’t think that catalysts are very important.

“….. alters the rate of a chemical reaction …..”

This means that catalysts make chemical reactions go faster. I am still looking for one which will make you do your homework faster, and another which will make me mark it faster. What about chemical reactions. Some of them go very slowly, your chemistry experiment might take hours, days, weeks, or ever years. Imagine if your chemistry teacher asked you to find out what gas is released from Hydrogen Peroxide: you might have to sit there watching your test tube for weeks; your chemistry teacher would keep on asking why you had not finished your work. Eventually you would have enough gas to test; so weeks later you would say “Oh, it is Oxygen Miss.” If you had put a little pinch of Manganese Dioxide into the test tube, the gas would be produced in a few minutes. So, you would be able to go long before the end of the lesson. Even better, you would still have the Manganese Dioxide catalyst which you would be able to sell back to your teacher to use with another class.

How about the chemical industry. Well they will make much more money if they can make their products quickly. The manufacturers of Nitric Acid use Platinum as a catalyst. Even though this is a very expensive metal, it does not cost too much to use it because they are only using small amounts of it.

“A catalyst is a substance …..”

This means that it is some kind of chemical substance! It could be a pure element; e.g. Platinum, Nickel; or it could be a pure compound, e.g. Manganese Dioxide, Silica, Vanadium V Oxide, Iron III Oxide; it coulb be dissolved ions, e.g. Copper ions, Cobalt II ions; or it could be a mixture, e.g. Iron-Molybdenum, or it could be a much more complicated compound such as protein (all enzymes are proteins; you learn about them in your biology; they are special cases.)

Enzymes are biological catalysts. They are slightly different in that they are easily denatured by heat. If you want to know more about enzymes, jump to the enzyme page (look at the Biology Index).

Most catalysts make chemical reactions go faster.

Chemists call such catalysts “positive catalysts” or “promoters”. However, sometimes we want a chemical reaction to go more slowly. So we choose a “negative catalyst”; we could call this an “inhibitor”. My wife put a negative catalyst in our central heating system. She did this to stop the iron bits from rusting. We did not have a problem with the Copper pipes (Copper does not rust), but we might have had a problem with the old Iron radiators: we wanted to stop them from rusting so we used an inhibitor. I think that we also have an inhibitor in the water cooling system of our car so that the car radiator does not rust. This is cheaper than buying a new car every year when the old one has got too rusty.

My baker puts an inhibitor into the bread he makes. This slows down the chemical reactions which make bread go stale. This is important since we only go shopping once a week. We used to put Lead in our petrol; this stops the engine from “knocking”. Now we have a better car which uses lead free petrol but the engine can burn it without knocking.

You might wonder how catalysts work.

There are two ways in which catalysts work. You already know that when two different molecules bump into each other, they might react to make new chemicals. We usually talk about “collisions” between molecules, it would be much simpler to say that the molecules bumped into each other. How fast a chemical reaction is depends upon how frequently the molecules collide. You have probably been told about the “kinetic theory” which is all about heat and how fast molecules move around. What catalysts are doing when they make a chemical reaction go faster is to increase the chance of molecules colliding. The first method is by “adsorption”, the second method is by the formation of intermediate compounds.


This occurs when a molecule sticks onto the surface of a catalyst. Make sure that you spell this word correctly; it is not the same as absorption. Here is an example: it is possible to use Platinum as a catalyst to make sulphur Trioxide from Sulphur Dioxide and Oxygen. Sulphur Trioxide is very important because it is used to make Sulphuric acid which is needed for car batteries. The molecules of the two gases (Sulphur Dioxide and Oxygen) get adsorbed (stuck onto) the surface of a Platinum catalyst. Because the two molecules are held so close together, it is more likely that they will collide and therefore react with each other. The Sulphur Trioxide easily falls off the catalyst leaving space for more Sulphur Trioxide and Oxygen.

Intermediate Compounds

Many catalysts, including all enzymes” work by forming intermediate compounds. What happens is very simple: the chemicals involved in the reaction combine with the catalyst making an intermediate compound, but this new compound is very unstable. When the intermediate compound breaks down it releases the new compounds and the original catalyst.

Well: if you have understood all this, it should be easy to memorise the definition of a catalyst given at the top of the page.