If iron were broken up into particles so small that the eye could not see them, would the properties of iron be found in those particles exactly as they are now?
Element
Some of the substances around us keep the properties of the original substance exactly as they were, in the particles formed when they are broken up by any process at all. Such substances are elements. Copper is an element, because copper cannot be broken up to make any other simpler substance, or, put another way, however much you break copper up, all you can ever get are pieces that still have the properties of copper. Iron, gold, silver, oxygen, hydrogen and sulphur are examples of elements.
A substance that cannot be broken down into a simpler substance by a chemical process is called an element. So far scientists have discovered 118 elements in all, of which 92 are natural and 26 are artificial.
Elements are found in the solid, liquid or gas state. For example, at ordinary temperature iron, gold and silver are elements in the solid state, bromine and mercury are elements in the liquid state, and hydrogen, oxygen and nitrogen are elements found in the gas state.
Notice two words in that definition that are doing a great deal of work, and that most students read straight past. The first is chemical. It does not say that an element cannot be broken up at all; it says it cannot be broken into anything simpler by a chemical process. You can hammer a piece of copper into dust, and the dust is still copper. You can melt it, and the liquid is still copper. Physical treatment never gets you anywhere new. The second word is simpler. An element is not simply small; a grain of sugar is very small and sugar is not an element, because sugar can be broken into carbon, hydrogen and oxygen, which are genuinely simpler things. So the test is not size. The test is whether anything simpler is hiding inside, and for an element the answer is no. This is why there are only 118 of them while the number of substances in the world is beyond counting: the elements are the ending point, the pieces that nothing else can be made out of.
Where elements sit in the family of matter
Atom
If elements go on being broken up by a chemical reaction, the smallest particle of them can be found, and this is called an atom. The smallest particle of an element that can take part in a chemical reaction is called an atom. When iron carries out a chemical reaction with some other substance, it is its atoms that do the reacting, and in the same way, when every element reacts chemically with another element, it is their atoms that react. The atoms of one and the same element are all of one kind, but the atom of one element is different from the atom of another element. For instance, the atom of the element oxygen is different from the atom of hydrogen. There are 118 kinds of atom for the 118 elements discovered so far.
Structure of an atom
An atom is the smallest particle of an element. An atom is made up of subatomic particles joined together, and the proton, the neutron and the electron are those subatomic particles. There are two parts in an atom, the nucleus and the shell. The central part of an atom is called the nucleus, and the protons and neutrons are packed together in it. The outer part around the nucleus of an atom is the orbit or shell, in which the electrons are found. The electrons stay in the shells and go round and round the nucleus.
| Particle | What it is |
|---|---|
| Proton | A subatomic particle carrying a positive charge. It stays in the nucleus of the atom. The mass of one proton is equal to the mass of one hydrogen atom, and this mass is taken as 1 atomic mass unit (amu). |
| Neutron | A neutral particle, that is to say one with no charge at all. It too stays in the nucleus of the atom. The mass of one neutron is equal to the mass of one proton, so the mass of one neutron is also taken as 1 atomic mass unit. |
| Electron | A particle carrying a negative charge. It goes round the nucleus of the atom in a definite shell. The mass of an electron is small compared with the proton and the neutron: the mass of one electron is about one part in 1837 of the mass of one proton. |
Comparing the three particles
| Subatomic particle | Symbol | Mass | Charge | Location |
|---|---|---|---|---|
| Proton | p⁺ | 1 amu | +ve | Nucleus |
| Electron | e⁻ | 1/1837 amu | -ve | Shell |
| Neutron | n⁰ | 1 amu | 0 | Nucleus |
Read the formula for atomic weight and something ought to look wrong. Atomic weight is the number of protons plus the number of neutrons, and the electrons do not appear in it at all. That is not carelessness. Look back at the table: a proton and a neutron are each 1 amu, while an electron is about one 1837th of that. In a carbon atom there are six electrons, and together they weigh about one three hundredth of a single proton. Leaving them out changes the answer so little that nobody bothers. Now put that beside a second fact. The nucleus, which holds all the mass, is minute compared with the shells, while the electrons, which contribute almost nothing to the mass, are spread all the way out to the edge of the atom. So almost all the mass of an atom lives in a tiny space at the centre, and almost all the volume of an atom is nearly empty. This is one of the strangest facts in science, and you have just worked it out from two numbers in a table.
Some elements and their particles
Elements are denoted by symbols. Here the symbols of some elements and the numbers of their subatomic particles are given in a table.
| SN | Element | Symbol | Protons | Electrons | Neutrons |
|---|---|---|---|---|---|
| 1 | Hydrogen | H | 1 | 1 | 0 |
| 2 | Helium | He | 2 | 2 | 2 |
| 3 | Lithium | Li | 3 | 3 | 4 |
| 4 | Beryllium | Be | 4 | 4 | 5 |
| 5 | Boron | B | 5 | 5 | 6 |
| 6 | Carbon | C | 6 | 6 | 6 |
| 7 | Nitrogen | N | 7 | 7 | 7 |
| 8 | Oxygen | O | 8 | 8 | 8 |
| 9 | Fluorine | F | 9 | 9 | 10 |
| 10 | Neon | Ne | 10 | 10 | 10 |
| 11 | Sodium (Natrium) | Na | 11 | 11 | 12 |
| 12 | Magnesium | Mg | 12 | 12 | 12 |
| 13 | Aluminium | Al | 13 | 13 | 14 |
| 14 | Silicon | Si | 14 | 14 | 14 |
| 15 | Phosphorus | P | 15 | 15 | 16 |
| 16 | Sulphur | S | 16 | 16 | 16 |
| 17 | Chlorine | Cl | 17 | 17 | 18 |
| 18 | Argon | Ar | 18 | 18 | 22 |
| 19 | Potassium (Kalium) | K | 19 | 19 | 20 |
| 20 | Calcium | Ca | 20 | 20 | 20 |
Check row twelve against your printed textbook. The book gives the symbol of magnesium as M, and that is a misprint. The correct symbol is Mg, with a small g, and it is written that way on every periodic table in the world. The reason is that plenty of elements begin with the letter M, including manganese, mercury and molybdenum, so a single M could not possibly tell them apart. That is exactly why most symbols use two letters, with the first one a capital and the second one small. Write Mg in your notes, and while you are looking at that column, notice two more oddities: sodium is Na and potassium is K, which do not match their English names at all. Those two come from their older Latin names, Natrium and Kalium, and the book quietly prints both names in the table for exactly that reason.
Atomic number and atomic weight
| Atomic number = number of protons = number of electrons |
| Atomic weight = number of protons + number of neutrons |
Look down the proton column of that table and you will see something remarkable: 1, 2, 3, 4, 5, 6 and so on, without a single gap, all the way to 20. The elements are not just a list of names, they are numbered, and the number is the count of protons. That is the whole reason the table can be written in this order. Now take the idea seriously. If an atom has 8 protons it is oxygen, and nothing else in the universe can be oxygen. Give it one more proton and it stops being oxygen and becomes fluorine, and its properties change completely. So the identity of an element is decided by exactly one number, the proton count, and by nothing else. Its colour does not decide it, nor its state, nor even its mass, because the neutron count can vary while the element stays the same. Notice too that the electron count matches the proton count in every row, and you already know why from the unit on electricity: a positive charge and a negative charge of equal size cancel out, and that is what makes an ordinary atom neutral.
Worked example: reading the table
| Question | Working |
|---|---|
| An element has 17 protons. Name it and give its symbol | Protons decide the identity, so 17 protons means chlorine, symbol Cl |
| How many electrons does that atom have? | 17, because in a neutral atom the electrons match the protons |
| What is its atomic number? | 17, since the atomic number is the number of protons |
| What is its atomic weight? | Protons + neutrons = 17 + 18 = 35 |
| An element has 10 protons and 10 neutrons. Name it | The protons alone answer it: 10 protons means neon, symbol Ne, atomic weight 20 |
Notice the pattern in every one of those answers. Whenever a question tells you the number of neutrons and asks which element it is, that information is not enough on its own, because the neutron count does not decide identity. But the moment you are given the protons, or the atomic number, the element is fixed and everything else follows from the table.
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