Composition of matter

Atoms are the building blocks of all substances

Singte atoms are far too small to be seen, even with the most powerful microscope. For example, about four billion sodium atoms would fit on the full stop at the end of this sentence. However, in spite of their small size, scientists have managed to find out a great deal about atoms. Atoms are tiny particles which are the building blocks of all substances, whether they are solids, liquids, or gases. For hundreds of years atoms were thought to be the smallest particles. Then it was discovered that atoms themselves are made up of even smaller particles called protons, neutrons, and electrons.

All atoms consist of a nucleus and a cloud of electrons that move around the nucleus. The nucleus itself is a cluster of two kinds of particles: protons and neutrons. All the particles in an atom are very light. Their mass is measured in atomic mass units, rather than grams. Protons and electrons also have electric charges.

Arrangements of electrons in an atom

Scientists have worked out a theoretical model of the atom in which the electrons are arranged in definite layers or levels of energy around the nucleus. Each layer or shell can hold a certain number of electrons. Different orbits or shells can hold different numbers electrons. Starting nearest the nucleus, they are numbered 1, 2, 3, and so on. If 'n’ is the shell number, the maximum number of electrons which that shell can hold is 2n²

The first energy shell, which is at the lowest energy level, can hold a maximum of two electrons. The second can hold a maximum of eight electrons. The outermost shell can hold no more than eight electrons.

Study the sodium atom carefully. 

The sodium atom has 11 protons, 11 electrons, and 12 neutrons.

The different levels for the electrons are called electron shells.

Notice how the electrons are arranged in the sodium atom:

(2) in the first shell

(8) in the second shell

(1) in the third shell.

The sodium atom is often written as Na (2, 8, 1)

2, 8, 1 is its electron arrangement, or electronic configuration

Atomic number

Look at the sodium atom again. It has 11 protons. This fact could be used to identify it because only a sodium atom has 11 protons. Every other sort of atom has a different number of protons. The number of protons in an atom is therefore an important number. It is called the atomic number. The atomic number is equal to the number of protons in an atom. The atomic number of sodium is 11.

The sodium atom also has 11 electrons. So it has an equal number of protons and electrons. The same is true for every sort of atom: every atom has an equal number of protons and electrons. Because of this, atoms have no overall charge.

Mass number

The electrons in an atom have almost no mass. So the mass of an atom is largely due to its protons and neutrons. For this reason, the number of protons and neutrons in an atom is called its mass number

The mass number is equal to the number of protons + neutrons in an atom.

A sodium atom has 11 protons and 12 neutrons, so the mass number of sodium is 23.

The sodium atom can be described in a short way:

the symbol for sodium is (Na)

the atomic number is (11)

the mass number is (23)

From this you can tell that the sodium atom has 11 protons, 11 electrons, and 12 neutrons.

Chemists often describe atoms in this way:

mass number (23)

symbol (NA)

atomic number (11)

Elements

An element is a substance that is made of only one kind of atom. Sodium is made of sodium atoms only so it is an element. Carbon, mercury, iron, copper, etc. are all elements. Altogether 117 different elements are known. Of these, 90 have been obtained from the Earth's crust and atmosphere, and 23 have been artificially made by scientists.

Every element has a name and a symbol.

Some elements like helium, neon, and argon do not react with anything. These elements have similar properties because their atoms have full outer shells of electrons.

Isotopes

The atoms of an element are not always identical. They can be different. Atoms of one element all have the same number of protons and electrons but they do not necessarily have the same number of neutrons. Atoms of the same element, with the same number of protons but different numbers of neutrons are called isotopes of that element. They behave in an unusual way that has nothing to do with their electrons. Most elements have more than one isotope.

Chlorine has two: 35≡17 CL and 37≡17 CL

Carbon has three isotopes: 12≡6 C, 13≡6 C, 14≡6 C

The carbon-14 atom is the one that behaves strangely. Its nucleus is unstable, because of the extra neutrons. In time, every carbon-14 atom throws a particle out of its nucleus, and becomes a nitrogen atom. This process is called decay. Carbon-14 is said to be radioactive. When it decays it gives out radiation from its nucleus.

Like carbon, many elements have radioactive isotopes, or radioisotopes, that occur naturally. Other radioisotopes are made in laboratories or in nuclear power stations.

Uses of radiation 

1. Engineers can check oil pipes for leaks by adding a radioactive substance to the oil.

2. Doctors use a weak beam of gamma rays to kill cancer cells. 

3. Plants absorb some carbon-14 atoms from the carbon dioxide in the air during the process of photosynthesis. When a plant or animal dies, it takes in no more carbon atoms. The carbon-14 atoms in it slowly decay. By measuring the radiation from them, the age of the dead remains can be worked out. This is known as carbon dating.

Valency

Scientists sometimes call the valency the 'combining power' of an element.

Depending on the arrangement of electrons around a nucleus, atoms of different elements may tend to lose or gain electrons. If this tendency is strong, then the element is reactive or unstable. If it is weak, the element is unreactive or stable.

The valency of an element depends on the number of electrons the element has in its outermost shell. If an element has four or fewer electrons in its outer shell, then the number of electrons is the same as the valency. If the element has more than four electrons, its valency is eight minus the number of electrons.

1: Hydrogen has a valency of 1+ because it has one electron in its outermost shell.

2: Calcium has a valency of 2+ because it has two electrons in its outermost shell.

3: Carbon has a valency of 4+ or 4- because it has four electrons.

4: Nitrogen has a valency of 3– because it has three electrons short of 8.

5: Oxygen has a valency of 2- because it has two electrons short of 8.

6: Chlorine has a valency of 1- because it has one electron short of 8.

Study the table below:

Valencies can be used to work out the chemical formulae of compounds.

Study the table of valencies above and use it to write the formula for each of the following:

1: magnesium chloride

2: sodium oxide

 3: silicon dioxide

4: calcium oxide

Formation of ions

The atoms of some elements can obtain full shells by losing or gaining electrons when they react with other atoms.

Losing electrons

The sodium atom has just one electron in its outermost shell. By losing this electron it can obtain a full outer shell. This results in the formation of a sodium ion.

From the diagram we can see that the sodium ion has 11 protons but only 10 electrons, so it has a charge of 1*. It is called a positive ion or a cation.

Gaining electrons

The chlorine atom has 7 electrons in its outermost shell. By accepting one electron from another atom it can obtain a full outer shell. This results in the formation of a chloride ion. From the diagram we can see that the chloride ion has 17 protons and 18 electrons, so it has a charge of 1-. It is called a negative ion or an anion.

Any atom becomes an ion if it loses or gains electrons. An ion is a charged particle, which contains an unequal number of protons and electrons. A bond formed between oppositely charged ions in a chemical compound is known as an Any atom becomes an ion if it loses or gains electrons. An ion is a charged particle, which contains an unequal number of protons and electrons. A bond formed between oppositely charged ions in a chemical compound is known as an ionic bond.

Ionic bond between magnesium and chlorine 

In the same way sodium chloride (Common salt) is formed by the joining up of sodium and chloride Jons. 

Covalent bonds

A molecule can, therefore, be defined as a small group of atoms which are held together by covalent bonds. 

Law of constant composition

This law states: When elements combine, they do so in a ratio of small whole numbers. For example:

 The formula for water is H₂O the ratio of H to is 2 : 1

The formula for carbon dioxide is CO₂ ; the ratio of carbon to oxygen is 1: 2

Chemical formula

The chemical formula tells us the number and types of atoms present in a molecule, it contains symbols of elements and their number in subscript e.g. H₂O, O₂, CaS