Topic 3

3.1 The periodic table

In chemistry the elements are classified by arranging them in the periodic table. In the periodic table each row is called a period and each column is called a group.

The elements in a group tend to show similar chemical properties.

Some of them are given names.

group I - alkali metals

Lithium, sodium and potassium are alkali metals. They are stored under oil because they react with water and oxygen.

They react with water producing hydrogen gas and an alkaline solution.

They must be less dense than water, since they float on water. Metals from group I have very low densities and low melting points compared with most other metals.

group II - alkaline earth metals

Magnesium and calcium are alkaline earth metals. They react slowly with water giving hydrogen and an alkaline solution.

Flame tests

A wire carrying a small amount of the sample under test is placed in a bunsen flame. The colour of the flame is observed.

compound

flame colour

lithium sulphate

red

copper sulphate

green

sodium sulphate

orange

potassium sulphate

lilac

strontium sulphate

brick red

Some metals in compounds can change the colour of a flame. This can be used to identify the metal in an unknown compound.

group VII - halogens

Halogen

Colour

Normal State

Fluorine

pale yellow

gas

Chlorine

green/yellow

gas

Bromine

brown

liquid

Iodine

grey

solid

The Halogens are reactive elements.

group VIII or 0 - noble gases

Helium, neon and argon are noble (or rare) (or inert) gases

The noble gases are extremely unreactive elements.

Transition metals

The transition metals are in a block in the centre of the periodic table. They are often used as catalysts and are useful in alloys.

3.2 Atomic structure

Every element is made up of very small particles called atoms. Atoms are made of three smaller particles. The light negative charged particles are called electrons. The heavier positive charged particles are called protons. The neutrons have no charge.

The very positive centre of the atom is called the nucleus.

Structure of atom (eg. Helium)

An atom is overall electrically neutral because the numbers of protons and electrons are equal.

The masses of atomic particles are measured in atomic mass units. On this scale the mass of a proton is taken to be 1 amu (atomic mass unit).

particle

mass (amu)

proton

1

neutron

1

electron

1/2000 (=0)

The electron is so light compared to a proton or neutron that its mass is usually considered as zero.

3.3 Electron arrangement

The electrons in an atom are arranged in energy levels.

The energy level nearest the nucleus can hold 2 electrons, the second level can hold 8 and the third level 18. The levels are filled starting at the lowest energy near the nucleus.

3.4 Atomic number

The atoms of different elements differ in the number of protons they have in their nuclues. The atomic number of an element is the number of protons in the nucleus of one of its atoms.

The number of electrons in the outermost energy level fixes the chemical properties of an element. Therefore elements in the same group (column) of the periodic table have the same number of outer electrons.

3.5 Nuclide notation

The mass number of an atom equals the number of protons plus neutrons in its nucleus.

All the information about an atom or ion can be shown by nuclide notation. An ion is an atom which has had electrons added or removed from it leaving it with an electrical charge.

example - nuclide notation for the sodium ion

The sodium ion above has atomic number 11 , it has 11 protons in its nucleus.

The mass number is 23. Since it has 11 protons it must therefore have 12 neutrons in its nucleus.

The chloride ion above has atomic number 17 , it has 17 protons in its nucleus.

The mass number is 37. Since it has 17 protons it must therefore have 20 neutrons in its nucleus.

The charge on the ion is - (one minus). One electron has been added so it now has 18 electrons.

3.6 Isotopes

Most elements exist as a mixture of isotopes.

Isotopes are atoms of the same element having the same atomic number but different mass numbers.

example - lithium

Isotopes differ in the number of protons in the nucleus.

3.7 Relative atomic mass

The relative atomic mass of an element is the average mass of one atom of the element, on a scale where the carbon-12 isotope has a mass of 12.00000 a.m.u.

Consider the element chlorine which has two isotopes

Isotope

Abundance

75%

25%