Science C4

Chemists have arranged the chemical elements into a table called the Periodic Table. This helps us to make sense of the different properties of the elements and their compounds. It also helps us to predict how they will behave in different situations.

There are several different versions of the Periodic Table, but all have a similar apperance. Each element is shown but its symbol, and sometimes also by its name.

In the Periodic Table, the elements are arranged in order of proton number, also called atomic number. This is the number of positive protons in each atom. It is shown as the number written below each element in the table. 

Putting element in this order gives a repeating pattern of their properties. In the Periodic Table each element is place beneath those with similar properties.

The Periodic Table gives us a great deal of information about each of the elements. Firstly, the name and symbol of each element are shown. If you know one of these two facts about an element, you can use the table to find the other.

As well as the proton number shown below each element, another number is shown above it. This is the relative atomic mass of the element . It is a comparative measurement of the mass of one atom of the element. You can use it to see how much heavier an atom of one element is compared with an atom of another element.

For example a magnesium atom has a relative atomic mass of 24. So we know it is twice as heavy as a carbon atom, which has a relative atomic mass of 12. 

The Periodic Table is divided into horizontal rows and vertical columns. The first row has only two elements: hydrogen and helium. The next row has eight elements. lithium to neon.

Across each row, the elements on the left are metals, while those on the right are non-metals, most of the elements are metals.

Each column in the table contains elements with similar properties called a group. Each has a group number, shown across the top of the table. So Group 1 contains the elements lithium (Li) to francium (Fr), and Group 7 contains the elements fluorine (F) to astatine (At).

The elements in Group 1 of the Periodic Table are called the alkali metals. They include lithium, sodium and potassium.

Lithium, sodium and potassium are all soft metals that are easily cut with a scalpel or knife. The freshly cut surface is shiny, silver colour, but this tarnishes quickly to a dull grey as the metal reacts with oxygen and water in the air. Pieces of such metals are stored in oil to prevent these reactions. The shiny surface of sodium tarnishes more quickly than that of lithium. And potassium tarnishes more quickly than sodium. This shows the increasing reactivity of the metals as we go down the group.

Because the alkali metals are so reactive, care has to taken when using the,. They must not be touched because they will react with the water in sweat on the skin. Gloves may be used, and goggles should be worn.

The alkali metals have low melting and boiling points compared to most other metals. Apart from the other alkali metals, only three metals indium, gallium and mercury have lower melting points than lithium. Lithium has the greatest melting point in the group, the melting points then decrease as you go down the group.

Boiling points show a very similar pattern to the melting point. Lithium has the highest boiling point and they decrease as you go down the group.

The density of a substance is a measure of how much mass it has for its size. Its measured in grams/cubic centimetre. For example gold and lead are very dense metals - even a small lump of either of them can still feel heavy. The alkali metals have low densities compared to most other metals. Lithium is shown to have the lowest density in the group. The densities then generally increase as you go down the group.

The alkali metals are very soft. Lithium is the hardest alkali metal and they become softer as you go down the group. 

All the alkali metals react vigorously with cold water. In each reaction, hydrogen gas is given off and the metal hydroxide is produced. The speed and violence of the reaction increases as you go down the group. This shows that the reactivity of the alkali metals increases as you go down Group 1.

Lithium - When lithium is added to water, lithium floats. It fizzes steadily and becomes smaller, until it eventually disappears.
Lithium + Water = Lithium Hydroxide + Hydrogen
2Li+2H20=2LiOH+H2

Sodium - When sodium is added to water, the sodium melts to forma a ball that moves around of the surface. It fizzes rapidly, and the hydrogen produced may burn and orange flame before the sodium disappears.
 Sodium + Water = Sodium Hydroxide + Hydrogen
2Na+2H2O=2NaOH+H2 

This equation applies to all the Group 1 metals when reacting with cold water.

The hydroxides formed in all of these equations previously mentioned dissolve in water to form alkaline solutions. These solutions turn universal indicator purple, showing they are strongly alkaline. Strong alkalis are corrosive, so care must be taken when they are used - for example, by using goggles and gloves. 

All of the alkali metals react vigorously with chlorine gas. Each reaction produces a white crystalline salt. The reaction gets more violent as you move down Group 1, showing how reactivity increases down the group.

Lithium - If a piece of hot lithium is lowered into a jar of chlorine, white powder is produced and settles on the sides of the jar. This is the salt lithium chloride.
Lithium + Chlorine = Lithium Chloride
2Li + Cl2 = 2LiCl

 Sodium - If a piece of hot sodium is lowered into a jar of chlorine, the sodium burns with a bright yellow flame. Clouds of *********** are produced and settle on the sides of the jar. This is the salt sodium chloride. The reaction of sodium with chlorine is similar to the reaction with lithium, but more vigorous.
Sodium + Chlorine = Sodium Chloride
2Na + Cl2 = 2NaCl 

The elements in Group 7 of the Periodic Table are called the halogens. They include chlorine, bromine and iodine. The halogens are diatomic - this means they exist as molecules, each with a pair of atoms. Chlorine molecules have a formula Cl2, bromine Br2 and iodine I2,

The halogens show trends in physical properties down the group.

The halogens have low melting and boiling points. This is a typical property of non-metals. Fluorine has the lowest melting point and boiling point. The melting points and boiling points then increase as you down the group.

Room temperature is usually taken as being 25 degrees. At this temperature fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. There is therefore a trend in state from gas to liquid to solid down the group.

The halogens become darker as you go down the group. Fluorine is very pale yellow, chlorine is yellow-green and bromine is red-brown. Iodine crystals are shiny purple - but easily turn into a dark purple vapour when they are warmed up.

When we can see a trend in the properties of some of the elements in a group, it is possible to predict the properties of other elements in that group. Astatine is below iodine in Group 7. The colour of these elements gets darker as you go down the group. Iodine is purple, and, as we would expect astatine is black. 

The halogens become less reactive as you go down the group. Fluorine at the top of the group, is the most reactive halogen. It is extremely dangerous, causing severe chemical burns on contact with skin.

The halogens react with metals to make salts called metal halides.
metal + halogen = metal halide
For example sodium reacts with chlorine to make sodium chloride (common salt).

The reaction between sodium and a halogen becomes less vigorous as we move down Group 7. Fluorine reacts violently with sodium at room temperature. Chlorine reacts very vigorously when in contact with hot sodium. Iodine reacts slowly with hot sodium.

Halogens are bleaching agents. They will remove the colour of dyes. Chlorine is used to bleach wood pulp to make white paper.

Halogens kill bacteria. Chlorine is added to drinking water at very low concentrations. This kills any harmful bacteria in the water, making it safe to drink. Chlorine is also added to the water in swimming pools.

Because the halogens are very reactive and poisonous, care must be taking when using them. Chlorine is used in a fume cupboard. Iodine should not be handled (it will damage the skin). Gloves may be used, and goggles should be worn.

The reactivity of the halogens decreases as we move down the group. This can be shown by looking at displacement reactions.

When chlorine (as a gas or dissolved in water) is added to sodium bromide solution the chlorine takes the place of the bromine. Because chlorine is more reactive than bromine, it displaces bromine from sodium bromide. The solution turns brown. This brown colour is the displaced bromine. The chlorine has gone to form sodium chloride.

If you look at the equation you can see that the Cl and Br have swapped places.

Chlorine + Sodium Bromide = Sodium Chloride + Bromine
Cl2 + 2NaBr = 2NaCl +Br2

This type of reaction happens with all of the halogens. A more reactive halogen displaces a less reactive halogen from a solution of one of its salts.

If you test different combinations of the halogens and their salts, you can work out a reactivity series for Group 7. The most reactive halogen displaces all of the other halogens from solutions of their salts, and is itself displaced by non of the others. The least reactive halogen displaces non of the others, and is itself displace by all of the others. It works just the same where you use sodium salts or potassium salts.

When chemicals react with each other, different chemicals are made. One of the best ways to describe what is happening is by writing a chemical equation.

A chemical equation tells you which chemicals reacted together (the reactants) and the new chemicals that were made in the reaction (the products). The simplest equation is a word equation. For example: Sodium + chlorine = sodium chloride. A symbol equation gives more information about what is happening in the reaction. 2Na + Cl2 = 2NaCl - Each of the reactants and products is shown as a formula. This formula shows how many atoms of element are present. The formula for sodium is Na - the same as its symbol. The formula for chlorine is Cl2, because the halogens exist as molecules of two atoms (diatomic molecules).

Each of the Group 1 halides has a formula with one symbol for the metal and for for the halogen. So, for sodium chloride the formula is NaCl. The numbers in front of the formulae are there to balance the equation. This gives the same number of atoms of each element on each side of the equation.

Sometimes it is useful to know whether the reactants and products in a chemical reaction are solids, gases, liquids or dissolved in water. We can *** state symbols to a symbol equation to show this.

• (s) - solid
• (l) - liquid
• (g) - gas
• (aq) - aqueous (dissolved in water)

So for the reaction between sodium and water, this is the symbol equation with state symbols :

2Na (s) + 2H2O (l) = 2NaOH (aq) + H2 (g)

Atoms are not the smallest particles of matter. Atoms are made up of even smaller, subatomic particles called proton, neutrons and electrons.

At the centre of every atom is a nucleus containing protons and neutrons. All atoms of the same element have the same number of protons. This number is used to arrange the elements in the Periodic Table, beginning with hydrogen, which has just one proton.7

Electrons are contained in shells around the nucleus. The total number of electrons is always the same as the number of protons in the nucleus.

These shells are also called energy levels. The number of shells, and the number of electrons in the outer shell, varies from one element to another. For example, a lithium atom has two shells with two electrons in the inner shell and one in the outer shell. A carbon atom also has two shells but with two electrons in the inner shell and four in the outer shell.

Protons and neutrons have the same mass, which is about 2000 times larger than the mass of an electron. Protons and electrons have an electrical charge. This electrical charge is the same size for both, but protons are positive and electrons are negative. 

Neutrons have no electrical charge; they are neutral.

Proton - Relative Mass = 1, Relative Charge +1

Neutron - Relative Mass = 1 Relative Charge 0

Electron - Relative Mass = 0.0005 Relative Charge -1

The coloured light given off by fireworks is produced as elements in the fireworks are heated up. By studying the light geven out by elements, scientists have found out about the structure of atoms, and even discovered new elements.

When the atoms of some metals are heated, they give off coloured light. The colour given off by each metal is different, and can be used to identify them.

A small piece of metal compound on the end of a piece of Nichrome wire is introduced into a hot Bunsen flame. The Bunsen flame shows a colour that is characteristic of the metal in the compound.

Lithium shows red 
Sodium shows yellow
Potassium shows lilac 

All atoms give off light when heated, although sometimes this light is not visible to the human eye. A prism can be used to split this light to form a spectrum, and each element has its own distinctive line spectrum. This technique is known as spectroscopy. Some examples of what line spectra look like are shown here:

Scientists have used line spectra to discover new elements. In fact the discovery of some elements, such as rubidium and caesium, was not possible until the development of spectroscopy. The element helium was discovered by studying line spectra emitted by the sun.

The number of protons in the atom of an element determines its place in the Periodic Table. The number of electrons in an atom is the same as the number of protons. These electrons are arranged in shells or "energy levels" around the nucleus. The arrangement of electrons determines the chemical properties of an element.

Electrons are arranged in shells at different distances around the nucleus. As we move across each row of the Periodic Table the proton number increases by one for each element. This means the number of electrons also increase by one for each element. 

Starting from the simplest element, hydrogen, and moving through the elements in order we can see how the electrons fill the shells. The innermost shell of electron is filled first. The shell can contain a maximum of two electrons. 

Next the second shell filsl with electrons. This can hold a maximum of eight electrons. When this is filled, electrons go into the third shell, which also hold a maximum of eight electrons. Then the fourth shell begins to fill.

The electronic structure of each element can be shown simply as the number of electrons in each shell. For example, lithium is 2.1, neon is 2.8.8, and calcium is 2.8.8.2.

The arrangement of electrons can also be shown using a dot and cross diagram. Electron shells are drawn as circles, with the electrons on each shown as dots or crosses. Here is an example:

Lithium atom: The black dot represents the nucleus, where the red dots represent the electrons.

The way electrons are arranged in an atom is called the "electronic structure". As you have seen there is a link between an element's electronic structure and its place in the Periodic Table. You can work out an element's electronic structure from its place in the Periodic Table. 

Moving across each period, the number of shells is the same as the period number. As you go across each period from left to right the outer shell gradually becomes filled with electrons. The outer shell contains just one electron on the left hand side of the table, but is filled by the time you get to the right hand side.

Moving down each group, the number of electrons in the outermost shell is the same as the group number. Each element in a group therefore has the same number of electrons in its outer shell.

Group 0 is a partial exception to this rule, since although it comes after Group 7 its is not called Group 8; and it contains helium which has only two electrons in its outer shell.