THE PERIODIC TABLE

Extracted from Chemistry for You (Lawrie Ryan. Published by Nelson Thornes, 2^nd ed. 2001)

Sorting out the elements

Scientists like to find patterns.

Around 200 years ago, scientists were discovering lots of new elements. However, they struggled to find any links between the different elements.

At the time, some substances, which were thought to be elements, were in fact compounds.

Other elements had not yet been discovered.

No wonder finding a pattern was tricky!

Finding the pattern

Real progress was made around 1865 by John Newlands.

He put the elements in order of their atomic mass.

He found that every eighth elements was similar.

Unfortunately, his pattern only worked for the first 15 elements known at that time. After that, he could see no links between the rest of the elements. Other scientists made fun of his ideas. They suggested that he could have done better by sorting the elements into alphabetical order!

In 1869 the problem was solved by a Russian called Dmitri Mendeleev.

He also tried putting the elements in order of their atomic mass.

He made a table of elements.

New rows were started so that elements which were alike could line up together in columns.

He wanted a table of regular (periodic) patterns.

However, Dmitri was not afraid to take risks. When the pattern began to go wrong. he would leave a gap in his table. He claimed that these gaps were for elements that had not yet been discovered. He even changed the order round when similar elements didn’t line up.

As you might expect, people doubted his “Periodic Table”.

However. he used his table to predict the properties of elements which could fill the gaps.

In 1886, the element germanium was discovered. The new elements matched Dmitri’s predictions.

Finally other scientists accepted his ideas.

►The Periodic Table

Although Mendeleev’s table was accepted, there was one thing that he could not explain.

Why did he sometimes need to change the order of atomic masses to make the pattern carry on? The answer lies inside the atoms. The atoms of elements in the Periodic Table are not arranged in order of mass. It is their number of protons (atomic number) which really matters.

Groups

There are 8 groups in the Periodic Table.

A group is a vertical column.

All the elements in a group have similar properties.

They are a chemical family

Look at the Periodic Table above:

Some groups have ‘special family’ names.

Can you find Group 7?. What is the group called?

Nave you met any elements from this ‘family’ before’?

Other groups are just known by their group number.

Groups are families of elements. The members of the family are similar but not exactly the same

Notice that the transition metals form a block on their own.

Periods

Periods are the rows across the Periodic Table.

You read the table like a book. Start at the top. and work your way down, reading from left to right. So there are 2 elements in the 1st period. H and He.

The 2nd period has 8 elements. starting with Li.

Which is the last element in the 2nd period?

Can you count how many elements are in the 3rd period

►Metal and non-metal elements

92 elements are found naturally on Earth.

Most can be sorted into 2 sets — the metals and the non-metals.

Let’s look at their properties.

Metals

Over three quarters of the elements are metals. The experiments below show us some typical properties of metals.

Properties of metals

A!! the properties shown below are physical properties. They describe the metal itself, not its chemical reactions.

Metals are important in all our lives. Can you think of a use to match each property shown below?

Some exceptions

Not all metals have these properties.

For example, the alkali metals (lithium, sodium, potassium) are soft metals. They have low densities and have low melting points.

Mercury also has a low melting point for a metal.

►Metallic bonding

Think of some of the things around your home that are made of metal.

Did you include all the wiring, any radiators, your hot-water tank, or your cutlery and pans? Do you know which metals these things are made from? Which properties make metals good for these uses?

We have looked at ionic and covalent bonding. But the atoms in a metal are held together in a different way.

Do you remember all the properties of metals?

Any ideas we have about the bonding and structure of metals must be able to explain their properties.

In general, metals:

• have high melting and boiling points

• conduct electricity and heat

• are hard and dense

• can be hammered into shapes (they are malleable)

• can be drawn out into wires (they are ductile).

We believe that metal atoms (or ions) are held together by a ‘sea’ of electrons. Look at the diagram below:

Each metal atom gives up electrons from its outer shell into the ‘sea’ or ‘cloud’ of electrons.

The electrons can drift about in the metal. These free electrons explain how electricity can pass through solid metals.

What happens when one end of the metal is made positive and the other end negative?

Properties of non-metals

Some exceptions

Carbon in the form of graphite is a good conductor of electricity. Carbon in the form of diamond has a very high melting point. Can you find out about any other ‘exceptional’ elements?

►Oxides of metals and non-metals

You have seen the physical properties of metals and non-metals. Now let’s look at some chemical properties. The chemical properties of a substance describe its reactions.

We can make oxides by reacting elements with oxygen.

Experiment 4.2 Burning elements in oxygen

Collect 4 large test-tubes of air.

Use tongs or a combustion spoon to heat your element in a Bunsen flame. Then plunge it into one of your tubes of oxygen gas.

When the reaction has finished, add a few drops of universal indicator solution. Then shake the tube gently.

Record your results in a table:

• Which oxide is slightly alkaline?

• Which oxides are acidic?

The elements burn more fiercely in pure oxygen than in air. This is because only about 20% of the air is made up of oxygen.

Metals burning

The metal oxides made are solids. For example,

magnesium + oxygen à magnesium oxide

Can you write the word equation for iron reacting with oxygen?

Non-metals burning

When non-metals burn, their oxides are often gases. For example,

carbon + oxygen à carbon dioxide

Try to write the word and symbol equations for sulphur reacting with oxygen.

(The product is sulphur dioxide gas. Its formula is SO₂)

Basic and acidic oxides

In the next experiment, you can test the pH of more oxides.

The oxides have been dissolved in water for you.

Experiment 4.3 Testing the pH of oxides

Collect 2 cm of each oxide solution in separate test-tubes.

Add 3 drops of universal indicator solution to each oxide solution.

Record your results in a table like the one below:

Which compounds in the table are metal oxides?

What do you notice about your results for the metal oxides?

What can you say about your results for the non-metal oxides?

Metal oxides

If a metal oxide dissolves in water, it makes

an alkaline solution. For example,

sodium oxide + water àsodium hydroxide

Some metal oxides do not dissolve in water. Iron oxide is insoluble. These metal oxides test neutral with universal indicator.

However, they do react with acid. Insoluble metal oxides are called bases. Look at the diagram opposite:

Non-metal oxides

Most non-metal oxides dissolve in water to form acids. For example,

carbon dioxide + water àcarbonic acid

One exception is the oxide of hydrogen.

What do we call this oxide ? What is its pH?

However, in general we can say:

Summary

• The Periodic Table arranges the elements in order of atomic number.

• Elements with similar properties line up in vertical columns. These columns are called groups.

• There are 8 groups in the Periodic Table.

• A row across the Periodic Table is called a period.

• The elements can be divided into metals and non-metals (with a few semi-metals or metalloids in between).

• Metals are good conductors of heat and electricity. They are shiny, malleable (can be hammered into shapes) and ductile (can be drawn out into wires). Most are hard, dense and have high melting points.

• Iron, cobalt and nickel are the magnetic metals.

• Most non-metals are gases. They have low melting and boiling points. They are poor conductors of heat and electricity. If solid, they are usually dull and brittle.

• In general, metal oxides are basic. Non-metal oxides are usually acidic.

►Questions

1. Copy and complete:

The elements line up in order of atomic.......... in the Periodic Table. There are .... groups. The elements in each group have.... properties.

Groups form v_ _ _ _ _ _ _ columns, whereas periods are h_ _ _ _ _ _ _ _ _ rows. Most of the

elements can be divided into 2 sets - the…………….and the non-metals. In general, metal oxides are ………...., and non-metal oxides are…………………

2. Copy and complete this table:

3. Not ah metals and non-metals have the properties listed in question 2. Can you name some exceptions and say why they are unusual?

4. Yasha and Eric tested solutions of some unknown oxides. Look at their results:

D did not dissolve in water, but did dissolve in dilute acid.

a) Which are the oxides of non-metals? How can you tell?

b) Which metal oxide is an alkali and which is a base? Explain your answer.

5. The numbers in this Periodic Table represent elements.

a) Which 2 elements are in the same group? Give the name and number of this group.

b) Which elements are in the 2nd period?

c) Which elements are metals?

d) Which group is element 7 in? Which period is it in?

e) Which element is a semi-metal (or metalloid)?