General periodicity.

For example all of the group one elements (the alkali metals) react vigorously with water to form water soluble alkalis with the subsequent production of hydrogen gas.

However, the speed of reaction increases down the group (lithium only reacts relatively slowly with water but francium and rubidium are explosive in the presence of water)

The rows across the periodic table are known as periods. The chemistry of the elements within each period will tend to change quite dramatically but we can still determine trends in some physical properties and the nature of some common reactions.

For example, as we will see, the atomic size of the elements across period three gradually decreases as the period is crossed.

In this pack, we will start by considering some of the major properties of elements that show periodic variation.

We will then consider the variation in these properties down a particular group. In this case we are going to look at group II of the periodic table.

Finally, we will consider the variation in the properties across period three of the periodic table.

Properties of the elements that show periodic variation.

Before we start to look in any detail at the physical properties of the elements, we should first define the physical properties that we want to consider.

1. Atomic radius.

Atomic radius is defined as the distance from the centre of an atom to the perimeter of the outermost electron shell.

2. First ionisation Energy.

First ionisation energy is described as the minimum energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous uni-positive ions.

For example ; in equation for the first ionisation energy of sodium would be :

Na(g)    ➔    Na+(g)    +    e-

3. Electronegavitity

Electronegativity is defined as the relative ability of an atom to attract the pair of electrons in a covalent bond.

For example ;

Hydrogen bonds with oxygen to form water through the formation of covalent bonds.

At GCSE you will have learned that the electrons within a covalent bond are shared equally between the atoms forming the bond. However, current understanding of chemistry suggests that this is only an approximation.

For example in the water molecule, oxygen atoms have a higher effective nuclear charge than hydrogen (oxygen has a more powerful nucleus) and therefore the oxygen atoms in the bond are able to gain a greater share of the electrons in the bond than the hydrogen atoms. This also means that the oxygen atoms in water carry a slightly negative charge (δ-) while the two hydrogen atoms are slightly positive (δ+) – See electronegativity diagram below.

Linus Pauling described oxygen as being more electronegative than hydrogen.

Pauling assigned each element with a number (in the range 0 to 4) that describes their relative electronegativities.

For example :

Oxygen = 3.5            Hydrogen = 2.1

Electronegativity Diagram.

The greater the difference in electronegativities between the atoms forming the bond the more the electrons in the bond are pulled away from one atom and towards the other.

Electronegativity is best described as the relative ability of an atom to attract the pair of electrons within a covalent bond towards itself.

4. Melting Point.

The melting point of an element is the temperature (cited in oC or K) at which the element passes from the solid state into the liquid state. This has the same numerical value as the freezing point of the element.

General periodicity – The blocks of the periodic table.

Before we look at group II and period 3, it is important that we should be able to find our way around the periodic table easily. To assist with this process the elements can be subdivided into 4 blocks (s, p, d, f)

The diagram below is a simplified version of the periodic table showing only the main areas. Compare this diagram with the main periodic table.

Activity

1. Which two groups in the periodic table are in the s – block as shown above?
2. Which groups are in the p – block?
3. What is another name that is given to the d – block elements?
4. There are two types of  f – block elements – can you name them?
5. Only one element in the p – block is a liquid at room temperature and pressure. Can you spot which element this is?
6. In fact, there is only one other element in the periodic table that is a liquid at room temperature. Which element is this? What block is this element in?

The block that an element is in depends on the electronic configuration of the element. Use the exercise provided below to determine the general rule for assigning elements to blocks.

(Be careful with this exercise, it is not as easy as it at first might seem!)

Exercise.

Step 1

Write down the full electronic configuration of the elements – lithium (3), magnesium (12), Chlorine (17) and cobalt (26).

Using a periodic table decide which block  each element is in, in the periodic table.

Step 2

Compare the data above to come up with a theory as to why each element  is in its own particular block in the periodic table.

Warning – be sure that you take good note of the electronic configuration of cobalt before you decide on your theory.

What was your answer?

In fact :

An element will belong to the block in the periodic table that denotes the highest energy subshell occupied by the element.