10. Explaining reactivity

How can we explain the reactivity of metals?

The metals in Group 1 of the periodic table all react with water. There is a pattern in the order of reactivity, with the metals becoming increasingly reactive as you go down the table. To explain why metals react in different ways, we have to go deeper inside the atom. Scientists used to believe that atoms were the smallest possible particles. Just over 100 years ago, there were some experiments on cathode rays

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which led to the development of the television tube. Measurements on the cathode rays showed that the beam was made of particles that were nearly 2000 times smaller than the lightest atoms. We now call these particles electrons. They carry the electrical current in a circuit and are the cause of all chemical reactions.

We can’t see inside an atom, but one way of helping to understand the evidence about the structure of the atom is to use a model. One simple model shows the atom as being like our Solar System with the planets orbiting the Sun. In this model the atom has a core (or nucleus) at the centre, with electrons going round it. The nucleus has a positive electrical charge and the electrons have negative electrical charges. The positive and negative charges attract each other. Each element has a different charge on the nucleus, linked to its position in the periodic table. The number of electrons in each atom can also be found from its position in the periodic table – for example hydrogen has one, oxygen has eight, and magnesium twelve.

In the Solar System, the force of gravity attracts the planets to the Sun. In an atom, it is the electrical force that attracts the electrons to the nucleus. In both our Solar System and the atom, the force gets weaker as the distance gets larger. The reactivity of an element depends, in part, on how strongly the electrons are attracted to the nucleus. The more electrons there are in an atom, the larger the atom is likely to be, and this will affect how easily the electrons on the outside of the atom can react. The more easily the electrons can react, the more reactive the metal will be.

Here is some data for Group 1 of the periodic table. In this group all the metals are reactive, and the reactivity increases as you go down the group. The atomic radius (radius of an atom) is measured in nanometres (1 nanometre (nm) = 0.000 000 001 m).

Questions

1 a What name is given to the central part of the atom?

b What electrical charge does it have?

2 a What are the particles that move around the core of the atom called?

b What electrical charge do they have?

3 Atoms are electrically neutral overall. What can we deduce from this about the size of the charge on the nucleus and the overall charge on the electrons?

4 Describe one way in which atoms are:

a similar to our Solar System.

b different from our Solar System.

5 a What is the rule for the attraction of electrical charges?

b Will an electron feel a greater force of attraction when it is close to the nucleus, or when it is further away?

6 a Plot a graph of atomic radius (vertical axis) against number of electrons in the atom (horizontal axis).

b Describe the relationship between the number of electrons and atomic radius.

c Describe the relationship between reactivity and atomic radius.

7 Two possible explanations for the reactions of metals are given below.

Which of these explanations best fits the evidence using the Solar System model of the atom?

Explain why.

8 An alternative model of the atom was called the 'fruit cake' model. This model suggested that the electrons were like individual raisins in a fruit cake and the positive charge made up the rest of the atom, like the cake.

a Compare the Solar System model with the fruit cake model. What are the similarities and differences?

b Can the evidence in the table for atomic size be explained equally well by both models, or is one better than the other? Explain your reasons, using diagrams if this is helpful.

c Can the evidence about reactivity be explained equally well using both models, or is one better than the other?Explain your reasons.

Can you...

> plot graphs to identify patterns and trends

> explain patterns in reactivity using models of atomic structure

> compare different models and select the best one to support an explanation.