4 Inside the atom

Before going on to see how atoms can link (bond) with each other, you need to look at atoms in a little more detail. Doubtless they are not like blocks of Lego! So what are they like?

In fact, every atom has a complex internal structure. Given the extremely small size of an atom, you may find it difficult to visualise any smaller bits inside it. However, you may already be familiar with some of the effects of one of these components - electrons. It is easy to do an experiment that shows the presence of electrons and, moreover, one of their important characteristics.

You should find that the paper is attracted to the comb. The explanation for this phenomenon is that the rubbing action transfers large numbers of the tiny electrons from the atoms of your clothes onto the plastic comb and vice versa. The plastic builds up static electricity which attracts the paper because the electrons have an electrical charge. You now need to look at the electrons in more detail.

Each electron carries a minute but standard amount of negative charge. Conventionally, chemists and physicists speak of an electron as having a charge of −1. The units do not matter in this case as the '−1' is a comparative amount such as a ratio: one electron has a charge of −1, two electrons a charge of −2 and ten electrons have a charge of −10.

Most objects - combs, people or atoms - do not usually have any net charge. They are described as electrically neutral. This is not a very hard concept to accept in the light of some intuitive ideas from mathematics or, indeed, bank balances! Something can be negative, positive or have a value of zero, which means no charge at all.

These positive particles are known as protons and each one carries the same amount of charge as an electron but has the opposite sign, +1.

Looking at a few atoms will put this important idea into perspective. The simplest element possible is hydrogen. It has just one proton and one electron. As +1 and −1 together give a charge of 0, it follows that the atom is neutral. Next there is helium, the light but non-flammable gas that is used to fill balloons at fairgrounds and for celebrations (Figure 11).

Figure 11 Helium-filled balloons

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Figure 11

Figure 11 Helium-filled balloons

Each helium atom contains two protons and two electrons: together, +2 and −2 equal 0. Table 3 summarises this and gives three more examples of elements that you have already met. Note, in passing, two points: first, atoms with three, four and five protons are omitted from Table 3 for simplicity: they do exist! Indeed, the 100 different elements contain atoms with, progressively, 1 up to 100 protons.

Second, most atoms also contain electrically neutral particles called neutrons. These do not greatly affect the chemistry of elements and are not discussed further in this course. They are mentioned here in case you have already heard of them and wonder why they are omitted.

The number of protons determines the identity of each element. Thus, if an atom has six protons it must be an atom of the element carbon. If it has seven protons it must be nitrogen, and so on. As the number of protons increases so the mass of the atom increases. The number of protons in an atom also determines the number of electrons in that atom and it is the protons and electrons that give the atom its unique characteristics. Note that electrons have very little mass compared with protons.

Chemists picture an atom as comprising a central atomic nucleus, which contains the protons, with electrons moving around it. In this picture (or model) the electrons are arranged in layers, like the layers in an onion. Figure 12 shows a simple representation of this for the element carbon.

Figure 12 Chemists' simple representation of the carbon atom. The smaller dots represent electrons moving in concentric orbits around the central nucleus containing the protons

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Figure 12

Figure 12 Chemists' simple representation of the carbon atom. The smaller dots represent electrons moving in concentric orbits around the...

Question 13

Using Table 3 and the information in the last few paragraphs, label Figure 12 by making a note of the appropriate charge for each dot.

Answer

Figure 13 shows a labelled drawing of a carbon atom. The small blue dots represent electrons in layers around the central nucleus shown by a larger orange circle. There is a charge of +6 in the nucleus and 6 electrons each with a charge of −1.

Figure 13 Labelled drawing of a carbon atom

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Figure 13

Figure 13 Labelled drawing of a carbon atom

The nature of atoms is described above. However, in Nature very few atoms ever exist entirely on their own: most atoms are joined to other atoms by some kind of bonding. In this course we consider two ways in which atoms can bond together. Both ways depend on 'interactions' between the outermost layer of electrons of each of the atoms that are bonding. One way is called covalent bonding (pronounced 'co-vay-lent') and the other is called ionic bonding (pronounced 'eye-on-ic'). They are discussed in Section 5 and Section 7 respectively.