“Are all atoms the same size?” “Are they all round or can you have wibbly-wobbly ones?” ”Does gravity make them round?” These questions came up in a fascinating discussion, after one of the participants had been looking at a diagram of an atom with “a small nucleus at the centre and particles flying round it”. She had expected there to be more diversity of shapes.
All atoms have fundamentally similar architecture but with vitally important differences within this – rather like human beings. Our scientific concept of what atoms are like is based on a huge variety of different experiments conducted over the past couple of hundred years. Each aimed to test one or other of the various properties of the atom – its size, its strength, its electrical charge, and so on. The results of these experiments surprised scientists by failing to converge on a single, unique representation of what an atom is. Instead scientists have been forced to concede there is no single, all-purpose way to describe the atom. They have to content themselves with a number of different models, each of which explains one aspect or another of the atom’s properties: there is no single reality that provides a complete description.
Atoms are normally too small to be imaged, except in some special cases where their outer shape can be discerned.
Here is an image of silicon atoms made using an ultra-powerful kind of microscope (‘scanning tunnelling microscope’). Each spot corresponds to one atom and is roughly 0.1 nanometres across. That means it would take 10 million of them in a row to fill a millimetre.
Experiments first carried out at the end of the 19th century revealed that atoms are composed of electrically charged particles. At the time it was assumed these would comprise a number of negatively charged particles embedded in a positively charged matrix – like plums in a plum pudding. But in a brilliant set of experiments, early in the 20th century, it was discovered that almost all the space occupied by an atom was in fact empty, a vacuum, and the positive charge was not spread out but confined to a tiny core at the centre. This was dubbed the nucleus, after the Latin for the kernel of a nut. The negative particles (named electrons) existed in some kind of cloud around this. The nucleus was itself composed of particles that were positively charged which exactly balanced the negative charge of the electrons. These were dubbed ‘protons’. Later developments suggested that the electrons were fast moving and travelled in orbits around the nucleus – a kind of ‘planetary’ model. Early developments in the quantum theory showed that the electrons lie in discrete orbits or, in three dimensions, ‘shells’, but not anywhere in between. This theory has given rise to the model commonly taught at school.
Most of the interior of an atom is empty, but it has a very hard positively charged core and highly energetic negative charge around it. This diagram is not to scale. The size of the nucleus is as a speck of dust to the Albert Hall.
Decades later, neutral particles were found inside the nucleus, in addition to the protons. They were dubbed neutrons.
This model helps us explain differences between each element. Each has a unique number of charged particles. The one depicted above is lithium, which has three protons and an equal number of electrons to balance out the charge. Nitrogen has seven each of these, and oxygen eight each. In fact, this is what gives us the Periodic Table of the Elements. As you move along it from left to right each element has one more proton and electron (see the end of this piece). The number of neutrons is close to the number of protons in each case (at least for the smaller atoms), but not identical with it.
As the quantum theory developed during the 20th century, it became clear that particles cannot be located precisely – there is an inherent uncertainty in their position. As a result, a more sophisticated model of the atom was built up, in which the chances of an electron being at a place was depicted, rather than an exact position. This gives a fuzzy image of the electrons as a smeared out ‘cloud’ rather than a set of points in precise orbits. This model indicates the density of the negative charge at any given point
Both these two models show that the negative charge (i.e. the totality of electrons) is arranged in discrete zones with gaps in between – a key feature of the quantum theory.
In more recent times, a third model has emerged which dispenses with any attempt at describing the position of particles inside an atom and simply assigns them to discrete levels of energy, in accordance with the quantum theory. As an example, the diagram shows (schematically) the 36 electrons of the element Krypton distributed across the various energy levels available. In reality the levels are not evenly spaced.
To round off this piece, we return to the questions posed at the beginning, about size and shape. If you take lots of atoms of one element, say gold, they are indeed all the same size. But if you take atoms of different elements they differ in size, quite significantly so, as you can see in this diagram.
The atoms get smaller as you progress rightwards along a row. This is because the number of charged particles in the atom of each element is increasing. This strengthens the attraction between the negative electrons and the positive protons at the core, contracting the overall size.
They also get bigger as you progress down a column. This is due to filling up of each shell at the end of each row and a jump to a new shell of larger radius at the beginning of the next row.
This takes us into a fascinating new question of why and how shells get filled at all: that will have to await another time, another blog. There’s plenty in this one to contemplate and gradually absorb!
Appendix
©Andrew Morris 14th April 2021
Getting to Grips with Science 