## Physics-SchoolUK.com

### The UK site for KS3 and KS4 Physics

KS4 Atomic Structure:
Although the word "atom" was first used over a thousand years ago, atoms were not known in any true sense until the final years of the 19th century. Since then our knowledge of the atom has expanded enabling us to do both great and terrible things.
Atoms and Isotopes
In this first section we need to grasp some fundamentals about the atom's structure and learn that ideas can change over time as new evidence arises.

#### The Structure of an Atom

Atoms are small

Having a radius of about 1 × 10-10 metres, atoms are very small.

Try to get a feel for it.
Look at a ruler and look at its millimetre, mm.
A millimetre is 1 x 10-3 metres. It is huge compared to to 1 × 10-10 metres.

To get to the size of an atom you would have to divide the millimetre by 10, seven more times!

The world’s best optical microscopes do not come near to being able to see atoms.

So, the world of atomic physics is a very small world. I hope you are now convinced of that.

Atoms - building blocks of matter

Atoms are the basic building blocks of all that we see around us and all that we don’t see like the gases of the air etc.
Atoms combine together to make molecules.
In other words, atoms are the building blocks of all matter whether, solid, liquid or gas.

But although they were at one time thought to be the smallest things (and the Greek word “atomos” means “can not be divided”), we now know that within the atom there are smaller things, smaller particles!

The Atom - what lies within?

The basic structure of an atom is a positively charged nucleus composed of:

protons (which are positively charged, hence the “p” in the word proton)
and neutrons (which have no charge, hence the “n” in the word neutron)

surrounded by electrons (which are negatively charged; the word electron was used long before anyone knew its exact charge and comes instead from the word "electric").

One way of trying to represent the Atom in a diagram is the following though we really can't "picture" it accurately.

The Atom - small outside, tiny inside!

If we agree that the radius of an atom is small, then we will be amazed at the radius of a nucleus.
It is less than 1/10 000 of the radius of an atom!

A common way to try to comprehend the relative sizes of the atom compared to its nucleus is to visualise a sport stadium like Wembley Stadium with a garden pea at the centre of the pitch. It is said that if the stadium represents the size of the atom, then the garden pea represents the size of the nucleus!

So, the nucleus is very, very……very small.

Here are two amazing things arising from the very small nuclear radius compared to the atomic radius:

1. The electrons that surround the nucleus are almost massless and spaced far apart from each other, so most of the atom’s mass is concentrated in the nucleus. ** You must remember this fact.

2. Since the distance between the nucleus and its electrons is like that between the garden pea and the outside edge of the stadium, then most of the atom is empty space. ** You don't need to remember this fact, but its interesting isn't it?

The Atom - and its electrons

The electrons are arranged at different distances from the nucleus.
The electrons that are closer to the nucleus are held much tighter to the nucleus compared to those that are further away. (There is an electrostatic force between the negatively charged electrons and the positively charged nucleus, which increases as the distance decreases.)
In the world of physics these different distances give rise to the description of different energy levels.
So, within an atom, we say that the electrons surround the nucleus at a range of different energy levels.

The different energy levels represent the amount of energy needed to remove the electron from the atom, to separate it from its nucleus, to cause “ionization”.

Sometimes the electrons can move to a different level but not be separated from the nucleus. The electron might:
1. absorb energy in the form of electromagnetic radiation which could move it further from the nucleus; to a higher energy level.

or, 2. the electron might release energy by the emission of electromagnetic radiation causing it to move closer to the nucleus, to a lower energy level.

#### Atomic number, mass number and isotopes

Atoms and charge

In their normal state, 99% of the time, atoms have no charge.
So, in their normal state they must have an equal number of protons in the nucleus and electrons surrounding the nucleus so that their charges (positive and negative) will cancel out.

However, an atom can become charged if it either gains or loses electrons.
If the atom gains an electron it becomes known as a Negative Ion, and if it loses an electron it becomes known as a Positive Ion.
(NOTICE, there is no mention here of any gaining or losing protons. Protons can not ever be gained or lost because they are held tightly within the nucleus. We discussed this idea in the section on Static Electricity, 4.2.5)

Atoms - atomic numbers

What makes one atom different from another?
Answer: its atomic number (nothing else).

If you are told that 2 atoms have the same atomic number then they are atoms of the same element, eg they are both copper atoms.
If you are told that one atom has an atomic number of 8 whilst another has an atomic number of 9 then they are not atoms of the same element.

So, atoms are defined by their atomic number.

What do we mean by "atomic number"?
The atomic number is simply the number of protons within the nucleus of a particular atom.

Here is a table of some atoms/elements and their atomic numbers, starting with the smallest number and ending with the largest known atomic number:

Atom/Element Atomic
Number
Hydrogen 1
Helium 2
Nitrogen 7
Aluminium 13
Copper 29
Tin 50
Tungsten 74
Einsteinium 99
Oganesson 118 NB. only 5 atoms of Oganesson have ever been detected !

Now that we know about the atomic number we can use it whenever we write the symbol for an element.

To display the atomic number with the element symbol we add it to the symbol (eg for Helium, He) like this:

So, we always add the atomic number at the bottom left of the element symbol.

Atoms - mass numbers

Although it would be reasonable to look at the table of atomic numbers and conclude that the atom of Tungsten must be a much bigger atom than that of Helium, it is not actually the atomic number that we use to make comparisons about the sizes of atoms, because the atomic number only tells us about the number of protons in the atom, it tells us nothing about the other particles, particularly the number of neutrons.

It the mass number that we use in order to make comparisons about the sizes of atoms because the mass number is:
the total number of protons and neutrons in the atom.

Here are the mass numbers for the atoms/elements listed above:

Atom/Element MASS
Number
Hydrogen 1
Helium 4
Nitrogen 14
Aluminium 27
Copper 64
Tin 119
Tungsten 184
Einsteinium 252
Oganesson 294

Just as we have a way of writing the atomic number with an element symbol, we have a way of writing the mass number with an element symbol.

To display the MASS number we add it to the symbol like this:

So, we alway add the mass number at the TOP left of the element symbol and the atomic number at the bottom left.

Atoms - and isotopes.

All atoms of a particular element will have the same atomic number since they all have the exact same number of protons (we stated this above; its what defines an element).
But they do not all have to have the same number of neutrons, so they do not all have to have the same mass number.

For example, the well known element Carbon can be found in two "versions" one with 6 neutrons and one with 8 neutrons, but in both "versions" the atomic number (the number of protons) is the same; it is 6.
So in one version the mass number is 12 and in the other it is 14.

What is the proper name for these "versions" of Carbon?

Atoms of the same element, which share a common atomic number, are called isotopes.

So, Carbon-12 is one "isotope" of Carbon
and Carbon-14 is another "isotope" of Carbon.

(In case you are interested: the "iso" part of the word means "same" and the "topo" part means "place", so iso-tope was used to indicate that these are elements which "share the same place on the periodic table.)

The presence of just one or two extra neutrons can affect the behaviour of an element dramatically; research into isotopes led to the discovery of a huge number of radioactive isotopes. For example, whilst Carbon-12 is completely stable, Carbon-14 with just 2 more neutrons, is radioactive!

Time to have a go at a few basic questions.

#### The development of the model of the atom (& the scientific method)

The model of the atom that we are familiar with today is known as The Nuclear Model. We have already learnt a lot about it and we can see why its called The Nuclear Model - it considers the atom as consisting of a positively charged nucleus surrounded by one or more negatively charged electrons.

But this "model" was not the model of the atom that scientist held to back in the early years of the 20th century (the early 1900's). At that time a prominent scientist J.J Thompson (who had discovered the electron, so he was a giant amongst scientists!) proposed the idea that an atom consisted of a ball of positive charge with negative electrons embedded in it.
Such an atom might look a bit like this:

Due to its appearance it became known as The Plum Pudding Model of the atom. The electrons were like "plums" in a "pudding" of spread out positive charge.
Notice there is no nucleus in this model.

So how did the nuclear model come to replace Thomson's plum pudding model?

A brief explanation:

Another scientist, Ernest Rutherford, decided to do an experiment to investigate further this plum pudding model; his experiment became known as the alpha particle scattering experiment.
In the experiment he fired alpha particles at a very thin piece of gold foil.
Rutherford expected that most of the alpha particles would pass straight through the gold foil, but to his surprise some were deflected back along their original path.
Rutherford knew that the alpha particles were positively charged, so the only way that he could explain this significant deflection was to suggest that the atoms of gold were not in a "plum pudding" arrangement, where the positive charge was spread out, making it weak in any single area, but instead, all of the positive charge of the atom was concentrated in one place, at the centre of the atom.
Rutherford called this centre, the Nucleus.

Thus, a new model of the atom was born! One were electrons orbited a small central positively charged nucleus much like the way our planets orbit the sun.

So, Rutherford's nuclear "solar system" model of the atom replaced Thompsons "plum pudding" model.

Fundamental to all of this was the Scientific Method - that new experimental evidence may lead to a scientific model being changed or replaced.
The evidence from Rutherford's experiment was sufficient to cause Thompsons model to be changed.

The development of the model of the atom did not end there! Because if you notice, Rutherford's model makes no mention of the interior of the nucleus and is vague about the role of the electrons.

Within a few years a scientist called Neils Bohr began to look more closely at the electrons and his theoretical work, combined with experimental observations of others, revised the place of electrons in the model. Instead of orbiting the nucleus like planets, Bohr proposed that they exist in "energy levels" at a variety of distances from the nucleus. This understanding of the electron remains today.

Others began to investigate the nucleus further and through experiments they noticed that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton was given to these particles.

Twenty years after the nucleus was first accepted as a credible idea, the experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus.

Today, the scientific method is at the root of all science. It allows science to move forward, to discard old ideas, to suggest new theories when new evidence arises.
(Isn't it good to know that science is not like many other areas of life where "opinion" or "tradition" or "received ideas" matter more than fact and proof!)