Additional Physics Topic 12 - Atomic Structure



The Basic Atom

All matter is made up of atoms.  Although we can go smaller than atoms, we will do that later.  The basic atom consists of a nucleus surrounded by electrons going round the nucleus in orbit.  Electrons are negatively charged.  Here is a Lithium atom:



The nucleus consists of:

The protons and neutrons are the nucleons.


The protons and neutrons have very nearly the same relative mass.  The neutron has slightly more mass than the proton, but at this level we are going to say that the relative mass of both the proton and the neutron is 1.  The mass of a proton or neutron in kilograms is about 1.6 10-27 kg.


The mass of an electron is about 1/1800 the mass of a proton.  The mass of an electron is about 9.1 10-31 kg.


The charges are as follows:





+ 1 e




- 1 e


The symbol e is often called the electronic charge.  Its value is 1.6 10-19 C.




Isotope Notation

Elements are often written like this:

A is the total number of nucleons.

Z is the total number of protons.


For lithium, the isotope notation is:


To work out the number of neutrons we take away the number of protons from the number of nucleons:


No of neutrons = A - Z


If the number of electrons is the same as the number of protons, the atom carries zero overall charge.  It is described as neutral.


Question 1

How many protons, neutrons, and electrons are there in this atom?



The nucleus is very tiny, about 1/10 000 the size of an atom.  It is the equivalent to the size of a pea on the floor of your school dining hall.


If we change the number of electrons, the atom is charged.  It becomes an ion:

Ions are NEVER made by adding or taking away protons.




Look at this atom:



It has six protons, six neutrons and six electrons.  It is an atom of carbon.


Now look at this atom:


Question 2

What is the same and what is different between this atom and the last atom?



Isotopes have the same number of protons, but different numbers of neutrons.  If we change the number of protons, we change the element completely.


Isotopes have the same chemical properties as the normal element.


Question 3

Now answer the interactive gap-fill exercise.




Models of the Atom

When the atom was first discovered, physicists gave a lot of thought as to what the atom consisted of. J J Thompson proposed the plum pudding model, in which electrons were placed like cherries in a matrix of positive charge. The neutron had not yet been discovered.



Most people thought that the atom was like this. Nobody had any reason to believe otherwise until Ernest Rutherford, a New Zealand physicist, proved otherwise in the early 1900s.


Rutherford bombarded a thin layer of gold atoms with alpha particles. He was using the alpha particles like bullets, expecting to see the atoms burst like watermelons. He described his experiment as firing artillery shells at tissue paper. To his surprise he found that:

This is what Rutherford found:


This alpha scattering showed some amazing facts about the nucleus:

The alpha particles are NOT refracted.


This discovery led to the idea of the nuclear atom. This was developed further by Neils Bohr, a Danish physicist (and goalkeeper of the Danish Olympic football team). It is the model of the atom shown at the start of this topic. The neutron was discovered twenty years later by an English physicist, Chadwick.


Since the nucleus is so small, the size of an atom is governed by the size of the electron shells. Therefore big atoms and small atoms are all roughly the same size, about 10-10 m in diameter.



The Strong Force (Extension Only)

If we have all the protons packed into one tiny space, there is an immense force of repulsion, since two positives will repel. A calculation will show that the repulsive force is about 30 N, a massive force on something so tiny. Therefore there must be something that stops the nucleus flying apart. The protons and neutrons are held together by the strong nuclear force:






  • Atoms have a nucleus surrounded by orbiting electrons;

  • Nuclei consist of protons and neutrons;

  • Isotopes have the same number of protons but different numbers of neutrons;

  • If the proton number changes, so does the element;

  • Some isotopes have unstable nuclei and are radioactive;

  • There are three kinds of radioactive emissions, alpha, beta, gamma.

  • Each of these has different properties.

  • Radioactive isotopes have half-lives;

  • Radioactive decay is exponential.