Atomic Structure and Isotopes
This topic covers the fundamental structure of atoms, which are the building blocks of matter. A solid grasp of protons, neutrons, electrons, and how they are described using nuclide notation is essential for understanding all subsequent nuclear physics concepts, including radioactivity and nuclear reactions.
Part of the ESAT Physics syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The atom consists of a tiny, dense, positively-charged nucleus containing protons and neutrons, surrounded by a cloud of negatively-charged electrons.
- Most of an atom's mass is concentrated in the nucleus, but most of its volume is empty space defined by the electron orbits.
- Protons have a relative charge of +1 and relative mass of 1. Neutrons have a charge of 0 and relative mass of 1. Electrons have a charge of -1 and a negligible relative mass (approx. 1/2000).
- In any neutral atom, the number of electrons is equal to the number of protons, resulting in an overall charge of zero.
- The strong nuclear force is responsible for holding the protons and neutrons together in the nucleus, overcoming the electrostatic repulsion between the positively charged protons.
Diagram
› Why does this happen?
Why do we think atoms are mostly empty space?
This comes from the alpha particle scattering experiment (also known as the Rutherford gold foil experiment). Scientists fired a beam of tiny, positively charged alpha particles at a very thin sheet of gold foil. Most of the particles went straight through without changing direction, which could only happen if the atoms were mostly empty space. However, a small number were deflected, and a tiny fraction even bounced straight back. This showed that there must be a very small, dense, positively charged centre to the atom – the nucleus – which repelled the positive alpha particles when they came too close.
Why doesn't the nucleus fly apart?
The nucleus contains positively charged protons packed very closely together. You know that like charges repel, so there is a huge electrostatic force pushing the protons apart. For the nucleus to be stable, there must be an even stronger force holding it together. This is called the strong nuclear force. It's an attractive force that works on both protons and neutrons, but it only acts over the tiny distances found inside a nucleus. It is strong enough to overcome the electrostatic repulsion and bind the nucleus together.
Formulae
Number of neutrons = A - Z Use this to find the number of neutrons in a nuclide when you know its mass number (A) and atomic number (Z).
Definitions
- Atomic Number (Z)
- The number of protons in an atomic nucleus. This number uniquely defines a chemical element.
- Mass Number (A)
- The total count of protons and neutrons (collectively known as nucleons) in an atomic nucleus.
- Isotopes
- Atoms of the same element (i.e., having the same number of protons) but with different numbers of neutrons. This means they have the same atomic number but different mass numbers.
- Nuclide
- A specific type of atomic nucleus, defined by its unique combination of protons and neutrons. For example, Carbon-12 and Carbon-14 are different nuclides.
- Ionisation
- The process by which a neutral atom becomes charged by either losing one or more electrons (creating a positive ion) or gaining one or more electrons (creating a negative ion).
Worked example
An ion has a mass number of 59 and contains 32 neutrons. If the ion has a charge of +2, how many protons and electrons does it contain?
- 1
Step 1:
Determine the number of protons (the atomic number, Z).
The number of protons is the mass number (A) minus the number of neutrons (N).
So, Z = A - N = 59 - 32 = 27 protons - 2
Step 2:
The number of protons defines the element and does not change when an ion is formed.
So the ion has 27 protons.
- 3
Step 3:
Determine the number of electrons in a neutral atom of this element.
In a neutral atom, the number of electrons equals the number of protons, which is 27.
- 4
Step 4:
Account for the ion's charge.
A charge of +2 means the neutral atom has lost 2 electrons.
So, the number of electrons in the ion is 27 - 2 = 25 electrons.
Answer: The ion contains 27 protons and 25 electrons.
Common mistakes
- ×Mistaking the mass number (A) for the neutron number (N). Remember, the mass number is the total of protons AND neutrons.
- ×Performing simple subtraction incorrectly under pressure when calculating the neutron number (A - Z). Always take a moment to double-check the arithmetic.
- ×Assuming the number of electrons equals the number of protons in an ion. This is only true for a neutral atom. The net charge tells you the electron deficit or surplus.
No-calculator tips
- ✓To quickly find the number of neutrons from nuclide notation like 'X-A', subtract the proton number Z from the mass number A. For example, for Cobalt-59 (Z=27), the neutron count is 59 - 27. You can do this as 59 - 20 = 39, then 39 - 7 = 32.
- ✓The mass number (A) is always greater than or equal to the atomic number (Z). If you calculate a negative number of neutrons, you have subtracted the wrong way around.
- ✓When comparing isotopes, focus only on the neutron count. The proton number must be the same by definition, so any difference in mass number is due entirely to the neutrons.