IB Chemistry home > Syllabus 2016 > Atomic theory > Relative mass and charge

Syllabus ref: 2.1

Atoms differ from one another suggesting that the arrangement of sub-atomic particles is different from atom to atom. One way that they differ is in terms of mass.

 

Particle mass

The actual masses of these sub-atomic particles are very small and to the nearest whole number measured relative to the mass of a carbon-12 isotope being equal to 12 units:

Particle
location
mass/amu
Proton
nucleus
1
Neutron
nucleus
1
Electron
energy shells
5 x 10-3

Although these values suggest that the protons and neutrons are identical they do, in fact, have very slightly different masses, which is only of concern to us when considering changes in the structure in nuclear chemistry. Compared to the mass of the protons and neutrons the electrons have negligible mass and can be ignored when carrying out calculations involving mass.

Protons have a mass of 1 atomic mass unit. They are all together in the nucleus, but they cannot repel one another because of the strong nuclear force exerted by the protons and the neutrons. You could consider the neutrons to be the nuclear glue that holds the nucleus together.

A full description of the electron is rather complicated as they behave both as particles in some circumstances and waves in others. This has lead quantum physicists to calling them wavicles - things that possess the characteristics of both particles and waves.

TOK: This picture of the atom is a model to help us understand the underlying structure. It does not imply that the atom is actually identical to this description, nor does it suggest that it is a simple arrangement of particles. It's just a model to help explain observed properties.

This model allows us to use the atomic theory successfully to explain many observations in the microscopic world. Its use in modern particular science has been refined by the introduction of another model, which is rather more difficult to understand, called the quantum model.


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Electrical charges

The forces that hold atoms together (and to one another) are largely electrical in nature (apart from the strong nuclear charge that binds the particles in the nucleus together).

Electrically charged particles have an associated electrostatic field, rather like a magnet has a magnetic field. If another electrical charge comes into this field it will feel a force of either attraction or repulsion, depending on whether the charge carried is opposite or the same.

Overall, atoms are neutral, which means that they must have as many positive charges as negative charges.

Protons carry a single positive charge and the electrons carry a single negative charge, so in the neutral atom there are always the same number of protons and electrons.

The electrons are tiny in comparison to the protons and neutrons. The overall charge on an atom is zero, the charges of the electrons cancel out the positive charges of the protons in the nucleus.

Summary of fundamental particle charge and location

Particle
location
charge
Proton
nucleus
1+ (positive)
Neutron
nucleus
none
Electron
energy shells
1- (negative)

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