Physics 6.1 - 6.6 - Atoms and isotopes

Atomic structure and scale

An atom cannot be inspected like an everyday object, so physicists use a model: a representation that captures the features needed to explain observations. A shell or orbit drawn as a line is part of that model, not a material track inside the atom, and an ordinary diagram cannot show the nucleus and the whole atom at their true relative sizes.

The model has two very different regions:

• At the centre is a positively charged nucleus containing protons and neutrons.
• Negatively charged electrons surround the nucleus.
• The nuclear radius is much smaller than the atomic radius, yet almost all the atom's mass is in the nucleus.

The last statement is not a contradiction. Protons and neutrons are the relatively massive particles and are both in the tiny nucleus; the surrounding electrons have much less mass.

Useful GCSE estimates are:

Here $1\,\mathrm{nm}=1\times10^{-9}\,\mathrm{m}$. Individual atoms, molecules and nuclei vary. A small molecule contains a few bonded atoms, so its size is on the atomic-to-nanometre scale; it is not always exactly ten times an atom. A named molecule and a stated radius or diameter are needed for an exact comparison.

Worked scale comparison

Suppose a model uses an atomic radius of $8.0\times10^{-11}\,\mathrm{m}$ and a nuclear radius of $8.0\times10^{-15}\,\mathrm{m}$. To compare like quantities, divide the larger radius by the smaller radius:

The metres cancel, so a ratio has no unit. As a sense-check, if that atom were enlarged to a radius of $100\,\mathrm{m}$, the nucleus would have a radius of about $100/10\,000=0.010\,\mathrm{m}$, or $1\,\mathrm{cm}$. A textbook nucleus must therefore be drawn far too large to remain visible.

Particle properties and neutrality

Relative mass and charge compare particles using convenient reference values. They have no unit: a relative mass of 1 does not mean $1\,\mathrm{kg}$, and a relative charge of $+1$ does not mean $1\,\mathrm{C}$.

A positron is the electron's antiparticle: it has the same mass as an electron but the opposite charge. It is not a proton, and it should not be placed beside the electrons in a model of an ordinary atom. Its role in nuclear radiation is developed beyond this lesson.

Neutrons do not affect the total charge because their relative charge is zero. For example, an atom with eight protons and eight electrons has

It is neutral whatever its neutron number. This cancellation does not mean the proton or electron is uncharged; it means their equal and opposite total charges balance.