The atom
Atomic structure
An atom has a small, positively charged nucleus at its centre. Negatively charged electrons surround the nucleus in orbits.
An atom is electrically neutral because the total positive charge of its nucleus balances the total negative charge of its electrons. The charges are equal in size and opposite in sign, so the atom has no overall charge.
Ion formation
An atom contains a positively charged nucleus surrounded by negatively charged electrons. In a neutral atom, the total positive and negative charges are equal, so the atom has no net charge.
An ion is an atom with a net electric charge. An atom becomes an ion when it gains or loses one or more electrons, changing the balance between its positive and negative charges.
- Losing electrons forms a positive ion. With fewer negatively charged electrons, the atom has more positive charge than negative charge.
- Gaining electrons forms a negative ion. The added electrons give the atom more negative charge than positive charge.
Only the number of electrons changes during ion formation. The charge of the nucleus remains the same.
Alpha-particle scattering
Alpha particles are positively charged particles. When a beam of them is directed at a thin metal sheet, their paths reveal how charge and mass are arranged inside the atoms of the sheet.
Scattering observations
- Most alpha particles pass through with little or no deflection.
- A small proportion are deflected through large angles.
- A very few are scattered backwards.
What the scattering reveals
Most particles pass through because atoms are mostly empty space. The large deflections are rare because the nucleus occupies only a very small part of each atom.
The nucleus contains a concentrated positive charge. When a positive alpha particle passes close to it, the two charges repel, changing the particle’s direction. A particle approaching the nucleus particularly closely can be deflected through a large angle or even sent backwards; it does not need to hit the nucleus for this to happen.
Backward scattering also indicates that most of the atom’s mass is concentrated in the nucleus. The nucleus is sufficiently massive to turn an alpha particle back without itself being displaced greatly.