5.1.1 The atom
- Syllabus
- 0625–2026–2027
- Topic
- 5.1.1
- Level
- —
Learning objectives
5.1.1.1Structure of an atom in terms of a• Describe the structure of an atom in terms of a positively charged nucleus and negatively charged electrons in orbit around the nucleus5.1.1.2How atoms may form positive ions by• Know how atoms may form positive ions by losing electrons or form negative ions by gaining electrons5.1.1.3Alpha-particle scattering by thin• Describe how alpha-particle scattering by thin metal supports the nuclear atom: (a) tiny nucleus with mostly empty space around it (b) nucleus contains most atomic mass (c) nucleus is positively charged
Describe the nuclear model of an atom
An atom has a very small, positively charged nucleus at its centre. Negatively charged electrons orbit around the nucleus. The nucleus and electrons occupy different regions of the atom.
| Region or particle | Location | Charge |
|---|---|---|
| nucleus | tiny central region | positive |
| electrons | outside and orbiting the nucleus | negative |
| space between | most of the atom's volume | no material filling it continuously |
A neutral atom has equal total positive and negative charge. Its nucleus is still positive and its electrons are still negative; neutral describes the combined charge of the whole atom.
Diagrams are models and are not drawn to scale. The nucleus is much smaller than the atom, while the electron region accounts for the atom's overall size.
Do not place electrons inside the nucleus or describe the nucleus as neutral. The detailed proton-and-neutron composition of the nucleus is a separate next objective.
Form positive and negative ions
An ion forms when an atom gains or loses electrons. The nucleus does not gain or lose positive charge during this process.
| Electron change | Result | Why |
|---|---|---|
| atom loses one electron | 1+ ion | one more unit of positive than negative charge |
| atom loses two electrons | 2+ ion | two more units of positive than negative charge |
| atom gains one electron | 1− ion | one more unit of negative than positive charge |
| atom gains two electrons | 2− ion | two more units of negative than positive charge |
Lose negative electrons → become positive. Gain negative electrons → become negative. The sign follows the imbalance left after the electron transfer.
A magnesium atom that loses two electrons forms Mg²⁺. A chlorine atom that gains one electron forms Cl⁻.
Ordinary ion formation changes the number of electrons, not the nucleus. Removing a proton is not how a positive ion forms, and adding positive charge to the nucleus is not required.
Infer the atom from alpha scattering
In the alpha-scattering experiment, a narrow beam of positively charged alpha particles is directed at a very thin metal foil. Detectors record whether each particle passes through or changes direction.
| Observation | Conclusion about the atom | Reasoning |
|---|---|---|
| most alpha particles pass straight through | the atom is mostly empty space | most particles meet no concentrated matter or charge |
| some alpha particles are deflected | positive charge is concentrated in the nucleus | positive alpha particles are repelled by positive nuclear charge |
| only a very small fraction are deflected through large angles or backwards | the nucleus is very small | very few particles pass close enough for a strong interaction |
| a few alpha particles reverse or change direction sharply | the nucleus contains most of the atom's mass | the massive nucleus remains almost stationary while the alpha particle changes momentum |
An alpha particle passing far from a nucleus is nearly undeflected. Passing closer produces stronger repulsion and a larger bend. A near head-on approach can send it back along or near its incoming path.
Together, the results support a nuclear atom: a tiny positively charged nucleus containing most of the mass, surrounded by mostly empty space.
The scattering evidence does not by itself show that the nucleus contains protons and neutrons. Match each observation only to the size, charge, mass concentration or empty-space conclusion it supports.