The Nuclear Model of the Atom
This topic covers the fundamental model of an atom used in chemistry. Understanding this structure is essential as it underpins all concepts of bonding, reactivity, and the periodic table.
Part of the ESAT Chemistry 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 small, dense, positively-charged nucleus at its centre.
- The nucleus contains protons and neutrons, which are collectively known as nucleons.
- Negatively-charged electrons are arranged outside the nucleus in specific, discrete energy levels, often called shells.
- An atom is mostly empty space; the diameter of the whole atom is tens of thousands of times larger than the diameter of its nucleus.
- The path of electrons is not a fixed orbit like a planet; shells represent regions of probability where an electron is likely to be found.
Diagram
Definitions
- Nucleus
- The positively charged central core of an atom, composed of protons and neutrons, containing nearly all its mass.
- Electron Shell
- A fixed energy level that electrons occupy as they move around the nucleus. Also known as a principal energy level.
- Subatomic Particles
- The constituent particles of an atom: protons, neutrons, and electrons.
Worked example
An analogy is often made between an atom and a sports stadium. If the nucleus of a hydrogen atom were scaled up to the size of a marble placed on the centre spot of a football pitch, which of the following would best represent the location of its single electron?
- 1
Recall the relative scale of the atomic nucleus and the overall atom.
The atom is mostly empty space.
- 2
The diameter of an atom is typically in the range of 20,000 to 200,000 times larger than its nucleus.
- 3
Evaluate the options based on this scale.
The football pitch represents the volume of the atom.
- 4
A marble on the centre spot represents the nucleus.
The electron would be found somewhere within the volume of the stadium, very far from the centre.
- 5
Therefore, the electron would be orbiting at a distance equivalent to the outer stands or seating area, not close to the centre spot.
Answer: The electron would be found somewhere in the highest tier of the stadium's seating.
Common mistakes
- ×Underestimating the scale: A common mistake is to think the nucleus is only slightly smaller than the atom, rather than orders of magnitude smaller. This relates to 'off-by-factor' errors in conceptual questions.
- ×Confusing particle locations: Incorrectly placing electrons inside the nucleus or protons/neutrons in the shells.
- ×Visualising shells as solid paths: Thinking of electron shells as fixed, hard tracks like train lines, instead of regions of probability corresponding to energy levels.
No-calculator tips
- ✓Use analogies for scale: To remember that an atom is mostly empty space, use the 'pea in a stadium' or 'marble on a football pitch' analogy. The nucleus is the pea/marble, and the whole stadium is the atom.
- ✓Focus on keywords: For descriptive questions, associate keywords: 'nucleus' with 'central, dense, positive', and 'electrons' with 'shells, energy levels, surrounding'.