P5.1.1P5. Nuclear Physics
The atom — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 2 of 8 1 quiz
Lesson summary
P5.1.1 The atom — COMBINED SCIENCE notes: Describe the nuclear model of the atom, the particles in it and the evidence from the alpha-particle scattering experiment.Atom = a small, dense, positively charged nucleus with negatively charged electrons orbiting around itSize — an ato…
- Nucleus contents
- Relative charge
- Neutral atom
- Positive ion
- Negative ion
- Net charge
Section 1 of 8Structure of the atom
Structure of the atom
Atom
Atom a small, dense, positively charged nucleus with negatively charged electrons orbiting around it
Size
- Size an atom has a radius of about 1 × 10-10 m
- About one hundred million atoms fit side by side across a thumbnail.
Nucleus
- Nucleus its radius is over 10 000 times smaller than the atom, but it holds almost all of the mass
Nucleus contents
- Nucleus contents positively charged protons and neutral neutrons, so the nucleus has an overall positive charge
Terminology
- Atom = nucleus plus electrons.
- Nucleus = protons and neutrons at the centre of the atom.
Section 2 of 8Particles in the atom
| Particle | Relative charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | about 1/2000 (negligible) | Orbiting the nucleus |
Particles in the atom
Relative charge
Relative charge the charge of a particle compared with the fundamental charge, so it has no units
- The fundamental charge is the size of the charge on a proton or an electron, 1.6 × 10-19 C.
Neutral atom
- Neutral atom it has equal numbers of protons and electrons, so the charges cancel
Section 3 of 8Ions
Ion
Ion an electrically charged atom or group of atoms formed by the loss or gain of electrons
Positive ion
- Positive ion formed when an atom loses electrons, so there are more protons than electrons
Negative ion
- Negative ion formed when an atom gains electrons, so there are more electrons than protons
Net charge
- Net charge the overall charge of the atom or ion
Nucleus unchanged
- Nucleus unchanged only electrons move when ions form
Section 4 of 8Rutherford's alpha scattering experiment
1
Geiger and Marsden, directed by Rutherford in 1909, fired a beam of alpha particles (He2+ ions) at a thin gold foil.
2
The plum pudding model predicted that the particles would pass through with only small changes of direction.
3
Most alpha particles passed straight through the foil.
4
Some alpha particles were deflected as they passed through.
5
A few alpha particles bounced back off the foil.
Section 5 of 8Conclusions from the experiment
| Observation | Conclusion |
|---|---|
| Most alpha particles pass straight through | The atom is mostly empty space |
| Some are deflected | The nucleus is positively charged and repels the positive alpha particles |
| A few bounce straight back | The nucleus is tiny and contains most of the mass of the atom |
1Replacing the plum pudding model
- Replacing the plum pudding model the bounce-back could not be explained by it, so the nuclear model was created
2First evidence
- First evidence this was the first evidence for the nuclear atom
Section 6 of 8More from the specification
More from the specification
Rutherford nuclear model derivation
- Rutherford nuclear model derivation analyzing alpha particle deflection patterns provided an experimental means to discover the compact positive atomic nucleus.
Atomic Structure and Nuclide Notation
- An atom is represented in nuclide notation as AZ X,
- where atomic number Z denotes proton count and mass number A represents total protons and neutrons.
Isotopes of an Element
- Isotopes are atoms of the same chemical element having identical atomic numbers but different numbers of neutrons,
- resulting in different physical masses.
Nuclear Fission Energy Release
- Fission occurs when a heavy unstable nucleus absorbs a neutron and splits into smaller nuclei, releasing substantial kinetic and thermal energy.
Nuclear Fusion in Stars
- Nuclear fusion is the process where light nuclei fuse under extreme core temperature and pressure to form helium nuclei, releasing radiant energy.
- (such as hydrogen isotopes)
Nuclear fission
- The splitting of a large nucleus into smaller nuclei, releasing energy.
Nuclear fusion
- Joining of hydrogen nuclei, releasing energy.
Nuclide
- A group of atoms with the same number of protons and neutrons.
Element
- A substance made of only one kind of atom.
Hydrogen
- Gas that gives a squeaky pop with a lit splint.
Isotopes
- Atoms of the same element with the same number of protons but different numbers of neutrons.
Product
- A substance made in a chemical reaction.
Reactant
- A substance used up in a chemical reaction.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Atom | A small, dense, positive nucleus surrounded by orbiting negative electrons. |
| Nucleus | The central part of the atom made of protons and neutrons, holding almost all the mass. |
| Ion | An atom or group of atoms with an electric charge because it has lost or gained electrons. |
| Relative charge | The charge of a particle compared with the fundamental charge of 1.6 × 10-19 C. |
| Alpha particle | A helium nucleus, He2+, used in the scattering experiment. |
| Plum pudding model | The earlier model of the atom, replaced by the nuclear model after the alpha scattering experiment. |
Section 8 of 8Exam technique
1
Saying the nucleus is the whole atom — the atom is the nucleus plus the electrons
2
Thinking the electron has the same mass as a proton — its mass is negligible by comparison
3
Thinking ions form by gaining or losing protons — only electrons are gained or lost
4
Thinking the alpha particles that bounced back hit an electron — electrons are far too light, and the rebound is caused by the tiny dense nucleus
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