Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Read the notes, then try the questions on paper.
Notes
Inside the atom
| Particle | Relative charge | Relative mass | Where |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | Very small | Energy levels (shells) |
- An atom has a radius of about 1 × 10−10 m. The nucleus is less than 1/10 000 of the radius of the atom, but contains almost all the mass.
- Atoms have no overall charge: the number of electrons equals the number of protons.
- Atomic number = number of protons. Mass number = protons + neutrons.
- Isotopes of an element have the same number of protons but different numbers of neutrons.
- Atoms become positive ions if they lose one or more outer electrons.
- Electrons can move to a higher energy level by absorbing electromagnetic radiation, and move down by emitting it.
How the model of the atom developed
- Tiny spheres: atoms were thought to be tiny spheres that could not be divided.
- Plum pudding model: after the electron was discovered – a ball of positive charge with negative electrons embedded in it.
- Nuclear model: the alpha particle scattering experiment showed the mass is concentrated in a tiny, positively charged nucleus.
- Bohr: electrons orbit the nucleus at specific distances (energy levels). His calculations agreed with experimental results.
- Later experiments showed the nucleus contains smaller positive particles – protons. About 20 years after the nucleus was accepted, James Chadwick showed that neutrons exist.
Alpha scattering results: most alpha particles went straight through the gold foil (the atom is mostly empty space); some were deflected (the nucleus is positively charged); a very few bounced back (the mass is concentrated in a tiny nucleus). Models change when new experimental evidence can’t be explained by the old model.
Questions
Q1 (F) Give the relative charge and relative mass of a proton, a neutron and an electron.
Show answer
Proton: charge +1, mass 1. Neutron: charge 0, mass 1. Electron: charge −1, mass very small.
Q2 (F) Why does an atom have no overall charge?
Show answer
It has the same number of electrons (negative) as protons (positive), so the charges cancel.
Q3 (F) Carbon-14 has an atomic number of 6 and a mass number of 14. How many protons, neutrons and electrons does an atom have?
Show answer
Protons = 6; neutrons = 14 − 6 = 8; electrons = 6.
Q4 (F) What are isotopes?
Show answer
Atoms of the same element with the same number of protons but different numbers of neutrons.
Q5 (F/H) Describe the plum pudding model of the atom.
Show answer
The atom is a ball of positive charge with negative electrons embedded in it.
Q6 (F/H) In the alpha scattering experiment, give each observation and what it showed about the atom.
Show answer
Most went straight through → the atom is mostly empty space. Some were deflected → the nucleus is positively charged. A very few bounced back → the mass is concentrated in a tiny nucleus.
Q7 (F/H) Why was the plum pudding model replaced?
Show answer
The results of the alpha scattering experiment couldn’t be explained by it – the plum pudding model predicted all the alpha particles would pass through with little deflection. A new model (the nuclear model) was needed to explain the new evidence.
Q8 (F/H) What is an ion, and how does an atom become a positive ion?
Show answer
An ion is a charged atom. An atom becomes a positive ion when it loses one or more outer electrons.
Q9 (H) What happens to an electron when an atom absorbs electromagnetic radiation?
Show answer
The electron moves to a higher energy level (further from the nucleus). When it falls back to a lower level, it emits electromagnetic radiation.
Q10 (H) An atom has a radius of 1 × 10−10 m and its nucleus has a radius of 1 × 10−14 m. How many times bigger is the atom than the nucleus?
Show answer
1 × 10−10 ÷ 1 × 10−14 = 1 × 104 = 10 000 times
