Atomic and Nuclear Physics

Question 1

3 observations and subsequent conclusions of the alpha particle experiment

Answer 1

1. Almost all alpha particles were undeflected - therefore the atom is almost entirely empty.
2. Some were measurably deflected - therefore all the positive charge seems to be concentrated in one “nucleus” (at the centre of the atom).
3. Some were deflected through very large angles - therefore almost all the mass of an atom is concentrated in this nucleus.

Question 2

Definition of the atomic mass unit

Answer 2

One twelfth the mass of a (neutral) carbon-12 atom.

Question 3

Mass defect (in words)

Answer 3

The difference between the sum of the individual nucleons that make up a nucleus and the mass of the nucleus

Question 4

Mass excess

Answer 4

The difference between the mass of the nucleus and the atomic mass number. (Can be negative and positive, depending on the binding energy per nucleon)

Question 5

Binding Energy

Answer 5

The minimum energy needed to separate a nucleus into its consituent nucleons.

Question 6

(Thermonuclear) Fusion

Answer 6

A nuclear reaction in which two light nuclei join together to form a heavier but more stable nucleus.

Question 7

(Induced) Fission

Answer 7

The splitting of a nucleus (into two large fragments and a small number of neutrons).

Question 8

Isotopes

Answer 8

Nuclei of the same element with the same number of protons but a different number of neutrons.

Question 9

Nuclide

Answer 9

A specific combination of protons and neutrons in a nucleus.

Question 10

Mass (or Nucleon) Number (NOT relative atomic mass)

Answer 10

The number of protons and neutrons in the nucleus of an atom.

Question 11

Atomic (or Proton) Number

Answer 11

The number of protons in the nucleus of an atom.

Question 12

Ionising ability (of radiation) and explanation of its order for alpha, beta and gamma radiation

Answer 12

Radiation which is more ionising is more likely to knock or pull electrons off of atoms. Alpha is the most ionising because it has a higher charge and higher mass (also, alpha particles tend to move more slowly than beta particles). Gamma rays are not charged so are the least ionising (beta in between).

Question 13

Penetrating power (of radiation) and explanation of its order for alpha, beta and gamma radiation

Answer 13

Penetrating power is the ability of ionising radiation to travel far in a certain medium before the intensity of the radiation becomes zero/negligible. This determines the range of the different radiation.
When alpha or beta radiation ionises atoms, it loses energy and so the intensity drops. Since alpha is more ionising, it is less penetrating.
Gamma radiation follows an inverse square law so its intensity also drops with distance (note that the energy of each photon does NOT decrease but the number density does). However, atoms can absorb the energy of single photons and this leads to gamma attenuation.

Question 14

Attenuation of gamma

Answer 14

The fall in intensity of gamma radiation as it passes through a certain material.

Question 15

Half-life

Answer 15

The half-life of an isotope is the mean time taken for half of the radioactive nuclei in a sample to decay.

Question 16

Spontaneous (of radioactive decay)

Answer 16

The decay of a particular nucleus is not affected by other nuclei or chemical reactions, or external conditions like pressure and temperature.

Question 17

Randomness (of radioactive decay)

Answer 17

One cannot predict when or how a specific nucleus will decay, but each nucleus has the same probability of decaying per unit time.

Question 18

The decay constant

Answer 18

The fixed probability that a nucleus will decay per unit time

Question 19

Activity (of a radioactive sample)

Answer 19

The rate at which nuclei decay, i.e. number of decays per second.