7.2 Nuclear Stability And Radioactivity

Question 1

What is the nucleon number?

Answer 1

Number of protons and neutrons in nucleus.

Question 2

What is the atomic number?

Answer 2

Number of protons in nucleus.

Question 3

What is a nuclide?

Answer 3

Nucleus with specific number of protons and neutrons.

Question 4

What is an isotope?

Answer 4

Nuclei with same proton number but different nucleon number.
Have same chemical properties - same number of electrons - different physical properties.

Question 5

What is the atomic mass unit?

Answer 5

1/12 of the mass of carbon 12.

Question 6

What is the nuclear notation?

Answer 6

A 
    X    
Z
A is nucleon number        
Z is proton number 
X is chemical symbol of element. 
Approximate mass of nucleus is A x atomic mass units.

Question 7

What is activity?

Answer 7

Rate of radioactive decay in Bq.

Question 8

Why may a nucleus be unstable?

Answer 8

If nucleus contains too many protons it is unstable because electrical repulsion force overwhelms strong nuclear force. Also unstable if contains too many neutrons compared to protons.

Question 9

How can a nucleus reduce its energy and become more stable?

Answer 9

Emission of particles and energy - radioactive decay.

Question 10

What is important about radioactive decay?

Answer 10

Random process - cannot be predicted which nucleus will decay and when it will happen.
Spontaneous process - cannot be induced to happen or prevented from happening.

Question 11

What are the three types of decay?

Answer 11

Alpha - emission of helium nucleus
Beta - emission of fast electrons
Gamma - emission of energetic photons

Question 12

What is important about the energies of alpha, beta and gamma particles?

Answer 12

Energies of alpha and gamma decay are discrete.

Energies in beta decay are continuous.

Question 13

What are the emissions ionising?

Answer 13

Knock electrons off atoms they collide with creating ions and radicals.

Question 14

How ionising are alpha, beta and gamma?

Answer 14

Alpha is most ionising and least penetrating.
Gamma is most penetrating least ionising.
Beta in the middle.

Question 15

What is the radioactive law decay?

Answer 15

Rate of decay is proportional to number of nuclei present: dN/dt is proportional to N. Implies exponential decrease in activity over time.

Question 16

What is half life T1/2?

Answer 16

Time interval which activity of sample is reduced by half.

Question 17

What is the mass defect?

Answer 17

Difference between mass of nucleus and sum of masses of its individual nucleons. Mass of nucleus will be smaller than the sum of masses of nucleons.

Question 18

What is the energy corresponding to the mass defect?

Answer 18

Binding energy of nucleus - minimum energy needed to completely separate its nucleons.

Question 19

What is important about binding energy?

Answer 19

Larger the binding energy of nucleus, more stable it is.

Question 20

Why do most nucleus have the same binding energy per nucleon of 8.5MeV?

Answer 20

Consequence of short range of strong nuclear force - in larger nucleus any nucleon is surrounded by same number of nucleons as any other so requires the same amount of energy to remove it.