Unit 1.2 Basic Ideas about Atoms (pt. 3 Half-life)

Question 1

Can you predict when a particular individual atom will undergo radioactive decay?

Answer 1

No.

Radioactive decay is a random process.

It is not possible to predict when a particular individual atom will undergo radioactive decay.

But, in a sample of many billions of atoms, it is possible to predict hoe many of those will decay in a particular period of time.

Question 2

Define half-life.

Answer 2

The time taken for the activity or number of isotopes remaining to halve is constant and is known as the half-life of that isotope.

Question 3

Why is half-life useful?

Answer 3

This property of radioactive decay is useful in determining the age of materials in radioactive dating.

E.g. The proportion of carbon-14 remaining in plant or animal remains or products can be used to determine the length of time that has passed since the organism was alive to date a fossil or piece of fabric.

Similarly, the proportion of potassium-40 remaining in a rock can be used to give an estimate of the age of the rock.

Question 4

List the four main processes that take place in a mass spectrometer.

Answer 4

1. Ionisation
2. Acceleration
3. Deflection
4. Detection

Question 5

Describe the process of ionisation in a mass spectrometer.

Answer 5

A beam of electrons is fired at the vaporised sample. The electrons collide with each other and are knocked out, making the atom positively charged.

Question 6

Describe the process of acceleration in a mass spectrometer.

Answer 6

Two negatively charged plates attract the positive ions, which accelerate toward them.

Question 7

Describe the process of deflection in a mass spectrometer.

Answer 7

Magnetic field causes the path of the ions to bend as it exerts a force at right angles to their motion.

Lighter particles tend to deflect more.

Question 8

Describe the process of detection in a mass spectrometer.

Answer 8

They collide with a detector which records the position where the particle hits.