Atomic Structure

Question 1

Dalton

Answer 1

• early 1800s
• said atoms are tiny solid spheres
• believed atoms couldn’t be broken down

Question 2

J. J. Thompson

Answer 2

• 1897
• discovered the electron using cathode ray experiments
• proposed the plum pudding model

Question 3

What is the plum pudding model

Answer 3

A ball of positive charge with negative electrons scattered through it

Question 4

Rutherford

Answer 4

• 1909
• carried out the alpha scattering experiment
• proposed the nuclear model

Question 5

Alpha scattering experiment

Answer 5

• fired alpha particles at thin gold foil
• most went straight through - atom is mostly empty space
• some deflected - nucleus is small and positively charged
• a few bounced straight back - nucleus must be very dense

Question 6

Nuclear model

Answer 6

Small dense nucleus surrounded by electrons

Question 7

Bohr

Answer 7

• 1913
• suggested electrons orbit the nucleus in fixed energy levels (shells)
• this matched experimental results and explained chemical behaviour

Question 8

Chadwick

Answer 8

• 1932
• discovered the neutron, a particle in the nucleus with no charge but similar mass to a proton

Question 9

Nucleus

Answer 9

• tiny - radius of about 1 x 10^-14m
• contains protons +1 and neutrons 0
• has the most mass

Question 10

Electrons

Answer 10

• orbit the nucleus in shells
• negatively charged (-1), very small mass

Question 11

Atoms

Answer 11

• have nucleus and electrons
• mostly empty space
• they’re neutral as theres the same number of protons and electrons

Question 12

Proton relative mass and charge

Answer 12

• relative mass: 1
• charge: +1

Question 13

Neutron relative mass and charge

Answer 13

• relative mass: 1
• charge: 0

Question 14

Electron relative mass and charge

Answer 14

• relative mass: very small
• charge: -1

Question 15

What is an isotope

Answer 15

• atoms of the same element (same protons) with different numbers of neutrons
• example: carbon-12 vs carbon-14
• some isotopes are unstable - radioactive decay

Question 16

Mass number

Answer 16

Protons + neutrons

Question 17

Atomic number

Answer 17

Number of protons

Question 18

Radioactive decay

Answer 18

• random process - can’t predict when a specific nucleus will decay
• decay happens to make the nucleus more stable
• releases radiation: changes atomic/mass number

Question 19

Alpha particles

Answer 19

• 2 protons + 2 neutrons (helium nucleus)
• low penetration
• high ionising power
• blocked by paper/skin

Question 20

Beta particles

Answer 20

• high-speed electron
• neutron= proton + e-
• medium penetration
• medium ionising power
• blocked by thin aluminium

Question 21

Gamma particles

Answer 21

• EM wave
• no mass or charge
• high penetration
• low ionising power
• blocked by thick lead/concrete

Question 22

What is half-life

Answer 22

• time it takes for half of the radioactive nuclei in sample to decay
• OR for activity (decays per second) to halve
• measured in Becquerels (Bq)

Question 23

Long half life

Answer 23

Decays slowly, remains radioactive for a long time

Question 24

Short half life

Answer 24

• rapid decay, but dangerous in short term

Question 25

What is radioactive decay

Answer 25

• The random process by which an unstable atomic nucleus loses energy by emitting in radiation in order to become more stable
• it happens spontaneously - we cannot predict exactly when a particular atom will decay
• the process continues until the atom reaches a stable form

Question 26

Irradiation

Answer 26

• definition: exposure to radiation, but no material contact
• example: being near a gamma source
• protection: shielding, limit exposure

Question 27

Contamination

Answer 27

• definition: radioactive material gets on/ in you or an object
• example: swallowing/inhaling radioactive dust
• protection: gloves, suits, tongs

Question 28

What happens to irradiated objects and contaminated materials

Answer 28

• irradiated objects do not become radioactive
• contaminated materials emit radiation until cleaned/decayed

Question 29

Uses of radiation

Answer 29

• Medical tracers
• radiotherapy
• sterilisation
• smoke alarms

Question 30

Medical tracers

Answer 30

• beta or gamma emitters used
• short half life so it doesn’t stay radioactive
• used to track organ function (e.g., kidneys, thyroid)

Question 31

Radiotherapy

Answer 31

• gamma rays used to kill cancer cells
• focused to avoid healthy tissue as much as possiblee

Question 32

Sterilisation

Answer 32

• gamma rays used to kill bacteria on medical equipment or food

Question 33

Smoke alarms

Answer 33

• use alpha emitters
• alpha particles ionise air to allow current
• smoke blocks the alpha particles - current drops - alarm sounds