Week 2

Question 1

as you go further down the periodic table…

Answer 1

the elements will become increasingly radioactive, nucleus is too big

Question 2

Electrostatic (Coulomb) repulsion

Answer 2

The repulsion between protons acts to push these nucleons apart over a long range

Intended to make a nucleus radioactive, unstable

Question 3

The strong nuclear force

Answer 3

a short range attraction between all nucleons
Make stable

Question 4

Radioactivity 1: In nuclides with too few neutrons…

Answer 4

the electrostatic repulsion overwhelms the strong nuclear attraction

Question 5

Radioactivity 2: As the nucleus gets larger…

Answer 5

the long-range electrostatic repulsion between protons accumulates and eventually overwhelms the strong nuclear attraction.

Nuclides with M > 208 (e.g. Uranium) are unstable

Question 6

Radioactivity 3: When there are too many neutrons…

Answer 6

the nucleus is also unstable.

This is explained by a nuclear form of quantum theory, the Nuclear Shell Model

Question 7

Noble gases are very stable because…

Answer 7

they have a complete shell

Question 8

alpha decay

Answer 8

lose 2 protons and 2 neutrons

α particle is simply a helium nucleus with mass number 4 and charge 2+

Question 9

beta decay

Answer 9

1 neutron lost and 1 proton gained

β(or β−) is an electron ejected from the nucleus. One neutron is changed into a proton in this nuclear reaction to balance the charge.

Question 10

positron decay

Answer 10

1 neutron gained and 1 proton lost

β+ is a positron ejected from the nucleus. One proton is changed into a neutron in this nuclear reaction to balance the charge

Question 11

electron capture

Answer 11

1 neutron gained, protons the same

Electrons fall into lower energy states to fill the vacancy left by the captured electron. A proton combines with the electron, forming a neutron.
Mass number stays the same

Question 12

neutron emission

Answer 12

Simple emission of a neutron, which changes M but leaves Z unchanged.

Question 13

gamma emission

Answer 13

No change in M or Z is associated with γ-emission on its own.

Question 14

nuclear stability depends on…

Answer 14

size of nucleus
(there are no stable nuclei heavier than Pb w/ A=208 and Z=82)

N:Z ratio
(near to 1, but “bends” towards more neutrons per proton as the nucleus gets larger)

Question 15

bad uses of radiation

Answer 15

Radiation sickness/burns
Cancer
Weapons

Question 16

good uses of radiation

Answer 16

Cancer therapy
Medical imaging

Question 17

radiation is…

Answer 17

high energy and produced by radioactive decay and causes ionisation of matter by ejecting electron from atoms

Question 18

the ionisation of a single molecule needs

Answer 18

10 eV

Question 19

how much energy in alpha radiation? (approx)

Answer 19

5 MeV

Question 20

how much energy in beta radiation? (approx)

Answer 20

1 to 0.05 MeV

Question 21

how much energy in gamma radiation? (approx)

Answer 21

1 MeV

Question 22

Why is radiation so bad for the body?

Answer 22

because the body is 50-70% water and reactions begin with water.

Water ionised to a cation and an electron

Question 23

Damage depends on 3 things…

Answer 23

1. type of radiation
2. length of exposure
3. source of exposure

Question 24

internal exposure to radiation

Answer 24

Ingestion or inhalation. Alpha and beta are most dangerous. Most gamma radiation escapes the body

Question 25

external exposure to radiation

Answer 25

alpha and beta **can't penetrate** through air and skin. **Gamma** radiation can penetrate skin - *more dangerous*

Question 26

Sievert (Sv)

Answer 26

unit that measures **biological effect** of *radiation*

Question 27

cancer therapy

Answer 27

*Focusing* ionising radiation onto the **tumour** OR Internal administration of a **radiopharmaceutical**

Question 28

Radiation for imaging

Answer 28

Uses radiation (**gamma**) emitted from within the body to **map** the body Computer-assisted tomography can give **3D** reconstruction of the body

Question 28

Technetium-99m imaging

Answer 28

Easily incorporated into many **drugs** Easily prepared from **Mo-99** Does **not** change its *chemistry* when it decays Emits only highly-penetrating **gamma** rays, not harmful alpha/beta particles

Question 29

PET imaging

Answer 29

Uses radionuclide that emits positrons: positron emitters are proton rich Within the body, **positron** reacts with an **electron**, producing two high energy **gamma** rays, which are *detected* outside the body FDG is used to observe parts of the body that use high levels of glucose (tumours, brain etc.)

Question 30

Light (photoelectric effect)

Answer 30

Light is electromagnetic radiation that has both **wave** and **particle** (photons) nature. The amount of **energy** (quantum) in each photon is determined by its **frequency**, ν (nu) or **wavelength**, λ (lambda).

Question 31

Black-body radiation

Answer 31

Proposed that energy is **quantised**: **E = hν** (ν = frequency of oscillation, h = Planck’s constant)

Question 32

Spectroscopic lines

Answer 32

**electrons** in *discrete* orbits Thus atom **cannot** lose energy continuously, but must do do in *quantum jumps* between different orbits Light emitted by an excited atomic gas consists of discrete wavelengths, not a continuous band.

Question 33

Bohr model

Answer 33

postulated a set of **circular orbits** for electrons with specific, *discrete* **radii** and **energies** and that electrons could move in each orbit **without** radiating energy

Question 34

Energies for H

Answer 34

values between −ER (n = 1) and −ER/4 (n = 2) **cannot** be observed as n *increases*, En approaches the energy of an unbound electron, or **0**

Question 35

Problems with Bohr model

Answer 35

1. According to classical physics, revolving charged particles **radiate energy** 2. Bohr’s model could only explain the emission spectra of `single-electron` atoms. It failed to predict the spectra of multi-electron atoms. 3. Bohr could offer `no reason` why an electron should have discrete orbits or energies.

Question 36

wavelength of a matter wave

Answer 36

**λ=h/mv** m is mass v is velocity h is Planck's constant

Question 37

Planck's constant

Answer 37

**6.626** x 10^**-34** J s

Question 38

Mechanics of waves

Answer 38

**Wave**-behaviour + **restricted motion** lead automatically to *discrete* energy levels or frequencies. Thus, the matter wave concept *explains* why electrons have **discrete energy levels**. No need for discrete orbits!

Question 39

Classical vs. quantum

Answer 39

quantum - has **large scale** (goes to subatomic objects) - **wave** is particles (dual nature)