Topic 2: Nuclear Chemistry

Question 1

Identify and describe 2 forces contributing to stability of a nucleus

Answer 1

• Electrostatic repulsion: long-range repulsion between protons
• Strong nuclear force: short-range attraction between nucleons

Question 2

Describe how the 2 forces mentioned in previous question influence decay of a radioactive nuclide

Answer 2

• Too few neutrons → electrostatic repulsion overwhelms
• As nucleus becomes larger with higher Z → electrostatic repulsion accumulates → more neutrons are needed to increase the nuclear force and stabilize the atom
• When Z gets too high, added neutrons can no longer stabilize the atom → electrostatic repulsion overwhelms

Question 3

Identify and describe 6 different decay mechanisms

Answer 3

• alpha decay: produces alpha particle/He nucleus with 2+
• Beta decay: ejects 1 electron, 1n –> 1p
• Positron decay: ejects 1 positron, 1p –> 1n
• Electron capture: 1p + 1e –> 1n, X-ray emitted
• Neutron emission: 1n emitted
• Gamma emission: high freq radiation ejected, A and Z unchanged

Question 4

What happens to N, Z and N/Z in 4 main mechanism of radioactive decay?

Answer 4

• alpha decay: 2n and 2p lost –> N↓, Z↓
• Beta decay: N↓, Z↑, N/Z↓
• Positron decay: N↑, Z↓, N/Z↑
• Electron capture: N↑, Z↓, N/Z↑

Question 5

What can happen to N/Z ratio in alpha decay?

Answer 5

Because neutrons and protons both go down, N/Z ratio can either increase or decrease depending on values of N and Z

Question 6

Identify 2 parameters determining the stability of a nucleus

Answer 6

• The size of a nucleus
• N/Z ratio

Question 7

Describe how N/Z ratio behaves in the zone of stability

Answer 7

Zone of stability has N/Z ratio near to 1 and increases

Bends towards more N per Z as nucleus gets larger

Question 8

State the rule for nuclear stability/radioactive decaying

Answer 8

Unstable isotopes must decay towards the zone of stability, finally falling below Bi-209

Question 9

Identify decay mechanisms suitable for nuclides out of zone of stability based on N/Z ration and explain why

Answer 9

If N/Z > zone of stability –> too many N and/or too few Z
–> N ↓ and/or Z ↑ –> Beta decay, neutron emission

If N/Z < zone of stability –> too few N and/or too many N
–> N ↑ and/or Z ↓ –> Positron decay, electron capture

Too large nucleons –> too many N and too many Z
–> N ↓ and Z ↓ –> alpha decay

Question 10

Explain why ionising radiation causes biological damage

Answer 10

When our body interacts with ionising radiation, most reactions happen with water.

Water is isonised to a cation and an electron which continue to react with more water to produce free radicals.

Free radicals, or single electrons, are very reactive and can damage DNA strands (genetic damage, cancer), cell membranes –> cells fall apart, and proteins –> without proper structure, dysfunctioned

Question 11

How does the type of radiation affect the level of radiation damage?

Answer 11

Higher ability to penetrate means there will not be much interaction, thus less damage

• Alpha: lowest energy, cannot penetrate through air and biological tissues –> high biological effectiveness
• Beta: relatively high energy, basically cannot penetrate through air and biological tissues –> higher biological effectiveness than gamma
• Gamma: same energy as Beta, able to penetrate through air and biological tissues –> lower biological effectiveness

Question 12

How does length of exposure affect the level of radiation damage?

Answer 12

• Short term (acute): radiation poisoning; high dose in short time –> acute cell damage/death –> death
• Long term (chronic): radiation-induced cancer; exposed in long term –> damage in genes/DNA –> cancer

Question 13

How does source of exposure affect the level of radiation damage?

Answer 13

• Internal exposure: inhale or ingest, a and B are most harmful
• External exposure: y radiation can penetrate skin –> more dangerous

Question 14

Name some sources of natural radiation people are exposed to

Answer 14

• Radon: naturally occuring radioactive gas –> can damage lungs (a decay) if inhaled
• K-40: B decay, present in some fruits and nuts, very low abundance
• Cosmic rays: high kinetic energy particles, mostly abosrbed high in upper atmosphere –> very small amount on the ground

Question 15

Identify and describe 2 ways of treating cancer with radiation (type of radiation used, half-life, example)

Answer 15

• External ionising radiation focused onto the tumour: y radiation so to penetrate air and skin
• Internally-administered cancer therapy: using radioactive drug or radiopharmaceuticals to target the tumour
  + use a or B emitters –> act strongly, but short-range effect –> minimize damage for neighbor cells
  + half-life: hours to days –> stay long enough to kill the tumour
  + example: iodine-131

Question 16

How does imaging by radiation work in general? The mechanism?

Answer 16

Inject a radioactive substance –> radioimaging uses radiation emitted from the body to map

Scintillation counting monitors the distribution of radiation as y rays leave the body –> localize the problem

Question 17

Identify and describe 2 types of radioimaging

Answer 17

• Gamma-ray imaging: y radiation, highly penetrating through air and skin –> camera can detect
  + half-life: hours –> only enough while imaging, decay after
  + eg: Technetium-99m
• Positron emission tomography (PET): use a radionuclide emitting positron which will react with an electron to produce high energy y rays

Question 18

What does m in Technetium-99m indicate?

Answer 18

A metastable nucleus, not completely stable or unstable, high level of energy which can decay to lower energy state and release high energy y rays

Question 19

How do matter and energy behave in quantum theory?

Answer 19

Matter and energy have both wave and particle properties

Particles are waves and waves are particles.

Question 20

Explain black-body radiation, how the result was different from the classical theory of energy is continuous and what Max Planck discovered

Answer 20

In classical theories, adding heat to a moving body causes its charges to oscillate and also emit electromagnetic radiation as continuous spectrum. That radition changes as the temperature changes.

Planck assumed that energy is quantised or quantifiable instead of continuous electromagnetic wave, so it gives of electromagnetic radiation in discrete levels, too. So, energy exists in individual units called quanta.

Question 21

Explain photoelectric effect and what Einstein proposed after such experiment

Answer 21

When light hits a metal surface, it can eject electrons from that metal but only if its frequency is higher than a threshold.

• In classical theories, light is only a wave traveling in space, so its energy is proportional to square of its amplitude. So, the results of experiment was predicted that:
  + increase amplitude –> increase light energy –> increase electrons’ kinetic energy
  + increase frequency –> increase rate at which electrons are emitted –> increased measured electric current
• Actually, the observation was totally opposite. So, Einstein proposed that light also has particle properties beside wave.
  –> light energy is proportional to its frequency
  + increase amplitude –> increase rate at which electrons are emitted –> increased measured electric current
  + increase frequency –> increase light energy –> increase electrons’ kinetic energy

Question 22

Describe properties of light and equation for amount of light energy

Answer 22

Light is electromagnetic radiation that has both wave and particle nature. Light is a stream of particles called photon.
Amount of energy in each photon is

E = hv = h*c/λ

h: Planck’s constant
v: frequency
λ: wavelength
c: speed of light

Question 23

Explain Bohr model and its effects on atomic emission spectra (spectrum of electromagnetic radiation emitted by electrons changing energy level)

Answer 23

Bohr proposed a planetary model of an atom with circular orbits of electrons surrounding the nucleus with discrete radii and energy level.

• When electron is in 1 of the orbits, it is in stationary state and has fixed energy. If it is the innermost orbit, it is the electron’s ground state.
• When adding energy, electrons abosrb that energy and can move to orbits with higher energy level and in excited state.
• When energy is removed, electrons return back to ground state emitting the corresponding amount of energy - a quantum of light/photon

–> an atom cannot lose energy continuously, but has to do so in quantum jumps between different orbits
–> light emitted by an excited atom gas, due to the atom changing energy in quantum jumps, has concrete wavelengths instead of continuous band

Question 24

What is wrong with Bohr model?

Answer 24

• Revolving charged particles radiate energy, so classical physics suggest that electrons should continually lose energy and revolve into the nucleus.
• Bohr’s model can only explain the emission spectra of single-electron atoms, not multi-electron ones.
• No reason for why an electron should have discrete orbits or energies

Question 25

What did De Brogile reason based on the photoelectric effect and what was the result equation?

Answer 25

Photoelectric effect shows that light can behave both as a wave and a particle, so matter like electrons should also have wave propoerties.

De Brogile reasoned that the momentum of a particle should be related to its wavelength just as photon and matter like electrons can have both wave and particle properties.

λ = h/p = h/mv
p: momentum
m: mass
v: velocity

Question 26

How does matter-wave concept explain why electrons have discrete energy levels?

Answer 26

Matter-wave concept suggests that with wave behaviors happening in restricted motion or space, electrons have discrete energy levels or frequencies.

Question 27

Equation calculating allowed energy of hydrogen atom

Answer 27

E(n) = -E(r)*1/n^2

E(r): Rydberg constant
n: quantum numbers

Question 28

As n/quantum number increases, energy of an electron and its average distance from the nucleus…

Answer 28

Energy of electrons approaches 0, energy of a free electron at rest
The average distance, in theory, approaches infinity

Question 29

How does light shooting at a metal surface give kinetic energy to electrons?

Answer 29

Light has energy and electrons also have energy allowing them to bind to the metal. If the frequency of light is above that of electrons –> the energy of light is enough to cancel out the binding energy of electrons –> electrons and ejected and the extra energy is given to electrons as kinetic energy.

Question 30

Explain the relation between absorption and emission of energy of electrons

Answer 30

When electrons absorb energy or photon of light, they are excited to higher energy elvels. When electrons emit energy or the corresponding amount of photon, they fall to lower energy levels.