P4 : Atomic Structure

What is the radius of an atom?

1 × 10^-10m

Where is the mass of the atom concentrated?

In the centre where the nucleus is

What 2 particles does the nucleus contain?

Protons and neutrons

What are the 3 subatomic particles called and what is their overall charge?

• Neutrons → 0

• Protons → +1

• Electrons → -1

Where are electrons located in an atom?

On energy levels

What is the relationship between energy levels and their energy in regards to distance from the nucleus?

The further the energy levels are from the nucleus, the higher energy they have

What happens when an electron absorbs electromagnetic radiation?

It can move from a lower energy level to a higher energy level

What happens to the electron when the atom emits electromagnetic radiation?

It returns back to the lower energy level

How can electrons change energy levels?

By emitting or absorbing electromagnetic radiation

What is this number called and what does it mean?

Atomic number → number of protons

What is this number called and what does it mean?

Mass number → number of protons and neutrons

What is an isotope?

An element which has a different mass number (different number of neutrons)

What is an ion?

When an atom has an unbalanced number of electrons (has either gained or lost electrons)

What were atoms thought to be before the discovery of electrons?

Tiny spheres that could not be divided

What did the discovery of the electron lead to?

The plum pudding model

Explain the plum pudding model

A ball of positively charge with negative electrons embedded into it

Who, when and how was the plum pudding model showed to be wrong?

• Earnest Rutherford

• 1909

• Alpha particle scattering experiment

Explain the alpha particle scattering experiment (3)

• Positively charged alpha particles were thrown at an extremely thin sheet of gold

• From the plum pudding model the particles were expected to pass straight through the sheet of gold or be only slightly deflected

• However even though most of them did go through, more were deflected and some came right back

What was the conclusion of the alpha particle scattering experiment + what new model was created?

• That the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged

• The nuclear model

What did Niels Bohr suggest?

That electrons orbit the nucleus in energy levels at specific distances

What did James Chadwick prove?

The existence of neutrons in the nucleus

What is the difference with the plum pudding model and the nuclear model?

• The plum pudding model is a ball of positive charge whereas the nuclear model has a positively charged nucleus in the middle

• The plum pudding model has electrons embedded into the positively charged ball whereas the nuclear model has electrons orbiting the positively charged nucleus in energy levels

Explain radioactive decay (2)

• Some isotopes have an unstable nucleus

• To become stable the nucleus gives out radiation

What type of process is radioactive decay?

Random

What is activity in terms of radioactive decay + what is it measured in?

• Activity is the rate at which a source of unstable nuclei decays

• Activity is measured in becquerel (Bq)

What is becquerel (Bq) of activity equal to?

1 decay per second

What is the count-rate?

Number of decays recorded each second by a detector (e.g. Geiger-Muller tube).

What are the 4 types of nuclear radiation?

• Alpha (α)

• Beta (β)

• Gamma (γ)

• Neutron (n)

What is an alpha (α) particle?

A helium nucleus emitted from the nucleus

• 2 protons

• 2 neutrons

What is a beta (β) particle?

A high speed electron released from the nucleus

When is a beta (β) particle released from the nucleus?

A beta particle is formed inside the nucleus when a neutron changes into a proton and releases a electron

What is a gamma (γ) ray?

A type of electromagnetic radiation released by the nucleus

What is the range in air for Alpha (α) particles, Beta (β) particles and Gamma (γ) rays?

• Alpha (α) → only a few centimetres (lowest range in air)

• Beta (β) → a few meters (medium range in air)

• Gamma (γ) → long distances (highest range in air)

What is the penetrating power for Alpha (α) particles, Beta (β) particles and Gamma (γ) rays + what are they absorbed by?

• Alpha (α) → low ~ a single sheet of paper

• Beta (β) → medium ~ sheet of aluminium

• Gamma (γ) → high ~ thick sheets of lead or meters of concrete

What is ionising power?

When radiation collides with atoms, that can cause the atoms to lose electrons and form ions

What is the ionising power for Alpha (α) particles, Beta (β) particles and Gamma (γ) rays?

• Alpha (α) → strongly ionising

• Beta (β) → moderately ionising

• Gamma (γ) → weakly ionising

What do nuclear equation show?

Radioactive decay by using elements

What form are nuclear equations written in?

atom before decay → atom after decay + radiation emitted

When an atom emits an alpha particle what happens to its mass number and atomic number?

• Mass number decreases by 4

• Atomic number decreases by 2

What is alpha decay represented as?

A helium nucleus

What would be the nuclear equation for alpha decay of Uranium?

When an atom emits a beta particle what happens to its mass number and atomic number?

• Mass number stays the same

• Atomic number increases by 1

What is a beta particle represented as?

What would be the nuclear equation for beta decay of Carbon?

What do gamma rays get rid of + what does this mean in terms of mass number and atomic number?

• Get rid of excess energy

• Means no change is mass number and atomic number

What is half life?

The time it takes for the number of radioactive nuclei in an isotope to halve

What does it mean when a source has a short-half life? (3)

• Activity falls quickly because the nuclei are very unstable and decay quickly

• Sources with a short half life are dangerous because they emit high amounts of radiation at the start

• But they quickly become safe

What does it mean when a source has a long half-life? (3)

• Activity falls slowly because most of the nuclei don’t decay for a long time

• It releases small amounts of radiation for a long time

• This is dangerous because nearby areas are exposed to radiation of millions of years

What is irradiation?

The process of exposing an object to nuclear radiation.

Does an irradiated object become radioactive?

No

What 3 ways can reduce the effects of radiation?

• Keeping sources in a lead-lined box

• Barrier methods or being in a different room

• Using remote controlled arms when handling a source

What is contamination?

is when unwanted radioactive isotopes end up on other materials

What can contamination lead to?

Radiation which is harmful

What 2 ways can reduce the effects of contamination?

• Gloves and tongs

• Protective suits

What type of radiation is the most dangerous outside of the body and why?

• Beta and gamma

• Because beta and gamma can penetrate the body and get to delicate organs

What type of radiation is the most dangerous inside of the body and why?

• Alpha

• Because it is highly ionising

What are the risks to using radiation? (3)

• Radiation can enter living cells and ionise atoms leading to tissue damage

• Lower doses do minor damage without killing cells : however can lead to the rise of mutant cells which divide uncontrollably ; this is cancer

• Higher doses kill cells completely causing radiation sickness (vomiting, tiredness + hair loss)

What are the 2 ways nuclear radiation is used in medicine?

• Exploring internal organs

• Controlling or destroying unwanted tissue

Why would a radioactive tracer be used to explore internal organs? (2)

• To check if an organ is functioning normally (thyroid and iodine-123)

• To see if a cancer has developed (killing cancer with gamma rays)

Why are gamma rays taken into the body? (4)

• Lowly ionising → least harm body tissue

• Highly penetrating → radiation passes out of body

• Short half life → radioactivity inside the patient quickly disappears

• Can be detected by a tracer (beta radiation can be detected too)

Why do people still use tracers even though they could potentially cause cancer? (2)

• Risk of cancer via tracer is low

• Tracers detect life-threatening conditions

  (benefits outweigh the risks)

Why do people undergo radiotherapy even though there are many side risks?

• It gets rid of cancer entirely (benefits outweigh the risks)

What is background radiation?

Low level radiation around us all the time

What are the 2 types of natural of background radiation?

• Rocks (granite)

• Cosmic rays

What are the 2 types of man-made of background radiation?

• Fallout from nuclear weapon testing

• Nuclear accidents

What 2 things increase how exposed you are to background radiation + give examples?

• Location (Cornwall)

• Occupation (cabin crew + airline pilot)

What is radiation does measured in?

In sierverts (SV)

1 Sv = 1000 mSv (millisievert)

What are the 2 types of nuclear reactions?

• Nuclear fission

• Nuclear fusion

What is nuclear fission?

The splitting up of a large unstable nucleus

What does nuclear fission release + how?

Releases energy from large unstable atoms by splitting them into smaller atoms

What does the nucleus have to do before splitting + what does that lead to?

Absorb a neutron which leads it to becoming unstable

What is the product of nuclear fission? (4)

• Gamma rays

• 2 smaller nuclei

• 2 or 3 neutron

• Energy

Why is nuclear fission a chain reaction?

Because the neutrons released after the initial unstable nuclei splits leads to other nuclei to absorb a nucleus and become unstable and split leading to a chain reaction

Where is the chain reaction of nuclear fission controlled?

In a nuclear reactor

How is the chain reaction of nuclear fission controlled?

By lowering control rods into the nuclear reactor which absorb neutrons which slow down the chain reactions and keep them under controlled

What happens if the chain reaction of nuclear fission isn’t controlled?

Lead to lots of energy being release as an explosion (this is how nuclear weapons work)

What is nuclear fusion?

The joining of two light nuclei to form a heavier nucleus / 2 light nuclei collide at high speed and fuse together to create a larger heavier nucleus (a hydrogen nucleus fuses together to produce a helium nucleus)

What are the products of nuclear fusion?

• A larger heavier nucleus

• Energy

Which one releases more energy, nuclear fission or nuclear fusion?

Nuclear fission

Why haven’t scientist been able to do nuclear fission?

The temperatures and pressures needed for fusion are very high which means that fusion reactors are really hard and expensive to build