[P4] Atoms and radiation

How much smaller is the atomic nucleus, compared to the whole atom?

10 000 times smaller

What is the approximate radius of an atom?

1 x 10⁻¹⁰ m (which is the same as 0.000 000 000 1 m)

What is the basic structure of an atom?

a positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons

What happens when electrons gain energy by absorbing electromagnetic radiation?

They move to a higher energy level (further from the nucleus)

What happens when electrons lose energy by releasing electromagnetic radiation?

They move to a lower energy level (closer to the nucleus)

How are the number of protons and electrons related in an atom?

same number of protons and electrons

How do atoms turn into positive ions?

by losing one or more electrons

What is the atomic number of an atom?

the number of protons in an atom

What is the mass number of an atom?

the total number of protons and neutrons in an atom

What are isotopes?

atoms which have the same number of protons but a different number of neutrons

𝗕𝗲𝗳𝗼𝗿𝗲 the discovery of electrons, what was the model of the atom? (2)

Atoms were tiny spheres that could not be divided

Each element was made of a different type of sphere

𝗔𝗳𝘁𝗲𝗿 the discovery of electrons, what was the model of the atom? (2)

Plum pudding model

atoms are balls of positive charge, with negative electrons embedded in it

What is the nuclear model of the atom?

The atom has a nucleus which contains most of the mass and is charged

What was Bohr's contribution to the model of the atom?

Electrons orbit the nucleus at specific distances (called energy levels)

His theoretical calculations agreed with experimental observations

What was Chadwick's contribution to the model of the atom?

He provided the evidence to show the existence of neutrons in the nucleus. This was about 20 years after the nucleus became an accepted scientific idea

Who calculated that electrons had fixed orbits?

Bohr

Who provided the evidence to show neutrons existed?

Chadwick

After the scattering experiment, how did ideas about the nucleus develop?

Experiments showed that the nucleus' positive charge was actually carried by more than one particle, which were named protons

How was the alpha scattering experiment conducted?

Scientists fired a beam of alpha particles at a thin sheet of gold foil and recorded where the alpha particles went

What were the results of the alpha scattering experiment?

The majority of the alpha particles went straight through the gold sheet, and some were deflected and a small number were deflected right back the way they came. The plum pudding model couldn't explain these observations.

In the alpha scattering experiment, a few alpha particles were deflected right back and some were deflected more than expected. What did the scientists deduce from this?

This showed that most of the atom's mass was concentrated in a central nucleus, and that the nucleus must be charged

In the alpha scattering experiment, most of the alpha particles passed straight through. What did the scientists deduce from this?

This showed that most of the atom is just empty space, rather than a solid sphere, and that the nucleus is very small relative to the size of the atom

What changes were made to the atomic model based on evidence from the alpha scattering experiment?

Most of the atom's mass was concentrated in a central nucleus, which is charged. Most of the atom is just empty space, and that the nucleus is very small relative to this space

What happens when new experimental evidence can't be explained by an existing scientific model?

The scientific model may be changed or replaced

What do unstable isotopes tend to do? (2)

They give out radiation from the nucleus as it changes to become more stable

This process is called radioactive decay

What is radioactive decay?

When the nucleus gives out radiation as it changes to become more stable

What types of radiation can unstable nuclei release? (4)

Alpha, beta, gamma, neutrons

What is alpha radiation? (2)

two neutrons and two protons

it is the same as a helium nucleus

What is the symbol for alpha?

α

What is the mass and charge of alpha particles? (2)

Relative mass: 4

Charge: +2

What is the ionising power of alpha radiation?

High

What is the range of alpha particles in air?

Low; a few centimetres

What materials stop alpha radiation?

Paper or skin (and anything thicker or denser than these materials)

What is beta radiation?

a high speed electron ejected from the nucleus as a neutron turns into a proton

What is the symbol for beta?

β

What is the mass and charge of beta particles? (2)

Relative mass: negligible

Charge: -1

What is the ionising power of beta radiation?

Moderate

What is the range of beta radiation in air?

several metres

What materials stop beta radiation?

Thin sheet of aluminium (and anything thicker or denser than this)

What is gamma radiation?

electromagnetic radiation from the nucleus

What is the symbol for gamma?

γ

What is the mass and charge of gamma rays?

Relative mass: 0. Charge: 0

What is the ionising power of gamma radiation?

Low

What is the range of gamma radiation in air?

Very high: it is not stopped by air

What materials reduce gamma radiation?

Several centimetres of lead or several metres of concrete. Gamma is not stopped completely.

What is a use of alpha radiation sources?

Smoke alarms release alpha radiation, which ionises air particles, causing a current to flow. If there is smoke in the air, alpha particles bind to the ions. This stops the current, causing the alarm to go off.

What is a use of beta radiation sources? (3)

Testing the thickness of sheets of metal

A beta source is placed on one side of the metal, and a detector on the other side

If the metal sheet is too thick, beta radiation will not meet the detector

How are beta and gamma sources used as medical tracers?

A radioactive source is injected or swallowed, and the beta or gamma emitted is followed by an external detector. This indicates whether organs are working as they should be.

Why are beta and gamma emitters used in medical tracers?

They can pass out of the body to be detected

How can radiation be measured?

With a Geiger-Muller tube and counter, which records the count-rate.

How does alpha decay change the mass and proton number?

The mass decreases by four. The number of protons decreases by two (this causes the charge to decrease by two)

What happens in the nucleus during beta decay?

A neutron in the nucleus turns into a proton and releases a fast-moving electron.

How does beta decay change the mass and proton number?

The mass does not change. The number of protons increases by one (this causes the charge to increase by one)

Why doesn't the mass of the nucleus change during beta decay?

the nucleus has lost a neutron and gained a proton - which both have the same mass

How does gamma decay change the mass and proton number?

It does 𝗻𝗼𝘁 cause the mass, proton number, or the charge of the nucleus to change

Why is there no change to the mass or proton number of an atom after gamma decay?

Gamma radiation is not a particle, it is an energy wave, and has no mass or charge.

Why are new elements formed in alpha and beta decay, but not gamma?

The number of protons changes in alpha and beta decay, but doesn't in gamma decay. Changing the proton number changes the type of element the atom is

How can alpha particles be shown in nuclear equations?

(4,2)α or (4,2)He

How are beta particles represented in nuclear equations?

(0, -1)e or (0, -1)β

How are gamma rays represented in nuclear equations?

(0,0)γ

What is the unit for the activity of a radioactive source?

Bequerel, Bq

What is the activity of a radioactive source?

the rate at which a source of unstable nuclei decays

What is the half-life of a radioactive isotope?

the time it takes for the number of nuclei of the isotope in a sample to halve, or the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half its initial level

What is contamination?

The presence of radioactive 𝘀𝗼𝘂𝗿𝗰𝗲𝘀 in/on an object.

What is irradiation?

Exposure to radiation.

What precautions are used to prevent contamination?

1) Gloves 2) Tongs 3) Protective suits

What precautions are used to prevent irradiation?

1) Keeping sources in lead-lined boxes. 2) Staying behind barriers/in a different room

What is ionisation?

When atoms gain or lose electrons; when ionising radiation causes the ionisation atoms 𝗹𝗼𝘀𝗲 electrons

What can ionisation of living cells lead to?

Mutations (which can cause cancer) or cell death

Which type of radiation is the most ionising?

Alpha

Why is contamination more dangerous than irradiation?

A contaminated object will be exposed to radiation for as long as it is contaminated, whereas, if irradiated, it stops being exposed to radiation as soon as it is no longer near the source.

What is peer review?

when research is evaluated by other scientists (who are experts in that area of science) before it is published

Why is it important that findings of studies into radiation are published?

so that people have enough information about the risks and benefits to make informed choices

Why are sources of alpha radiation more dangerous inside the body than outside the body?

Alpha radiation is strongly ionising and can cause cell mutations or cell death. Inside the body it is almost certain to be absorbed by living cells. Outside it is easily absorbed by thin barriers (e.g. skin) so is unlikely to reach living cells

What are natural sources of background radiation?

Certain rocks and cosmic radiation from space

How many millisieverts are in 1 sievert?

1000

How do we use nuclear radiation in medicine?

• exploration of internal organs • control or destruction of unwanted tissue

What happens to nuclei during nuclear fission?

A larger atomic nucleus is split into smaller nuclei

What happens to nuclei during nuclear fusion?

Two small, light nuclei join together to make one heavy nucleus

How is nuclear fission induced?

An unstable nucleus absorbs a neutron

How is nuclear fission propogated?

The neutrons emitted when a nucleus breaks down are then absorbed by other nuclei

What are the conditions required for nuclear fusion to take place?

High temperatures and pressures

Where does nuclear fusion take place naturally?

In stars

What generates more energy: nuclear fission or nuclear fusion?

Fusion

What is the role of control rods in fission reactors?

To absorb neutrons and control the speed of reactions