Topic 4: Atomic Structure

Question 1

Describe John Dalton’s atomic model

Answer 1

In 1804, John Dalton agreed with Democritus that matter was made up of tiny spheres (“atoms”) that couldn’t be broken up, but he reckoned that each element was made up of a different type of “atom”.

Question 2

Describe J.J. Thompson’s atomic model

Answer 2

Nearly 100 yrs after Dalton, Thompson discovered that particles called electrons could be removed from atoms. So Dalton’s theory was not quite right. He also suggested that atoms were spheres of positive charge with negative electrons stuck in them - The Plum Pudding Model

Question 3

What experiment did Rutherford do in 1909 and what conclusions did he come to?

Answer 3

He tried firing a beam of alpha particles at a thin gold foil - the alpha scattering experiment.

From the plum pudding model, they expected the particles to pass straight through the gold, or only be slightly deflected.
Although most did go straight through, some were deflected at steeper angles than expected and a few were sent back the way they came - something the plum pudding model couldn’t explain.

Because a few were deflected back, they realised that most of the mass of an atom is concentrated at the centre in a tiny nucleus.
This nucleus must have a positive charge since it repelled some of the alpha particles
They also realised that most of an atom is just empty space because nearly all of the particles passed straight through.

Question 4

What did Neils Bohr state about an atom?

Answer 4

He stated that electrons orbiting the nucleus do so at certain distances called energy levels. His theoretical calculations agreed with the experimental data.

Question 5

What did James Chadwick prove in 1932?

Answer 5

He proved the existence of the neutron, which explained the imbalance between the atomic and mass numbers.

Question 6

The radius of an atom

Answer 6

1 x 10^-10

Question 7

The radius of a nucleus (atomic)

Answer 7

1 x 10^-14, less than 1/10,000 of the radius of an atom

Question 8

the number of protons = …?

Answer 8

The number of electrons

Question 9

What are isotopes?

Answer 9

Isotopes of an element are atoms with the same number of protons but a different number of neutrons.

Question 10

Define radioactive decay

Answer 10

Radioactive decay is when unstable isotopes decay into other elements and give out radiation as they try to become more stable.

Question 11

The mass number of an atom = …?

Answer 11

The number of protons + number of neutrons.

Question 12

Alpha Particle Radiation:

Answer 12

x2 Protons x2 Neutrons
\+2 Charge
Up to 10cm in air
Stopped by paper
Strongly ionising

Question 13

Beta particle radiation:

Answer 13

Consists of fast-moving electrons released by the nucleus 
-1 charge
Around 1m in air
Stopped by 5+mm of aluminium
Weakly ionising

Question 14

Gamma Ray Radiation

Answer 14

Consists of electromagnetic waves
No charge
Can travel km's in air
Stopped by about 60cm of lead or a couple of metres of concrete.
Very weakly ionising

Question 15

The form of nuclear equations

Answer 15

atom before decay —-> atom after decay + radiation emmitted

Question 16

The golden rule for nuclear equations:

Answer 16

the total mass and atomic numbers MUST be EQUAL on both sides

Question 17

Alpha decay:

Answer 17

Large unstable nuclei can emit an alpha particle
Four particles are removed from the nucleus so THE MASS NUMBER FALLS BY 4
x2 protons are removed from the nucleus so THE ATOMIC NUMBER DROPS BY TWO
As the atomic number has fallen by 2, A NEW ELEMENT HAS FORMED.

Question 18

Beta decay:

Answer 18

Unstable nuclei can also emit a beta particle
An energetic electron formed when A NEUTRON CHANGED INTO A PROTON
Meaning there is NO CHANGE to the total number of particles in the nucleus (The atomic mass is unchanged)
As a neutron changes into a proton, THE ATOMIC NUMBER INCREASES BY ONE
As the atomic number has increased by 1, A NEW ELEMENT HAS FORMED.

Question 19

Gamma decay:

Answer 19

Unstable nuclei can emit a gamma ray
Due to the protons and neutrons getting rid of some energy as the nucleus becomes more stable.
There is NO CHANGE to the makeup of the nucleus - the number of protons and electrons stays the same.

Question 20

IN SHORT: Alpha decay

Answer 20

Larger No. (Mass no.) = -4

Atomic number = -2

Question 21

IN SHORT: Beta decay

Answer 21

Larger No. (Mass no.) = NO CHANGE

Atomic number = +1

Question 22

Define Half-Life

Answer 22

Half-Life = The time it takes for the number of radioactive nuclei in an isotope to decrease by half

Question 23

For individual atoms…?

Answer 23

radioactive decay is a random process

Question 24

For a large number of atoms…?

Answer 24

radioactive decay is predictable

Question 25

Where does background radiation come from?

Answer 25

Radioactivity of naturally occurring unstable isotopes which are all around us - in the air, food, building materials and in the rocks (Radium)
Radiation from space, known as cosmic rays. These mostly come from the Sun. The Earth’s atmosphere protects us from most of this radiation.
Radiation due to human activity eg: fallout from nuclear explosions or nuclear waste. But this is a tiny proportion

Question 26

The radiation dose tells you…?

Answer 26

The risk of harm to body tissues due to exposure to radiation

Question 27

What is the radiation dosage measured in?

Answer 27

Sieverts (Sv) although background radiation is usually measured in millisieverts (mSv) (1Sv = 1000mSv)

Question 28

The two types of radiation are:

Answer 28

Electromagnetic and nuclear

Question 29

Define Irradiation

Answer 29

Exposing an object to nuclear radiation, the object DOES NOT become radioactive.

Question 30

Define Contamination

Answer 30

Unwanted materials containing radioactive atoms on or in other materials.

Question 31

Outside the body…?

Answer 31

Beta and gamma sources are the most dangerous. This is because beta and gamma can penetrate the body.
Alpha is less dangerous because it cannot penetrate the skin and is easily blocked by the small air gap.
High levels of irradiation from all sources is dangerous, but especially from beta and gamma sources

Question 32

Inside the body…?

Answer 32

Alpha sources are the most dangerous because they do all their damage in a very localised area. So contamination, rather than irradiation, is the major concern when working with alpha sources.
Beta sources are less damaging as radiation is absorbed over a wider area, and some pass out of the body altogether. Gamma sources are the least dangerous inside the body, as they mostly pass straight out - and have the lowest ionising power.

Question 33

What are the risks of using radiation?

Answer 33

Radiation can enter living cells and ionise atoms and molecules within them. This can lead to tissue damage

Lower doses tend to cause minor damage without killing the cells. This can give rise to mutant cells which divide uncontrollably. Cancer

Higher doses tend to kill cells completely, causing radiation sickness (leading to vomiting, tiredness and hair loss) if a lot of cells are hit at once.

Question 34

Describe uses of radioactive tracers in medicine

Answer 34

Certain radioactive isotopes can be injected into people (or swallowed) and their progress around the body can be followed using an external detector, converting the reading to a display showing where the strongest reading is coming from.

Iodine-123 is absorbed by the thyroid gland just like normal iodine-127, but it gives out radiation which can be detected to indicate whether the thyroid gland is taking in radiation.

Question 35

Isotopes taken into the body are usually…?

Answer 35

GAMMA (never alpha), so that the radiation passes out of the body without causing much ionisation. They should also have a short half-life, so the radioactivity within the patient quickly disappears.

Question 36

How can cancer be treated with radiation?

Answer 36

Through radiotherapy
Since high doses of ionising radiation will kill all living cells, it can be used to treat cancers.
Gamma rays are directed carefully at just the right dosage to kill the cancer cells without damaging too many normal cells. (In a machine called a gamma knife)
Radiation-emitting implants (usually beta-emitters) can also be put next to tumours.

Question 37

What is Nuclear Fission

Answer 37

A type of nuclear reaction that’s used to release energy from large and unstable nuclei (e.g. uranium or plutonium) by splitting them into smaller nuclei.

Question 38

How does nuclear fission work?

Answer 38

A neutron is fired at Uranium-235 (Plutonium-239 could also be used), causing it to become unstable and split into two daughter nuclei as well as releasing energy.

More neutrons are also released and this becomes a chain reaction as they go onto hit more Uranium atoms and then they release more neutrons and so on…

Question 39

If left uncontrolled, what happens to a nuclear fission reaction?

Answer 39

BOOM

Question 40

How does a nuclear fission power station work?

Answer 40

A nuclear fission power station works by using fuel rods made out of uranium, to initiate a chain reaction of neutrons and uranium, to produce two daughter nuclei as well as energy. This energy goes from internal energy to thermal energy that’s used to heat water and the steam produced is used to power a generator.

If this reaction is left uncontrolled, the power plant would go into meltdown. That’s why there are control rods, made out of boron, which can be moved up and down to control the flow of neutrons.

Question 41

Describe nuclear fusion

Answer 41

It’s the opposite of nuclear fission.
In nuclear fusion, two light nuclei collide at high speed and fuse together to create a larger, heavier nucleus.
E.g. two isotopes of hydrogen (deuterium (Hydrogen
+ 1 neutron) and tritium (Hydrogen + 2 neutrons)) can fuse to create a helium nucleus + a neutron and releasing energy
The heavier nucleus produced by fusion does not have as much mass as the two separate light nuclei did. Some of the mass of the lighter nuclei is converted into energy which is then released as radiation.

However, fusion is still in research stages as and is very costly to produce.

Question 42

Advantages of nuclear power

Answer 42

1kg of uranium can release the same amount of energy as 3000kg of coal
Power stations release no CO2
Accidents are extremely rare

Question 43

Disadvantages of nuclear power

Answer 43

Uranium is a fuel which will run out one day
Nuclear waste must be carefully stored for thousands of years
Accidents have severe environmental impacts
Decommissioning nuclear power plants are expensive.