atomic structure (p4)

Question 1

what is the radius of an atom compared to the nucleus?

Answer 1

atom: 1 x 10^-10m
nucleus: less than 1/10,000th the radius of the atom.

Question 2

how can electromagnetic radiation affect electrons?

Answer 2

by absorbing electromagnetic radiation, electrons can move up an electron shell, to a higher energy level. it can then release the electromagnetic radiation back out, and then descend back to its original electron shell.

Question 3

describe ionisation:

Answer 3

ionisation is where ionising radiation is so strong that it’s able to knock one of the outermost electrons off the atom. this leaves more protons than electrons, making it a positive ion.

Question 4

describe the development of the model of the atom:

Answer 4

• democritus thought that everything was made up of small molecules
• john dalton believed everything was made up of small, solid spheres, and that different spheres made up different elements
• j.j. thompson believed that atoms where general balls of positive charge, with small, discrete spheres of negative charge inside (plum pudding model)
• ernest rutherford created the nuclear model, showing a nucleus of positive charge, surrounded by a cloud of negative charge (this would collapse in on itself)
• niels bohr discovered that electrons orbited the nucleus on electron shells
• james chadwick discovered neutrons in the nucleus

Question 5

describe radioactive decay:

Answer 5

when an isotope’s nucleus is unstable, it can emit radiation. this radioactive decay is totally random - the activity is the rate at which it decays (measured in becquerels Bq).
- 1 Bq = 1 decay per second

Question 6

how can we measure the radioactive decay at a source?

Answer 6

we can do this by using a geiger-muller tube - the count-rate is the number of decays recorded each second by the detector

Question 7

what is the difference between activity and the count-rate?

Answer 7

activity is the rate at which the unstable nucleus decays, whereas the count-rate is the rate at which radioactive emissions are detected.

Question 8

describe alpha radiation:

Answer 8

• the same as a helium nucleus (2 neutrons, 2 protons).
• large, travel 5cm through the air before they stop.
• easy to stop (single sheet of paper)
• very strongly ionising

Question 9

describe beta radiation:

Answer 9

• an electron which is ejected from the nucleus at a very high speed (formed inside the nucleus when a neutron changes into a proton and an electron).
• travel 15cm in air before stopping.
• stopped by a few mm of aluminium
• quite strongly ionising

Question 10

describe gamma radiation:

Answer 10

• a type of electromagnetic radiation from the nucleus.
• travels several metres in air before stopping.
• stopped by several cm of lead
• weakly ionising

Question 11

what is the half-life of an isotope?

Answer 11

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

Question 12

what is irradiation?

Answer 12

exposing an object to nuclear radiation (e.g. alpha, beta, gamma). some medical equipment is sterilised using gamma radiation. this does not make the object radioactive, as it only comes in contact with the radiation, not the radioactive isotope

Question 13

what can ionising radiation cause in humans?

Answer 13

it can cause cancer.

Question 14

what is radioactive contamination?

Answer 14

this is where unwanted radioactive isotopes end up on other materials. this is hazardous, as the radioactive atoms decay and release ionising radiation.

Question 15

what are the different levels of danger when a person comes into contact with different types of radiation?

Answer 15

alpha radiation: strongly ionising, but easily stopped by dead skin cells. dangerous if inhaled or swallowed. can damage cells badly once inside.

beta radiation: quite ionising and can penetrate the skin and into the body.

gamma radiation: weakly ionising. can penetrate the body but likely to pass right through it.

Question 16

what are two natural sources of background radiation?

Answer 16

1. radioactive rock (e.g. granite). in cornwall, for example, this is a major source of background radiation.
2. cosmic rays from space (very high energy particles which travel through space and crash into the Earth’s atmosphere). this can be created by a supernova.

Question 17

what are two human sources of background radiation?

Answer 17

1. fall out from nuclear weapons testing - has released radioactive isotopes into the environment for decades.
2. nuclear accidents - radioactive isotopes are released by nuclear accidents, which takes decades to clear up.

Question 18

what are some uses of nuclear radiation?

Answer 18

• radiotherapy, for the treatment of cancer. at high doses, radiation kills cancer cells, or slows their growth by damaging their DNA. these can then be removed by the body.
• tracers (an alternative to surgery). can be injected into the body, and its path can be traced by a radiographer. beta and gamma radiation is used, as they can pass out of the skin, and be detected.
• smoke detectors. emit a constant stream of alpha particles. a sensor detected this stream of alpha particles, creating a small current. if smoke comes between the source and the detector, the current is decreased, and an alarm goes off.

Question 19

what is nuclear fission?

Answer 19

• splitting up of large and unstable nuclei into smaller nuclei, which releases lots of energy.
• either spontaneous (rare), or by absorbing a neutron which splits the nucleus.
• when the nucleus splits, it forms two smaller, daughter nuclei, along with two or three neutrons, energy, and gamma radiation.
• these neutrons can then be absorbed by more nuclei, triggering fission again.
• chain reaction. this can be stopped by control rods, which absorb the neutrons released by the split nuclei, and slow down the reaction.

Question 20

give examples of when nuclear fission is controlled, and when it’s not controlled:

Answer 20

• a controlled chain reaction is used to release energy in a nuclear reactor.
• the explosion in a nuclear weapon is caused by an uncontrolled fission chain reaction.

Question 21

what is nuclear fusion?

Answer 21

• when two smaller, lighter nuclei (e.g. hydrogen) fuse together to make a larger, heavier nucleus.
• however, some of the mass of the nuclei can be converted into a massive amount of energy, which is released as radiation.
• nuclear fusion is NOT a chain reaction.
• releases lots of energy (creates stars).