atomic structure & radioactivity Flashcards
What is the radius of an atom?
- 1 x 10⁻¹⁰
Describe the basic structure of an atom
- a positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons
Where is most of the mass of an atom concentrated?
- in the nucleus
What is the radius of a nucleus?
- less than 1/10000 of the radius of an atom
How are electrons arranged?
- the electrons are arranged at different distances from the nucleus (different energy levels)
How can electron arrangements change?
- the absorption of electromagnetic radiation; they move further from the nucleus (a higher energy level)
- the emission of electromagnetic radiation; they move closer to the nucleus (a lower energy level)
- if an electron gains enough energy, it can leave the atom to form a positive ion
Why do atoms have no overall electric charge?
- the number of electrons is equal to the number of protons in the nucleus
- therefore they are neutral atoms
Define atomic number
- the number of protons in an atom of an element
What do all atoms of a particular element have in common?
- they have the same number of protons
Define mass number
- the total number of protons and neutrons in an atom
Define isotopes
- atoms of the same element that have the same number of protons but a different number of neutrons
When do atoms turn into positive ions?
- if they lose one or more outer electrons
What happens when new experimental evidence of the atom is released?
- this may lead to a scientific model being changed or replaced
What did scientists believe about the atom before the discovery of the electron?
- atoms were thought to be tiny spheres that could not be divided
Who discovered the electron and what did this lead to?
- JJ Thomson - the plum pudding model of the atom
Describe the plum pudding model
- it suggested that the atom is a ball of positive charge with negative electrons embedded in it
Describe what happened during the alpha particle scattering experiment
- Most particles when straight through
- Some particles were slightly deflected
- Few particles were deflected by >90 degrees
What did the results from the alpha particle scattering experiment lead to?
- the conclusion that the mass of an atom was concentrated at the centre (nucleus), with most of the atom being empty space
- and that the nucleus was charged.
- this nuclear model replaced the plum pudding model
What was wrong with Rutherford’s nuclear model?
- the electrons in the cloud close to the nucleus would get attracted and cause the atoms to collapse
How did Niels Bohr adapt the nuclear model?
- he adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances.
- the theoretical calculations of Bohr agreed with experimental observations
What did later experiments of the atom lead to?
- the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles,
- each particle having the same amount of positive charge;
- these particles are known as protons
What did James Chadwick discover?
- he provided the evidence to show the existence of neutrons within the nucleus.
- this was about 20 years after the nucleus became an accepted scientific idea
Describe radioactive decay
- some atomic nuclei are unstable
- the nuclei gives out radiation as it changes to become more stable
- this is a random process called radioactive decay
Define activity (give its units)
- the rate at which a source of unstable nuclei decays
- measured in becquerel (Bq)
Define count-rate
- the number of decays recorded each second by a detector
- (e.g. Geiger-Muller tube)
What ways can nuclear radiation be emitted by?
- an alpha particle
- a beta particle
- a gamma ray
- a neutron
What do alpha particles consist of?
- two neutrons and two protons
- it is the same as a helium nucleus
What are beta particles?
- high speed electrons ejected from the nucleus as a neutron turns into a proton
What are gamma rays?
- electromagnetic radiation from the nucleus
Why is count rate and activity not the same thing?
- because count rate takes into account background radiation
Describe alpha particle range in air
- they are relatively large so they can only travel a few centimetres in air, before they collide with air particles and stop
Describe alpha particle penetration
- they are relatively large so have a very low penetrating power
- they can be absorbed by a single sheet of paper
Describe alpha particle ionising power
- they have a great charge and a high mass so are strongly ionising * they can easily knock electrons off atoms and form ions
How are beta particles formed?
- when a neutron is converted into a proton and an electron
- the proton remains in the nucleus and the electron is emitted
Describe beta particle range in air
- they have a very low mass (they’re electrons) so they can travel a few metres in air
Describe beta particle penetration
- they have a moderate penetration power
- they’re absorbed by a few millimetres of aluminium
Describe beta particle ionising power
- they are moderately ionising
Describe the structure of gamma rays
- they are not particles
- they’re photons, so they don’t have mass or charge
- when gamma rays are emitted from the nucleus, the nucleus remains unchanged
Describe gamma ray range in air
- due to their lack of mass
- they can travel several km in air without being absorbed
Describe gamma ray penetration
- they have a very high penetrating power
- they’re absorbed by several centimetres of lead
Describe gamma ray ionising power
- relative to alpha and beta radiation, gamma rays are weakly ionising
What are nuclear equations used for?
- they’re used to present what is produced when a radioactive nucleus decays
What does alpha decay cause?
- the mass number to decrease by 4, and the atomic number to decrease by 2;
- this causes both the mass and charge of the nucleus to decrease
What does beta decay cause?
- the mass number to remain unchanged, and the atomic number to increase by 1
- this causes the charge of the nucleus to increase, but the mass of the nucleus stays unchanged (because electrons have very little mass)
- essentially gains a proton
What does the emission of nuclear radiation perhaps cause?
- a change in the mass and/or the charge of the nucleus
What do alpha particles consist of?
- 2 protons and 2 neutrons
What do beta particles consist of?
- 1 electron
Explain the emission of a gamma ray
- because gamma rays have no charge and no mass, they do not cause the mass or the charge of the nucleus to change
- therefore, if you are given a question to represent gamma radiation, you simply draw the same element + the gamma symbol (γ) with NO change in the structure of the nucleus
Describe radioactive decay
- random
Define half life
- the half-life of a radioactive isotope is the time it takes for the number of nuclei of the isotope in a sample to halve
- or the time taken for activity or count rate of a sample to decay by half
Can it be predicted when any one nucleus will decay?
- it cannot be predicted when any one nucleus will decay,
What does half-life enable to be predicted?
- but the half-life is a constant that enables the activity of a very large number of nuclei to be predicted during decay
Describe the risk of short half-life
- the source presents less of a risk as it does not remain strongly radioactive
- this means initially it is very radioactive, but quickly dies down
- so it presents less of a long term risk
Describe the risk of long half life
- the source remains weakly radioactive for a long period of time
- e.g. Americium used in smoke alarms - emitted into air around the alarm and doesn’t reach far because alpha is weakly penetrating, if smoke reaches alarm, amount of alpha particles in surrounding air drop and causes alarm to sound
- it is suitable because it will not need to be replenished and weak activity means it won’t be harmful to anyone
Define net decline
- initial number - number of X half lives divided by the initial number
Define radioactive contamination
- the unwanted presence of materials containing radioactive atoms ending up on other materials
What are hazards in radioactive contamination affected by?
- the hazard from contamination is due to the decay of the contaminating atoms
- the type of radiation emitted affects the level of hazard
Define irradiation
- the process of exposing an object to nuclear radiation
- the irradiated object does not become radioactive (it doesn’t re-emit this radiation)
How can we kill bacteria present on equipment?
- by irradiating equipment that we want to sterilise by exposing it to gamma radiation, for example, and kill any bacteria present
hazard of ionising radiation
it can increase the risk of cancer because it can knock electrons off atoms in DNA molecules, leading to mutations that lead to uncontrolled cell division
What are the hazards of radioactive isotopes?
- people who work with radioactive isotopes are at risk from being irradiated with alpha, beta, or gamma radiation
- they must take precautions
Name the types of shielding for people who work with radioactive isotopes
- since alpha radiation has a low penetrating power, just gloves can protect against it
- beta and gamma are more penetrating, so they can be shielded against using a lead-lined apron
What precautions can people take who work with radioactive isotopes can take? Describe how they work
- shielding absorbs the radiation before it can enter the body and cause any harm
- monitoring devices measure how much radiation we have received; while this doesn’t prevent radiation from getting into out body, it tells workers when they have been exposed to too much radiation and then they can stop working
What are the hazard of radioactive contamination?
- the contaminating atoms will decay and this can be hazardous as it emits ionising radiation
- with contamination, you have the radioactive source on or in you - hence you could receive a large dose of ionising radiation, which can be dangerous
What are the hazards of alpha particles with radioactive contamination?
- being contaminated with alpha is not dangerous if it is on the skin’s surface because dead cells not the skin absorb alpha radiation as it has a low penetrating power
- if inhaled or swallowed, this protection is gone and the alpha is strongly ionising, so can cause serious damage
What are hazard of beta particles with radioactive contamination?
- beta particles are quite ionising and can penetrate into the body via the skin
- they cause less damage than alpha particles
What are the hazards of gamma rays with radioactive contamination?
- gamma rays are weakly ionising but can penetrate through the body and are likely to pass straight through
- hence gamma rays are less hazardous than alpha and beta, which can stay in the body
Explain the importance of studying effects of radiation on humans
- the findings can be published and shared with other scientists so that the findings can be checked by peer review and have their validity verified
Compare contamination with irradiation
CONTAMINATION
* lasts long period of time
* source of radiation is transferred to object
IRRADIATION
* lasts for a short period of time
* source emits radiation which reaches object
* doesn’t cause object to become radioactive
* a contaminated object will be radioactive for as long as the source is on or in it whereas irradiation doesn’t cause an object to become radioactive.
once an object is contaminated, the radiation cannot be blocked from it whereas irradiation can be blocked with suitable shielding
it can be very difficult to remove all of the contamination whereas with irradiation it stops as soon as the source is removed
Define a background radiation
- weak radiation that can be detected from natural / external sources
Name examples of background radiation
- natural sources - rocks, cosmic rays from space
- man-made resources - fallout from nuclear weapons testing + accidents, medical rates
What does the level of background radiation and radiation dose depend on?
- they may be affected by occupation and/or location
Name the unit for radiation dose
- sieverts (Sv)
What does 1 sievert =?
- 1000 millisieverts (mSv)
Radioactive isotopes have a very [ ] range of half-life values
- wide
Why is half life used by scientists?
- scientists cannot tell when particular nuclei will decay due to their random nature
- so they use statistical methods to tell when half the unstable nuclei in a sample will have decayed
Why do hazards associated with radioactive material differ according to the half-life with short half life?
- if a substance has a short half-life it, it will release its radiation much more quickly (because it takes a shorter amount of time for the amount of radioactivity to reduce by half) and this can be dangerous as it will release a lot of radiation in a short amount of time;
- however, it also means that after a while it will lose all of its radioactivity and become stable
Why do hazards associated with radioactive material differ according to the half-life with long half-life?
- if something has a longer half life, it will release less radiation per second
- however, it will remain radioactive for a longer time
How are nuclear radiations used in medicine?
- to explore internal organs
- to control or destroy unwanted tissue
How is radioactive decay used in medical tracers?
- Half-life of 6hrs
- Decays into a safe isotope that can be excreted by the body
- It is injected/swallowed and there is enough time for it flow through the body and be detected before it decays away but is only present for a short enough time to minimise any harm from radiation.
- It is a gamma emitter, so can pass through the body tissue without being
absorbed (as it is the most penetrating)
How is radioactive decay used in chemotherapy?
*Gamma emitters are used to emit gamma rays, which are directed onto certain areas of the body with cancerous cells, which absorb the energy and die, controlling the disease
* It is used to control any other unwanted tissue too
* However, as it is hard to direct accurately, surrounding healthy cells may also be irradiated, and their destruction causes unhealthy side effects
Define nuclear fission
- the splitting of a large and unstable nucleus (e.g. uranium or plutonium)
Describe spontaneous fusion
- rare
What usually needs to happen for nuclear fission to occur
- the unstable nucleus must first absorb a neutron
Describe the process of nuclear fission
- the nucleus undergoing fission splits into two smaller nuclei, roughly equal in size
- and emits two or three neutrons plus gamma rays.
- energy is released by the fission reaction - all of the fission products have kinetic energy
What can happen after nuclear fission takes place?
- the neutrons may go on to start a chain reaction
- this is when the nucleus absorbs the neutron and becomes unstable - it splits reassign another neutron and produces more energy
- the chain reaction is controlled in a nuclear reactor to control the energy released.
What must happen for a chain reaction to occur?
- a neutron needs to be absorbed initially before it splits, because it needs extra energy before it can split and release lots of energy
How is a nuclear weapon explosion caused?
- the explosion caused by a nuclear weapon is caused by an uncontrolled chain reaction, as it will increase at an exponential rate
Define nuclear fusion
- the joining of two light nuclei to form a heavier nucleus
Describe the process of nuclear fusion
- When two small nuclei fuse to form a heavier nucleus it releases a lot of energy
What can happen in nuclear fusion?
- some of the mass may be converted into the energy of radiation
Name a natural fusion reactor
- the sun
The sum of the masses of the two nuclei is ….. than the mass of the heavier nucleus
- more
Which is more efficient at producing energy, fusion or fission?
- fusion would be a more efficient way of producing energy compared to fission
Why is fusion not more commonly used to produce energy?
- however no design has been produced that could accomplish positive net energy on earth
