atomic structure Flashcards
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
they may change with:
- 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)
describe the amount of sub-atomic particles in an atom and the charge
the number of electrons is equal to the number of protons in the nucleus; atoms therefore have no overall electric charge
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 knowledge was before the discovery of the electron
atoms were thought to be tiny spheres that could not be divided
what did the discovery of the electron lead to
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
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) and that the nucleus was charged. this nuclear model replaced the plum pudding model
what did Niels Bohr do
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 do
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
what is radioactive decay (one word)
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
alternate definition of a half life
the half-life is the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half its initial level
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
hazard 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
types 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
precautions that people 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
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
hazard 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 he 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
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
hazard 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
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
what does background radiation come from
- natural sources such as rocks and cosmic rays from space
- man-made resources such as the fallout from nuclear weapons testing and nuclear accidents
what does the level of background radiation and radiation dose depend on
they may be affected by occupation and/or location
unit for radiation dose
sieverts (Sv)
1 sievert =
1000 millisieverts (mSv)
radioactive isotopes have a very [ ] range of half-life values
wide
why is half life used
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
differences between irradiation and contamination
- 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
why do hazards associated with radioactive material differ according to the half-life; 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; 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
define nuclear fission
the splitting of a large and unstable nucleus (e.g. uranium or plutonium)
spontaneous fusion is [ ] (one word)
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; describe this
the neutrons may go on to start a chain reaction; the chain reaction is controlled in a nuclear reactor to control the energy released. the explosion caused by a nuclear weapon is caused by an uncontrolled chain reaction
define nuclear fusion
the joining of two light nuclei to form a heavier nucleus
what can happen in nuclear fusion
some of the mass may be converted into the energy of radiation
