atomic structure (p4) Flashcards
what is the radius of an atom compared to the nucleus?
atom: 1 x 10^-10m
nucleus: less than 1/10,000th the radius of the atom.
- most of the mass of the atom is concentrated in the nucleus.
describe the basic structure of an atom:
positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons.
- electrons are arranged at different energy levels away from the nucleus.
how can electromagnetic radiation affect electrons?
- by absorbing electromagnetic radiation, electrons can move up an electron shell, to a higher energy level, as they have enough energy. the electron has become ‘excited’
- it can then release the electromagnetic radiation back out, and then descend back to its original electron shell soon after
describe the numbers of protons and neutrons in an atom:
- the number of electrons is equal to the number of protons. atoms therefore have no overall electric charge.
- all atoms of a particular element have the same number of protons: atomic number (on the bottom).
- total number of protons and neutrons in an atom is called the mass number (on the top).
what are isotopes?
- atoms of the same element that have the same number of protons but different numbers of neutrons.
- these atoms are called isotopes of that element.
describe ionisation:
ionisation is where ionising radiation is so strong that it’s able to knock one of the outermost electrons off the atom, as it has absorbed enough energy to leave the atom. this leaves more protons than electrons, making it a positive ion.
describe the development of the model of the atom:
- democritus thought that everything was made up of small molecules that couldn’t be broken down any further, and are separated by empty space (atomic theory)
- john dalton believed everything was made up of small, solid spheres, that could not be divided, 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 compact nucleus of positive charge, surrounded by a cloud of negative charge (this would collapse in on itself) - from the alpha particle scattering experiment.
- niels bohr discovered that electrons orbited the nucleus on electron shells
- later experiments led to the idea that the positive charge of the nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge: protons.
- james chadwick discovered neutrons in the nucleus. this was about 20 years after the nucleus became an accepted scientific idea.
why would Ernest Rutherford’s model collapse in on itself?
the surrounding negative electrons would be attracted to the positive nucleus, causing them to rush inwards
describe the alpha particle scattering experiment in more detail:
- scientists took a piece of gold foil (can be hammered until it’s just a few atoms thick)
- fired tiny alpha particles at the gold foil.
- alpha particles have a positive charge
- most of the alpha particles passed straight through the gold foil, without changing direction.
- sometimes, an alpha particle was deflected (changed direction), as it passed through, and sometimes, an alpha particle would simply bounce straight back off the foil.
- the fact that most of the alpha particles went through the gold foil told scientists that atoms are mainly empty space.
- immediately, they knew the plum pudding model must be wrong.
- because some of the alpha particles were deflected, they knew the centre of the atom must have positive charge (any alpha particle that comes close to the positive
centre of an atom is repelled). - because some alpha particles bounced straight back, the scientists were told that the centre of an atom must contain a great deal of mass.
describe radioactive decay:
- when an isotope’s nucleus is unstable (only one or two of an element’s isotopes tends to be stable, the rest are unstable), it can emit radiation in order to become more stable (either in the form of alpha, beta or gamma radiation, or by just emitting neutrons) and decay into another element
- 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
what does it mean if we say a material is radioactive?
it consists of unstable isotopes that can decay
how can we measure the radioactive decay at a source?
we can do this by using a geiger-muller tube and counter, which records all of the decays that reaches it each second - the count-rate is the number of decays recorded each second by the detector, and can be used to estimate the activity
what is the difference between activity and the count-rate?
activity is the rate at which the unstable nucleus decays (measured in becquerels), whereas the count-rate is the rate at which radioactive emissions are detected by a detector (e.g. a Geiger-Muller tube).
describe alpha radiation:
- the same as a helium nucleus (2 neutrons, 2 protons). no electrons, overall charge of 2+
- large, travel 5cm through the air before they stop, easily stopped by collisions with other molecules
- easy to stop (single sheet of paper)
- very strongly ionising, due to large size and strong charge
describe beta radiation:
- an electron which is ejected from the nucleus at a very high speed (formed inside the nucleus when a neutron decays into a proton and an electron. the proton stays in the nucleus).
- charge of -1, virtually no mass
- travel several metres in air before stopping.
- stopped by a few mm of aluminium
- quite strongly ionising
describe gamma radiation:
- a type of electromagnetic radiation from the nucleus, coming in the form of a wave
- often emitted after alpha or beta radiation, as a way of the nucleus getting rid of a bit of extra energy
- travels several metres in air before stopping.
- tend to pass straight through materials, due to no mass or charge, can’t collide with the atoms. therefore weakly ionising
- stopped by thick lead or concrete
what is neutron radiation?
if a nucleus contains too many neutrons, making it unstable, it can throw out a neutron to increase its stability
what are nuclear equations, and what can they be used for?
represent radioactive decay.
how are alpha particles represented in a nuclear equation?
4 He
2
how are beta particles represented in a nuclear equation?
0 e
-1
what does radon form when it undergoes alpha decay?
219 radon = 215 polonium + 4 He
86 84 2
- when radon decays, both its mass number and atomic number decreases.
what does carbon form when it undergoes beta decay?
14 carbon = 14 nitrogen + 0 e
6 7 -1
- beta decay does not cause the mass number to change (as although it’s lost an electron, it’s gained a proton, so overall the mass hasn’t changed), but causes the atomic number to increase, as it’s gained a proton
do gamma rays cause decay?
the emission of a gamma ray does not cause the mass or charge of a nucleus to change, as it’s pure energy and it itself has no mass or charge
what is the equation for neutron emission?
- use beryllium 9 as an example
- subtract one from the mass number, as it lost one neutron
9 Be = 8 Be + 1n
4 4 0