Atomic Structure
Combined Science
- Created by: abbienoice
- Created on: 23-11-20 20:28
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- Atomic Structure
- Development of the Atom Model
- Current Model of the Atom
- the nucleus is tiny but takes up most of the mass of the atom - contains protons(+charge) and neutrons(neutral charge)
- overall positive charge
- radius is 10,000 times smaller than an atom
- negative electrons whizz around the outside of the nucleus
- radius of atom = 1 x 10^-10
- number of electrons = number of protons
- if electrons gain energy by absorbing EM radiation they move to a higher energy level - further from the nucleus
- if electrons release EM radiation they mover to a lower energy level nearer the nucleus
- if one or more electrons leave the atom becomes a positively charged ion
- the nucleus is tiny but takes up most of the mass of the atom - contains protons(+charge) and neutrons(neutral charge)
- Rutherford
- replaced the plum pudding model with the nuclear model
- alpha scattering experiment - tried firing a beam of alpha particles at thin gold foil
- expected particles to pass straight through the gold sheet or only be slightly deflected
- most of the particles went straight through the sheet but some were deflected more than expected, some were deflected back the way they had come
- because a few particles were deflected back, the scientists realised that most of the mass of the atom must be concentrated at the center in a tiny nucleus
- the nucleus must have positive charge - it repelled the positive alpha particles
- nearly all the alpha particles passed straight through - most of the atom is empty space
- J J Thomson
- discovered the electrons
- suggested atoms were spheres of positive charge with tiny negative electrons stuck to them like fruit in a plum pudding
- plum pudding model
- Current Model of the Atom
- Half-life
- the time taken for the number of radioactive nuclei in an isotope to halve
- graph*
- the half life is found from the graph by finding the time interval on the bottom axis corresponding to a halving of the activity on the vertical axis
- radioactivity
- radioactive substances always give out radiation from their nuclei
- radiation can be measured with a Geiger-Muller tube and counter - recorded the count-rate - number of radiation counts reaching it per sec
- cant predict exactly which nucleus in a sample will decay next or when anyone of them are going to decay - completely random
- half life can be used to make predictions about the radioactive sources - even though their decays are random
- when a radioactive nucleus decays to be come stable the activity will decrease
- for some isotopes it takes a few hours before all the unstable nuclei have decayed - others last millions of years
- problem with trying to measure it - the activity never reaches 0 - why we have to use half-life
- can be used to find the rate a source decays - its ACTIVITY
- measured in becquerels - Bq
- can also be described as the time taken for the activity to fall to half its initial value
- Dangers of Radiation
- irradiation
- exposure to radiation
- objects near radiation are irradiated by it
- irritating something doesnt make it radioactive
- methods of reducing irradiation - keeping sources in lead-lined boxes and standing behind barriers
- contamination
- radioactive particles getting onto objects
- the contaminated atomsmight decay, releasing radiation which can cause harm
- dangerous as radioactive particles could get inside your body
- gloves and tongs should be used when holding sources - some use protective suits
- ionising radiation can enter living cells and ionise atoms within them - damages the cells or kills them
- different types of radiation cause different amounts of harm
- outside the body - beta and gamma are most dangerous
- they can penetrate the body and get to the delicate organs
- alpha is less dangerous as it cant pernitrate the skinand is easily blocked by a small air gap
- high levels of irradiation from all sources are dangerous - but especially beta and gamma
- inside the body - alpha sources are most dangerous
- they do all their damage in very localised areas - contamination rather than irradiation is the major concern
- beta sources are less damaging as radiation is absorbed over a wider area and some passes out of the body
- gamma sources are the least dangerous inside the body as they mostly pass straight out
- outside the body - beta and gamma are most dangerous
- irradiation
- Isotopes
- isotopes of an element are atoms with the same number of protons but a different number of neutrons
- all elements have different isotopes - usually only one or two stable ones
- unstable isotopes tend to decay into other elements and give out radiation as they try to become stable - radioactive decay
- radioactive substances spit out one or more types of ionising radiation from their nucleus. theycan also release neutrons to rebalance the protons and neutrons
- ionising radiation is radiation that knocks electrons off atoms creating + ions. the ionising power of a radiation source is how easily it can do this
- types of radiation
- Gamma
- waves of electromagnetic radiation released by the nucleus
- penetrate far into materials without being stopped and will travel far in the air
- weakly ionising - cause they pass through rather than collide with atoms
- if hits something it damages it
- absorbed by thick sheets of lead or meters of concrete
- Alpha
- made from alpha particles being emitted from the nucleus
- absorbed by a sheet of paper
- dont penetrate far into materials and are stopped quickly
- strongly ionising - because of size
- Beta
- a fast-moving electron released by the nucleus
- virtually no mass
- charge -1
- moderately ionising
- pernitrate moderately far into materials before colliding and have range in air of a few meters
- absorbed by a sheet of aluminum
- for every beta particle emitted a neutron in the nucleus has turned into a proton
- Gamma
- nuclear equations
- a way of showing radioactive decay by using element symbols
- atom before decay -> atom after decay + radiation emitted
- the total mass and atomic numbers must be equal on both sides
- alpha decay
- alpha particles are made up of two protons and two neutrons
- when an atom emits an alpha particle its atomic number reduces by 2 and its mass number reduces by 4
- charge of nucleus decreases
- an alpha particle can be written as a helium nucleus
- beta decay
- a neutron in the nucleus turns into a proton and releases fast-moving electrons
- the number of protons increases by 1 - this increases the positive charge of the nucleus
- the mass of the nucleus doesnt change
- written as 0e-1 in nuclear equations
- gamma decay
- a way of getting rid of excess energy from a nucleus
- theres no change to the atomic mas or atomic number of the atom
- Development of the Atom Model
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