GCSE Physics – Topic 4 – Atomic Structure – Flashcards

1)The nuclear model that resulted from the alpha particle scattering experiment was a positively charged nucleus surrounded by a cloud of negative electrons 2) Niels Bohr said that the electrons orbiting the nucleus do so at certain distances called energy levels. His theoretical calculations agreed with the experimental data. 3) Evidence from further experiments changed the model to have a nucleus made up of a group of particles (protons) which all had the same positive charge that added up to the overall charge of the nucleus 4) About 20 years after the idea of a nucleus was accepted, in 1932 James Chadwick proved the existence of the neutron, which explained the imbalance between the atomic and mass numbers

Atoms are very small, having a radius of about 1 x 10-10 meters

The basic structure of an atom is a positively charged nucleus composed of both protons and neutrons, surrounded by negatively charged electrons

In an atom, the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge.

The number of protons in an atom of an element is called its atomic number. All atoms of a particular element have the same number of protons. The total number of protons and neutrons in an atom is called its mass number

1)In 1804 John Dalton agreed with Democritus that matter was made up of tiny spheres (atoms) that couldn't be broken up, but he reckoned that each element was made up of a different type of atom 2)Nearly 100 years later, J.J Thomson discovered particles called electrons that could be removed from atoms. So Dalton's theory wasn't quite right - atoms could be broken up. Thomson suggested that atoms were spheres of positive charge with tiny negative electrons embedded in it, like the fruit in a plum pudding - hence this model

1) In 1909 scientists in Rutherford's lab tried firing a beam of alpha particles at thin gold foil - this was the alpha scattering experiment. From the plum pudding model, they expected the particles to pass straight through the gold sheet or only be slightly deflected.

1) Although most of the particles did go straight through the sheet, some were deflected more than they had expected, and a few were deflected back the way they had come - something the plum pudding model couldn't explain 2) Because a few alpha particles were deflected back, the scientists realised that most of the mass of the atom was concentrated at the centre in the nucleus. This nucleus must also have a positive charge, since it repelled the positive alpha particles 3)They also realised that because nearly all the alpha particles passed straight through, most of an atom is just empty space. This was the first nuclear model of the atom.

1) Atoms with the same number of protons but a different number of neutrons 2) All elements have different isotopes but there are usually only one or two stable ones.

1) All elements have different isotopes but there are usually only one or two stable ones. 2) The other unstable isotopes tend to decay into other elements and give out radiation as they try to become more stable 3) Radioactive substances spit out one or more types of ionising radiation from their nucleus - alpha beta and gamma. They can also release neutrons when they decay

Ionising radiation is radiation that knocks electrons off atoms, creating positive ions. The ionising power of a radiation source is how easily it can do this

1) Alpha radiation is when an alpha particle is emitted from the nucleus. An a-particle is two neutrons and two protons (like a helium nucleus) 2) They don't penetrate very far into materials and are stopped quickly - they can only travel a few cm in air and are absorbed by a sheet of paper 3) Because of their size they are strongly ionising 4) A-radiation is used in smoke detectors - it ionises air particles, causing a current to flow. If there is smoke in the air, it binds to the ions - meaning the current stops and the alarm sounds

1) A beta particle is a fast moving electron released by the nucleus. Beta particles have virtually no mass and a charge of -1 2) they are moderately ionising 3) They also penetrate moderately far into materials before colliding and have a range in air of a few metres. They are absorbed by a 5mm thick sheet of aluminium 4) for every beta particle emitted, a neutron in the nucleus has turned into a proton 5) Beta emitters are used to test the thickness of sheets of metal, as the particles are not immediately absorbed by the material like alpha radiation would be and do not penetrate as far as gamma rays. Therefore, slight variations in thickness affect the amount of radiation passing through the sheet

1) Gamma rays are waves of electromagnetic radiation released by the nucleus 2) they penetrate far into materials without being stopped and will travel a long distance in air 3) this means they are weakly ionising because they tend to pass through rather than collide with atoms. Eventually they hit something and do damage 4) they can be absorbed by thick sheets of lead or metres of concrete

When it is emitted it causes the atomic number to go down by 2 and its mass number to go down by 4.

When it is emitted it causes the atomic number to go up by 1 and there is no change in mass number.

There is no change in atomic or mass number when gamma rays are emitted.

1) Radioactive substances give out radiation from the nuclei of their atoms 2) This radiation can be measured with a Geiger-Muller tube and counter which records the count rate - the number of radiation counts reaching it per second 3)Radioactive decay is entirely random, so you can't predict exactly which nucleus in a sample will decay next, or when any one of them will decay

The rate at which a source decays is its Activity - becquerels, Bq

*The half-life is the time taken for the number of radioactive nuclei in an isotope to halve* 1)Each time a radioactive nucleus decays to become a stable nucleus, he activity as a whole will decrease. 2) For some isotopes it takes just a few hours before nearly all the unstable nuclei have decayed, whilst others last for millions of years 3) The problem with trying to measure this is that the activity never reaches zero, which is why we have to use the idea of half life to measure how quickly the activity drops off

A short half-life means the activity falls quickly, because the nuclei are very unstable and rapidly decay. Sources with a short half-life can be dangerous because of the high amount of radiation they emit at the start, but they quickly become safe

A long half-life mean the activity falls slowly because most of the nuclei don't decay for a long time - the source just sits there, releasing small amounts of radiation for a long time. This can be dangerous because nearby areas are exposed to radiation for millions of years

1) Radioactivity of naturally occurring unstable isotopes which are all around us - air, food, building materials, rocks 2)Radiation from space - cosmic rays. These come mostly from the sun. The Earths atmosphere protects us from much of this radiation 3) Radiation due to human activity, e.g. FALLOUT FROM NUCLEAR EXPLOSIONS or nuclear waste. But this represents a tiny proportion of the total background radiation

When an object is exposed to radiation. Keeping sources in lead lined boxes, standing behind barrier or being in a different room and using remote-controlled arms when working with radioactive sources are all ways of reducing irradiation

1) If unwanted radioactive atoms get onto or into and object, then the object is said to be contaminated 2) these contaminating atoms might then DECAY, releasing radiation which could cause you harm 3) contamination is especially dangerous because radioactive particles could get inside your body 4) gloves and tongs should be used when handling sources to avoid particles getting stuck to your skin or under your nails. Some industrial workers wear protective suits to stop them breathing in particles

Radiation dose tells you the risk of harm to body tissues due to the exposure to radiation. It's measured in sieverts (Sv). Your radiation dose is affected by where you live and whether you have a job that involves radiation

Beta and gamma are the most dangerous because they can penetrate the body and get to delicate organs. Alpha is less dangerous as it can't pass through your skin.

Alpha because they do all their damage in a very localised area, so contamination is high when working with alpha sources. Beta and Gamma are less ionising and are less dangerous.

1) Certain radioactive isotopes can be injected into people and their progress around the body can be followed using an external detector. A computer converts the reading to a display showing where the strongest reading is coming from 2) One example is the used of Iodine-123, which is absorbed by the thyroid gland, but gives out radiation which can be detected to indicate whether the thyroid gland is taking in the iodine as it should 3) Isotopes which are taken into the body like this are usually gamma emitters, so that the radiation passes out of the body without causing too much ionisation. They should have a short half-life so that the radioactivity inside the patient quickly disappears

1)Radiotherapy is used to kill cancer cells in the body. Gamma rays are directed carefully and at the right dosage to kill the cancer cells without damaging too many normal cells. Radiation-emitting implants can also be put next to or inside tumours. Patients can feel ill if any normal cells are killed.

1)Radiation can enter living cells and ionise atoms and molecules within them, which can lead to tissue damage. 2)Low doses tend to cause minor damage without killing the cells but can cause cancer by giving a rise to mutant cells which divide uncontrollably. 3)High doses tend to kill cells completely, causing radiation sickness.

1) Tracers can be used to diagnose life threatening conditions, while the risk of cancer from one use of a tracer is very small 2) Whilst prolonged exposure to radiation poses future risks and causes many side effects, many people with cancer choose to have radiotherapy as it may get rid of their cancer entirely. For them, the benefits outweigh the risks 3) Perceived risk is how risky a person thinks something is. It's not the same as the actual risk of a procedure and the perceived risk can vary from person to person

It is when a nucleus releases two or three neutrons which go on to cause further fission, resulting in a chain reaction.

Uranium-235 and Plutonium-239

Energy is used to heat up water to make steam which drives a turbine which turns a generator.

Barium and Krypton. They are highly radioactive substances which need to be disposed of safely.

1) Spontaneous fission rarely happens. Usually, the nucleus has to absorb a neutron before it will split. 2) When the atom splits it forms two new light elements that are roughly the same size and that have some energy in their kinetic energy stores. 3) 2 or 3 neutrons are also released when an atom splits. If any of these neutrons are moving slow enough to be absorbed by another nucleus , they can cause more fission to occur. This is a chain reaction 4) the energy not transferred to the kinetic energy stores of the products is carried away by gamma rays. 5) the energy carried away by the gamma rays and in the ke stores of the remaining free neutrons and other decay products, can be used to heat water, making steam to turn turbines and generators 6) the amount of energy produced by fission in a nuclear reactor is controlled by changing how quickly the chain reaction can occur. This is done using control rods which, when lowered and raised inside a nuclear reactor, absorb neutrons. This slows down the chain reaction and controls the amount of energy released. 7) uncontrolled chain reactions quickly lead to lots of energy being released as an explosion - this is how nuclear weapons work

Energy released in the sun. Energy is released when two nuclei are fused together to form a heavier nucleus

Helium, a harmless gas. On the other hand it has technical difficulties as very high temperatures are needed to start the fusion of nuclei.

1) Nuclear fusion is the opposite of nuclear fission 2) In nuclear fusion, two light nuclei at high speed and join to create a larger, heavier nucleus. E.g. hydrogen nuclei can fuse to produce a helium nucleus 3) this heavier nucleus does not have as much mass as the two separate light nuclei did. Some of the mass of the lighter nuclei is converted to energy. This energy is then released as radiation 4) Fusion releases a lot of energy 5) So far scientists haven't found a way of using fusion to generate energy for us to use. The temperatures and pressure needed for fusion are so high that fusion reactors are really hard and expensive to build