AQA GCSE Physics: Atomic Structure

• What is the radius of an atom?

  ~1x10⁻¹⁰m (very small)
• What is the basic structure of an atom?

  - an overall positively charged nucleus (composed of both protons and neutrons)- nucleus is surrounded by negatively charged electrons at different energy levels (different distances from the nucleus)

  
• What is the radius of the nucleus in comparison to the radius of the atom?

  <1/10000 of the radius of an atom
• Where is most of the mass inside an atom?

  - most of the mass of an atomis concentrated in the nucleus
• What is the charge of protons, neutrons, and electrons?

  protons (+) positiveneutrons (0) neutralelectrons (-) negative
• How does the energy change in energy levels?

  - energy levels which are further from the nucleus are at a higher energy than those which are closer to the nucleus
• Why might the electron arrangements change (why might the electrons change which energy level they are orbiting the nucleus from)?

  - if the atom absorbs of electromagnetic radiation, an electron can move to a higher energy level (move further away from the nucleus)- when the atom emits electromagnetic radiation, the electron returns back to the lower energy level (move closer to the nucleus)
• Why do atoms have no overall charge?

  - in an atom, the number of negative electrons is equal to the number of positive protons => charges cancel out
• What is the atomic number of an atom?

  - the number of protons in an atom of an element

  
• What is the 'mass number' of an atom?

  - the total number of protons and neutrons in an atom

  
• What are isotopes?

  - isotopes are atoms of the same element that have different numbers of neutrons
• What is an ion?

  - a charged particle formed when an atom, or number of atoms, loses/gains electrons
• How are the 2 types of ions formed?

  - a positive ion is formed when an atom loses electrons- a negative ion is formed when an atom gains electrons
• Why may a scientific model be changed or replaced?

  new experimental evidence
• What did the Ancient Greeks believe about atoms?

  - before the discovery of the electron, the Greeks thought atoms were TINY SPHERES THAT COULD NOT BE DIVIDED
• What was discovered in 1897?What did this discovery prove wrong?What model was made from this discovery?

  - scientists discovered that atoms contain tiny negative particles called electrons=> showed that atoms are not spheres that cannot be divided (they have an internal structure)=> plum pudding model
• What did the plum pudding model suggest about atoms?

  - suggested that the atom is a ball of positive charge with negative electrons embedded in it

  
• Why did scientists use gold as the foil in the alpha-scattering experiment?

  - gold could be hammered into a very thin piece (just a few atoms thick)
• What 3 results happened when alpha particles were fired at the gold foil in the alpha-scattering experiment?

  A. Most of the alpha particles went straight through without changing directionB. Some alpha particles were deflected (changed direction) as they passed throughC. Some alpha particles bounced straight off
• What 3 conclusions did the alpha-scattering experiment lead to?

  A. Atoms are mainly empty space (plum pudding model had to be wrong)B. Centre of an atom must have a positive charge that repelled the positive alpha particlesC. Mass of the atom must be concentrated in the centre (now called the nucleus)
• After the alpha-scattering experiment, what new model replaced the plum pudding model?

  the nuclear model v1

  
• How did Niels Bohr adapt the nuclear model?

  - he suggested that electrons orbit the nucleus at specific distances (now called energy levels/shells)- his theoretical calculations agreed with experimental observations by other scientists

  
• What was discovered to cause the positive charge of any nucleus several years after Bohr adapted the nuclear model?

  - later experiments led 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- protons
• How did James Chadwick's experimental work on the atom lead to a better understanding of isotopes? (Exam Q)

  - Chadwick provided the evidence to show the existence of neutrons- isotopes have the same number of protons but with different numbers of neutrons
• What is radioactive decay?

  - some atomic nuclei are unstable (eg. certain isotopes)- the nucleus gives out radiation as it changes to become more stable- radioactive decay is a random process => we cannot predict when it will happen
• What is 'activity' in terms of nuclear radiation?What is activity measured in?

  - activity is the rate at which a source of unstable nuclei decays- measured in becquerel (Bq)(1Bq = 1 decay per second)
• What is 'count-rate' in terms of nuclear radiation?

  - the number of decays recorded each second by a detector
• What instrument can be used to detect and measure the count-rate?

  Geiger-Muller tube

  
• What are the 4 types of nuclear radiation that can be emitted by an unstable nucleus? What are their symbols?

  - alpha particle (α)- beta particle (β)- gamma ray (γ)- neutron (n)
• What is an alpha particle, in terms of nuclear radiation?

  - this consists of two neutrons and two protons- it is the same as a helium nucleus
• What is a beta particle, in terms of nuclear radiation?

  - a high speed electron ejected from the nucleus - a beta particle is formed inside the nucleus when a neutron turns into a proton and an electron
• What is a gamma ray, in terms of nuclear radiation?

  - a type of electromagnetic radiation from the nucleus
• What is a neutron, in terms of nuclear radiation?

  - when a neutron is ejected out of the nucleus
• How do alpha particles, beta particles, and gamma rays differ with their range in air?

  - alpha particles are large- they can travel ~5cm in air before they collide with air particles and stop- beta particles can reach ~15cm in air before stopping- gamma radiation can travel several metres in air before stopping
• How do alpha particles, beta particles, and gamma rays differ with their penetrating power?

  - alpha particles is stopped by a single sheet of paper (relatively easy to stop)- beta particles is stopped by a few millimetres of aluminium- gamma radiation is stopped by several centimetres of lead
• What is 'ionising power'?

  - the ability to lose electrons and form ions when radiaiton collides with atoms
• How do alpha particles, beta particles, and gamma rays differ with their ionising power?

  - alpha particles are very strongly ionising (they can produce a lot of ions when they collide with a material)- beta particles are quite strongly ionising (not as ionising as alpha particles)- gamma radiation is weakly ionising
• What does a nuclear equation show?

  - a nuclear equation shows what is produced when a radioactive nucleus decays
• What may be caused by the emission of the different types of nuclear radiation?

  a change in the mass and/or the charge of the nucleus
• How can we represent an alpha particle in a nuclear equation?

  - the symbol for helium- 2 protons and 2 neutrons (mass number is 4 and atomic number is 2)