Atomic Structure Models: Thomson & Rutherford

Development of the Model For The Structure Of An Atom

• Each element is made up from a particular type of atom, and different elements have different atoms.
• Atoms are indestructible.
• Compounds are made up from differing arrangements of atoms.
• Could not explain the discovery that atoms contain electrons, and these could be released from the atom.

Thomson's Model

• An atom consists of a sphere of positive charge in which the negatively charged electrons are evenly spread, like a plum pudding.
• The positive charge is exactly balanced by the negative charge and so the atom is neutral.
• Could not explain heated atoms emitting light of different colours.
• Could not explain atoms showing group similarities which led to the construction of the Periodic Table.

Rutherford's Model

• Atoms are electrically neutral.
• Atoms contain particles called electrons that can be made to leave the atom.
• Considers atomic masses.
• Electromagnetic energy is produced when charged particles accelerate.
• Atoms heated to high energies emit light, the colour of which is specific to the individual atom.
• Atoms show group similarities which led to the construction of the Periodic Table.
• Based on the results of an experiment in which alpha particles (positively charged) were fired at a sheet of thin gold foil.

Rutherford's Prediction of the Structure of the Atom

• Most of the atom must consist of empty space because firing alpha particles towards gold foil makes most of them go straight through without deflection.

• The "matter" of an atom must be concentrated into a very small volume compared to the total volume of the atom; this "matter" was called the nucleus.

• The gold nucleus repelled the alpha particle because firing alpha particles towards gold foil deflects some of them away from the gold nuclei, implying it must have the same type of charge as the alpha particle.

• The nucleus must contain positively charged particles and these were called protons.

• The gold nucleus had to make the alpha particle bounce back without itself-moving, therefore firing alpha particles towards gold foil deflects very few of them back from the gold foil due to it being very massive in comparison.

• The mass of the atom is all in the nucleus

• The nucleus must also contain other particles which must be neutral, due to the mass of the nucleus being greater than the scattering experiment predicts which means the neutral particles were called neutrons.

• There must be negatively charged particles orbiting the nucleus to balance the positive charge of the nucleus because an atom is electrically neutral and as most of the mass of the atom is in the nucleus, these must be very light.

• There must be electrons orbiting the nucleus.

Rutherford's Model Shortcomings

• It cannot explain an orbiting electron accelerating (centripetal acceleration) and is therefore constantly losing energy in the form of electromagnetic energy because if it is losing energy it should eventually collapse into the nucleus.
• It cannot explain light emitted by heated atoms being coloured, not white.

Atoms and Nuclei

• The sub-atomic particles which make up an atom: electron, proton, and neutron.
• Electron: a negatively charged particle with negligible mass.
• Proton: a positively charged particle with mass 1 mass unit (1 amu is 1.661×10-27 kg).
• Neutron: an uncharged particle with mass 1 amu.
• The protons and neutrons in the nucleus are held together by nuclear forces.
• A is the nucleon (or mass) number which shows the important information about an atom in its symbolic representation which is a (top) X (bottom) with Z being the atomic number, and X is the symbol for the element.
• The atomic number is the number of protons in the nucleus, and hence is also the number of electrons orbiting the nucleus.
• The nucleon number is the total number of particles (protons and neutrons) in the nucleus, and hence is also the mass of the atom in atomic mass units.
• Isotopes are elements which have atoms with the same atomic number, but a different mass number.
• Isotope atoms have the same number of protons and electrons (and so are the same element), but a different numbers of neutrons.

Radioactivity

• A radioactive element spontaneously emits radiation such as alpha particles, beta particles, or gamma rays.
• All radiation emission particles:
• Can penetrate materials which are opaque to light.
  • Ionise the air through which they travel.
  • Can cause certain compounds to fluoresce (glow).
  • Destroy or change the structure of cells.

Alpha Particles

• Alpha particles are helium nuclei and so consist of two protons and two neutrons.
• Alpha particles are positively charged and so are deflected by a magnetic field.
• Alpha particles have a speed of about 1/10 the speed of light.
• Alpha particles have a relatively large mass and so are only slightly deflected by a magnetic field.
• Alpha particles have little penetrating power (about 5cm in air, can be stopped by a sheet of paper).
• Alpha particles ionise far more strongly than other types of radiation.

Beta Particles

• Beta particles are electrons.
• Beta particles are negatively charged and so are deflected by a magnetic field.
• Beta particles have a speed of about 9/10 the speed of light.
• Beta particles have a negligible mass and so are strongly deflected by a magnetic field.
• Beta particles have far greater penetrating power than alpha particles (about 30 cm in air, can be stopped by a thin sheet of aluminium).
• Beta particles are less likely to ionise than alpha particles.

Gamma Rays

• Gamma rays are electromagnetic radiation of high frequency (short wavelength).
• Gamma rays have no charge and so are not deflected by a magnetic field.
• Gamma rays have the same speed as light.
• Gamma rays are the most penetrating of the three types of radiation (about 30cm of steel, a few cm of lead).
• Gamma rays are far less likely to ionise than alpha or beta particles.

Radioactive Decays

• Nucleon number is conserved which is the nucleon number of the original atom must equal the sum of the nucleon numbers of the products.
• Charge is conserved which is the atomic number of the original atom must equal the sum of the atomic numbers of the products.
• Mass / energy is conserved which is during radioactive decay mass ⇔ energy conversion may take place (E = mc²), but the total mass/energy before the reaction should be the same as the total mass/energy after the reaction.
• For a particular piece of radioactive material, the number of radioactive atoms left decreases exponentially with time.
• The time required for half the remaining radioactive atoms to decay is the half life.
• The element left after radioactive decay is often a different element from the original and the element which is left is called the daughter element.

Half-Life

• The heavier elements tend to have several isotopes.
• The nuclei of some isotopes are unstable and therefore tend to decay to the stable nuclei with the emission of radiation.
• In a given sample of radioactive substance there will be a finite, but unknown, number of unstable nuclei (say N).
• The rate at which unstable nuclei decay to the stable nuclei depends on the size of N.
• For all radioactive substances the time it takes for the number of unstable nuclei, N, be reduced to 12 N unstable nuclei (regardless of the original value of N) is a fixed time - called the half life.
• The idea of half-life is best illustrated on a graph of number of unstable nuclei, N, against time.
• The number of unstable nuclei in a sample of radioactive substance is very difficult to measure, but because it is proportional to the rate of decay (which can be easily measured using a geiger counter), a graph of this rate against time is the one usually drawn to determine half-life, T.