7.16 rutherford’s nuclear model of atom and Geiger and Marsden’s experiment explanation

Before Geiger and Marsden’s experiment – plum pudding model of atom existed

If atoms were like that, the alpha particles which have a positive charge would pass straight through the gold foil without any deflection as the negative charge via the electrons is negligible and and there is no concentration of positive charge that would cause the positive alpha particles to repel and deflect

This apparatus was used to test the hypothesis of the plum pudding model of atom. A thin beam of alpha particle was shot through the very thing gold foil. The inside of the equipment was coated with zinc sulphide photographic film which would change color if alpha particles hit it. This allowed the scientist to see where the alpha particles went after they hit the gold foil.

A vacuum was created inside the apparatus to remove all the air so that the alpha particles are not obstructed by the air.

When the experiment was carried out, Geiger and Marsden discovered that not all alpha particles went straight through the thin gold leaf. Some of the deflected a little bit and very few came back to the source. With this information the concluded that there was a concentration of positive charge in the atoms which was causing the positively charged alpha particles to repel and delect back. The also concluded that only a small part of the atom had concentration of positive charge and the rest was empty space as the detected particles were very few and most other particles went straight through.

The deflection of alpha particles increases if we reduce the speed of the particles or increase the positive charge in the atoms, by having an element that has more protons.

7.13 uses of radioactivity

• Radiotherapy
• Medical tracing – checking for radioactive substances in human body
• Non medical tracing – checking for leaks in pipes
• Radiotherapy – killing cancer cells
• Radioactive dating of archaeological specimens and rocks

7.19 chain reaction of Uranium 235 nuclei

When a neutron collides with a uranium 235 nucleus it forms uranium 236 which undergoes fission. This fission of U-235 also releases neutrons that can collide with more uranium 235 nuclei if they are around hence triggering them to undergo fission as well. This repeats over and over again forming a chain reaction.

7.5 Properties of alpha, beta particles and gamma radiation

Alpha (α)

• Made up of 2 protons and 2 neutron
• Strongly ionizing
• Low penetrating power – can be stopped by paper or several cm of air.

Beta (β)

• Fast moving electron
• Moderately ionizing
• Moderately penetrating – can be stopped by 1 m of air or several cm of aluminium
• Emitted when a neutron turns into a proton and releases an electron

Gamma (γ)

• Very high frequency electromagnetic waves
• Weekly ionizing
• Strongly penetrating – can be stopped by several cm of lead or 1 m of concrete

7.4 Alpha, beta and gamma rays

Ionisation is when an atom loses or gains an electron. This effectively damages the atoms by making ions.

Alpha, beta and gamma rays are 3 types of ionizing radiation.

7.3 atomic number, mass number and isotopes

Atomic number – number of protons in the nucleus of the atom

Mass number – total number of particles in the atom nucleus

Isotopes – Atoms with same proton configuration but different neutron configurati

7.2 Structure of an atom and notation

Atoms of protons and neutrons in the nucleus and electrons outside the nucleus.

Different isotopes of elements will have different number of neutrons in the nucleus but the same number of protons.

Carbon 14

C-14

14C

Have a look at this image: https://upload.wikimedia.org/wikipedia/commons/1/18/Atomic_number_depiction.jpg

All are isotopes of carbon with total of 14 sub-atomic particles in the nucleus of the atom.

14 (total) - 6 (protons) = 8 neutrons