Topic 2 Revision Notes

Particles Revision Notes (topic 2)

1. Atomic Structure

- Atoms are made up of Protons, Neutrons and Electrons: inside every atom there is a nucleus containing protons & neutrons. Orbiting this core are the electrons.

- What you need to remember about these particles:

• Relative charge: P (+1), N (0), e (-1)
• Relative mass: P&N (+1), e (0.0005)

- Proton number is the number of protons in a nucleus: this defines the element (no 2 elements have the same proton number). In a neutral atom, the number of protons = number of electrons. The chemical behaviour and elements reactions depend on the number of electrons, so the proton number tells you a lot about the chemical properties. (A particle w/ a different no. of electrons to protons is called an ion).

- The nucleon number is the total number of protons and neutrons: this is also called the mass number. Since each proton and neutron have relative mass of 1, and the electrons weigh virtually nothin, the nucleon number is the same as the atom's relative mass.

- Isotopes have the same number of protons, but different number of neutrons: changing the number of neutrons doesnt affect the atoms chemical properties. The number of neutrons affects the stability of the nucleus though. Unstable nuclei may be radioactive and decay over time into different nuclei that are more stable.

- Radioactive isotopes can be used to find out how old things are: All livin things contain the same % of radioactive carbon-14 taken in from the atmosphere. After they die, the amount of carbon-14 inside them decreases over time as it decays to stable elements.

->Scientists can calculate the approximate age of archaeloical finds made from dead organic matter (eg wood, bone) by using the isotopic data (amount of each isotope present) to find the percentage carbon 14 thats left in the object.

- Specific Charge: = charge (C)/mass(kg). i.e. the charge to mass ratio.

2. Stable and Unstable Nuclei 

- The strong nuclear force: holds the nucleus together by counteracting the electrostatic forces of repulsion between nucleons. short-range attraction up to approximately 3 fm, very-short range repulsion closer than approximately 0.5 fm. (at very small separations it must be repulsive or it would crush the nucleus to a point)

The force works equally between all nucleons.

1) the snf is repulsive for very small separations of nucleons

2) as the nucleon separaton increases past about 0.5fm, the snf becomes attractive. It reaches a maximum attractive value an then falls rapidly towards 0 after about 3fm.

3) the electrostatic repulsive force extends over a much larger range (indefinitely)

- Alpha decay happens in very big nuclei: eg uranium and radium. The nuclei of these atoms are just too big for the snf to keep them stable. When an alpha particle is emitted, proton number decreases by 2 and nucleon number decreases by 4. 

• Alpha particles have a very short range - only a few cm in air. This can be…