topic 1

simple model of the atom

atom is formed of 3 constituents protons neutrons and electrons. at the centre of an atom is a nucleus formed of protons and neutrons, therefore they are known as nucleons whereas electrons orbit the nucleus in shells

specific charge

charge/mass

how is the proton number denoted

Z

nucleon number

proton and neutrons denoted by A

isotopes

atoms with the same number of protons but different numbers of neutrons

isotopic data

AQA can give you information about the abundance of different isotopes.

For example, imagine an element has:

Isotope	Mass	Abundance
X-10	10	20%
X-11	11	80%

The average mass can be calculated using a weighted mean:

average mass=(10×20)+(11×80)100

=200+880100

10.8

The important idea is that the more abundant isotope has a greater influence on the average.

the four fundamental forces

-strong nuclear

-weak nuclear

-electromagnetic

-gravity

forces within the atomic nucleus

electromagnetic force -causes protons to repel each other due to their positive charges

strong nuclear force-holds protons and neutrons together in the nucleus

gravitational force-attracts nucleons together due to their mass

The electromagnetic repulsion between protons is much stronger than their gravitational attraction. So an additional strong attractive force is needed to hold the nucleus together - the strong nuclear force

experimetns show the strong nuclear force:

-at distance less than about 0.5 fm, the strong nuclear force is repulsive

-above 0.5fm, the strong nuclear is attractive

-the strong nuclear force is most attractive at distances of around 1fm

-above 3fm the strong nuclear force has negligible effect

-acts on both protons and neutrons

-short range attraction up to approximately 3fm ( upto about 3fm attractive)

-very short range repulsion closer than approximately 0.5fm

stable nucleus

if the strong nuclear force is strong enough it will not decay

unstable nucleus

if the strong nuclear is too weak it will decay in order to be stable

why is the strong nucleus force important inside the nucleu

It is important because:

• Protons are all positively charged, so they repel each other due to the electrostatic force.

• The strong nuclear force provides an attractive force between nucleons (protons and neutrons), helping to overcome this repulsion.

• It is a short-range force, so it only acts significantly when nucleons are very close together.

alpha decay

-emission of an alpha particle

-alpha particles are helium nuclei, comprising of two protons and two neutrons. they are very ionising but only have a range of few centimetres in air

-they are emitted from very large nuclei like uranium to reduce size

general equation for alpha decay is represented as

beta minus decay

Beta-minus particles are high speed electrons with a range of several meters in air.

During beta-minus decay a neutron in a neutron rich nucleus decays into a proton, emitting an electron (beta minus particle) and an electron antineutrino.

general decay equation for beta-minus decay;

beta-plus decay

a beta-plus particle is a positron the antiparticle of the electron

during beta plus decay a proton in. a proton-rich nucleus decays into a neutron a beta-plus particle and an eelctron neutrino

beta plus decay

why was the neutrino hypothesised

The neutrino was hypothesised because electrons emitted in beta decay have a range of kinetic energies. The neutrino carries away the missing energy, allowing conservation of energy to be satisfied.

Discoveries in beta decay

Originally scientists thought only electrons were emitted. But energy appeared to disappear during beta decay.

In 1930 Wolfgang Pauli hypothesised an additional near-massless neutral particle was produced - later named the neutrino.

Observing the antineutrino decades later provided evidence for Pauli's particle and led to revisions in theories of fundamental particles and processes within the atom.

descrbe how to demonstrate the range of alpha particles usin a cloud chamber, spark counter of geiger counter

Using a Geiger counter

1. Place an alpha source in front of a Geiger–Müller tube.

2. Keep the source and detector at a fixed separation initially.

3. Measure the count rate.

4. Increase the distance between the source and detector in small, measured steps.

5. Record the count rate at each distance.

6. The count rate falls as the distance increases.

7. At a particular distance, the count rate falls to approximately background level.

8. This distance gives an estimate of the range of the alpha particles in air.

If using a cloud chamber

You would look for the tracks produced by alpha particles in the supersaturated vapour. The tracks are relatively short, thick and straight. The length of the tracks can be measured to demonstrate the limited range of alpha particles.

what is the neutrino

The electrons emitted during beta decay have a range of kinetic energies. The neutrino carries away the remaining energy, allowing conservation of energy.

Explain why alpha particles have a much shorter range in air than beta particles.

• Alpha particles have a greater charge.

• Alpha particles have greater mass.

• Alpha particles cause stronger ionisation.

• Therefore, they lose kinetic energy more rapidly and have a shorter range.