What did John Dalton’s atomic theory say?
- All matter is made up of tiny particles called atoms
- Atoms are tiny, hard spheres that cannot be broken down into smaller parts
- Atoms cannot be created or destroyed
- The atoms in an element are all identical
What did JJ Thompson’s atomic theory say?
Described atoms as a pudding made of +vely charged material with -vely charged electrons (plums) scattered through it. This was due to his investigation of the mass of particles in the rays where he found they were about 1800 times lighter than hydrogen, discovering subatomic particles (electrons).
What experiment did Ernest Rutherford perform and what were the results?
- Directed beam of alpha particles at very thin gold leaf suspended in a vacuum
- Most could pass through (proving large gaps within atom)
- Some hit nucleus and bended away slightly
- Very small number hit the nucleus and were reflected at a >90° angle
What did Rutherford conclude from his experiment?
- Most of atom is empty space (as most particles were deflected less than 10°)
- Nucleus is positive (as some particles were deflected 10-90°)
- Most an atom’s mass is in the nucleus
What did Bohr’s atomic theory say?
- Atom consisting of a small, positively-charged nucleus “orbited” by negatively-charged electrons
- Electrons exist around the nucleus in shells that have a set size and energy
- Energy of shell is related to its size
- Radiation is absorbed or emitted when an electrons moves from one shell to another
What is the summary of the 4 atomic models?
Dalton (billiard ball): solid spheres that could not be divided.
Thompson (plum pudding): negatively charged electrons embedded in a positively-charged sphere.
Rutherford: positive nucleus in the center of a scattered cloud of electrons. Later discovered protons (positively-charged particles in nucleus).
Bohr (modern atomic model): electrons arranged in energy levels at certain distances from the nucleus. Helped explain why atoms absorb or emit only certain wavelengths of light.
What are the characteristics of the nucleus?
- Made up of protons and neutrons
- +vely charged because of protons
- Dense (contains nearly all mass of atom in a tiny space)
What are the characteristics of electrons?
- Very small, light and -vely charged
- Found outside of nucleus, orbiting in shells
What are isotopes?
Atoms of the same element with the same number of protons but different number of neutrons (same atomic number, different mass number).
What are the characteristics of electrons shells?
- Each shell at certain distance from nucleus
- Distance determines energy an electron orbiting a certain shell will posses
- Atom’s nth shell can accommodate 2n² electrons (up to 20th element 3rd shell only takes 8)
What happens when an electron moves between shells?
If electrons absorb enough energy they can jump to higher energy shells.
When they fall back to lower orbits they emit energy as E/M radiation.
How do we calculate the energy difference of electrons in energy level diagrams?
ΔΕ = hv
v - frequency
h - Planck’s constant
What is ionisation?
When an atom gains so much energy that one or more of the electrons can escape from the atom altogether.
Why are the emission and absorption spectra different for every atom?
Every atom has different energy levels.
What is radioactivity?
The spontaneous emission of radiation in the form of particles or high energy photons resulting from a nuclear reaction.
What is background radiation?
The low-level ionising radiation we are continuously exposed to, coming from naturally radioactive substances in the environment and from outer space (cosmic rays).
What are the main types of background radiation?
- Radon gas
- Radioactive food
- Hospital treatments
- Cosmic rays
What is the main source of background radiation and what is it?
Radon gas is a radioactive gas produced by rocks that contain small amounts of uranium.
It diffuses into the air from rocks and soil and can build up in houses (so ventilation is very important). Amount of radon in the air depends on the type of rock and how much uranium it contains. Rock type and building stone vary around the country and so does the amount of radon.
Why is radon dangerous?
If radon decays within our lungs, the alpha radiation will bombard lung tissue and the radon precipitates, becoming trapped. Trapped polonium and decay products continue to emit alpha particles inside the lungs for the remainder of that person’s life.
What does radiation from cosmic rays consist of?
+vely charged ions from protons to iron and larger nuclei derived from outside the Solar System.
How does a photographic film detect radioactivity and how is it used?
- Absorbs radiation
- The more it absorbs, the darker it becomes
- Films need to be developed in order to give exact amount of radiation the film has been exposed to
- People working with radioactivity wear film badges (dosimeters) to check the amount of radiation they are exposed to
- Newer dosimeters use material that changes colour without the need of any processing
How does a Geiger-Muller tube detect radioactivity and how is it used?
- Radiation passing through the tube ionises gas inside it
- This allows a short pulse of current to flow
- Tube is connected to a counter
- Measures the pulses of current or clicks each time radiation is detected
- Count rate is the number of clicks per second
How does the Geiger-Muller tube work?
Filled with inert gas (such as helium, neon or argon) at low pressure, to which high voltage is applied. Tube briefly conducts electrical charge when a particle or photon of incident radiation makes the gas conductive by ionisation. Tube is connected to counter to count pulses of a current or tube may give a click each time radiation is detected.
What is a radioactive substance?
Substance is unstable and continuously gives out radiation at random intervals.
