Topic 4 - Atomic Structure Flashcards
In an atom what particles are equal
Protons and electrons
Protons
Stay the same for each element and the number of them is called the atomic number
Mass number
Protons + neutrons
Isotopes
Atoms that have different numbers of neutrons
Why do atoms turn into positive ions
When they lose electrons as electrons have a negative charge
Plum Ludding
Positive charged ball with embedded electrons
Alpha particle scattering experiment
They fired tiny alpha particles at gold foil a few atoms thick.
They saw most of the particles passed straight through the foil
Sometimes the particle was deflected as it passed through the foil
Sometimes the particle bounced straight back from the foil
What did scientists conclude from the alpha particle experiment
- Because most of the particles went straight through the foil, they are mostly empty space
- Because some were deflected it meant the centre of the atom must have a positive charge which repelled the alpha particles
- Because some bounced straight back the mass of the atom must be concentrated in the centre (the nucleus)
Nuclear model
Mostly empty space, with a tiny positive charged nucleus in the middle and negative electrons around the edge.
Radiation decay
Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable.
Activity
the rate at which a source of unstable nuclei decays
Count-rate
The number of decays recorded each second by a detector
An alpha particle
-Large mass
-Positive charge
-Two neutrons and two protons
-Very ionising due to large mass and charge
-Range in air is 3-5cm
-Is easily blocked by paper and skin
-Two protons two neutrons (same as helium nucleus)
A beta particle
-Small mass
-Negative charge
-Quite ionising due to small mass and charge
-Less easily blocked, absorbed by thick aluminium and thin lead
-A high speed electron ejected from the nucleus as a neutron turns into a proton
-Range of air is tens of centimetres
Gamma Ray
-No charge or mass
-Weakly ionising due to no mass or charge
-Electromagnetic radiation from the nucleus
-Air range is several metres
-Difficult to block, absorbed by thick lead, several metres of concrete.
In a nuclear equation an alpha particle may be represented by what symbol
(4)
He
(2)
In a nuclear equation an beta particle may be represented by what symbol
0
e
-1
Half-life
The time it take for the number of nuclei of the isotope in a sample to halve
what is the radius of an atom
1x10^-10
Radius of the nucleus
less than 1/10,000 of the radius of the atom
How do electrons move between energy levels
If they gain energ (e.g. absorb electromagnetic radiation) they transfer to the highest energy level as it’s furthest away from the nucleus, and if it loses energy it moves back to the lower level
Relative mass of each particle types
proton - 1
neutron - 1
electron - 1/2000 (negligible)
What was discovered about the atom in 1897
Atoms contain tiny negative particles showing particles are not spheres that cannot be divided, they have an internal
What did Niels Bohr discover
electrons orbit the nucleus at specific distsnces
What did James Chadwick discover
Particles with no charge that the nucleus contain
20 years before Chadwicks discovery what did experiments show
The idea that the positive charge in a nucleus could be subdivided into particles called protons
What is activity measured in
Becquerels
1 Bq = 1 decay per second
How can you measure activity of a radio active source
Using a Geiger-Muller tube
Why can’t scientists predict when a nucleus will decay
Decay is a random process
Second definition for half life (count rate)
The time it takes for the count rate from a sample containing the isotope to fall to half its initial level
Irradiation
Exposing an object to nuclear radiation
Example of safe irradiation
-A syringe to be sterilised packaged in plastic to stop bacteria entering
-Placed by a radioactive isotope that’s behind a lead wall to protect workers
-Gamma radiation kills any bacteria present
Why do irradiated objects not become radioactive
risks of ionising radiation
-Can increase the risk of cancer
How can you be shielded from the different types of radiation
-Alpha you can wear gloves
-Gamma and beta you need a lead apron
Radioactive contamination
-When unwanted radioactive isotopes end up on other materials
-This is hazardous as the radioactive atoms decay and emit ionising radiation
Alpha sources harm irradiation and contamination
-Less dangerous with irradiation as it can’t penetrate the skin and easily is blocked by a small air gap
-In the body alpha sources are most dangerous because they do their damage in a very localised area.
Beta source harm irradiation and contamination
-Outside the body one of the most dangerous sources because they can get to the body and damage delicate organs
-Less damaging inside the body as radiation is absorbed over a wider area
Gamma source harm irradiation and contamination
-Most dangerous outside the body as can penetrate body and get to delicate organs
-Least dangerous outside of the body as they mostly pass straight out and they have the lowest ionising power
What is the nuclear equation for alpha particle
4
He
2
What happens when a beta decay occurs
-A neutron in the nucleus turns into a proton and releases a fast-moving electron
-The nucleus’ positive charge increases
-Because the nucleus loses a neutron the mass doesn’t change as neutrons and protons have the same mass
What is the nuclear equation of a beta particles
0
e
-1
