Topic 5- Atomic Physics Flashcards
Describe the structure of a nucleus
The nucleus is positively charged and is
made of protons (positive) and neutrons
(neutral).
What are the relative charges of protons,
electrons and neutrons?
Protons: +1
Electrons: -1
Neutrons: 0
What are the relative masses of protons,
electrons and neutrons?
Protons: 1
Electrons: 0 (0.0005)
Neutrons: 1
Describe Rutherford’s experiment
● Alpha particles (charge +2) were fired at a thin
sheet of gold foil
● Most particles went straight through
● Some particles were deflected by small angles
(< 90º)
● A few particles were deflected by large angles
(> 90º)
What are the conclusions of Rutherford’s
experiment?
● Most of the atom is made of empty space: most of the alpha particles passed straight through the gold foil
●The nucleus has a positive charge (protons) as some of the positive alpha particles deflected.
● The nucleus is dense and concentrated and located in a very small volume within the atom. Few alpha particles deflected at a small angle from their original and very few deflected at a large angle
Describe Rutherford’s atomic model
There is a positive nucleus at the centre of an atom, with negative electrons existing in a “cloud”/region around the nucleus.
(1913)
What is an isotope?
Atoms that are the same element with the same number of protons. However they have a different number of neutrons so therefore different masses.
Describe the key features of the plum pudding model. Include the following details: a) where is the negative charge; b) where is the positive charge
atom is a sphere of positive charge with electrons (negative charge) dotted inside like raisins in a pudding
What do all atoms of the same element
share?
The same number of protons (atomic
number).
What is nuclear fission?
heavy unstable nucleus (eg. uranium or plutonium) splits into two smaller nuclei and releases energy
2/3 neutrons are also released in the process
- the energy from the neutrons powers a nuclear reactor
What is nuclear fusion?
- two small nuclei fuse to form a
different element (heavier nucleus)
eg. hydrogen nuclei can fuse to produce a helium nucleus - releases a lot of energy
- requires HIGH temperatures and pressures (how energy is released in the stars)
What does Z represent?
The atomic (proton) number; the number of protons in the nucleus of an atom.
What does A represent?
The nucleon number; the numbers of
protons and neutrons in the nucleus of
an atom.
This is also known as the atom’s mass.
describe the structure of an atom
- central nucleus made of protons and neutrons
- electrons orbit at high speed around the nucleus
- protons +, electrons (-), neutrons 0 (normally electron n = proton n.; so total charge is 0)
- protons and neutrons -> nucleons
- electrons held in orbit by force of attraction between opposite charges
- protons and neutrons bound tightly together by strong nuclear force
what is the difference between Rutherford’s and Thomson’s model (
in Thomson’s model, the positive and negative charges are spread throughout the atom
how does the Rutherford model of the atom explain why
a) most alpha particles go straight through the foil
b) some alpha particles are deflected at large angles
a) nucleus is extremely small
b) repelled by highly concentrated charge of nucleus
why do the results of Rutherford’s experiment suggest the nucleus has a positive charge
alpha particles are positively charged and repelled by like charges
define nuclide
a nucleus with a specific combination of protons and neutrons
- characterised by the mass number (A) and the atomic number (Z)
each different type of atom, lithium 7 for example, is called a nuclide
why does radioactive decay cause a rise in temperature
emitted particles transfer energy to surrounding atoms when they collide with them
what is meant by
a) fission
b) chain reaction
a) splitting of heavy unstable nucleus into 2 lighter nuclei
b) when emitted particles (neutrons) trigger further fission and so on
give an example of
a) a controlled chain reaction
b) an uncontrolled chain reaction
a) energy release in nuclear reactor
b) explosion in nuclear bomb
where does plutonium-239 come from and why is it so dangerous
formed in reactor when U-238 is bombarded by neutrons
its toxic and dust can get into the lungs
from this data how can you tell energy is being released by the reaction (uranium-235 absorbs a neutron creating a nucleus which splits to form barium-141, krypton-92 and three neutrons)
mass/kg neutron 1.673x10^-27 uranium-235 nucleus 390.250x10^-27 barium-141 nucleus 233.964x10^-27 krypton-92 nucleus 152.628x10^-27
total mass of products is slightly less than total mass of U-235 nucleus and neutron
how do you maintain a chain reaction in a nuclear reactor and how do you control the rate of reaction
- a moderator is needed (eg. graphite) to slow down the neutrons or too many get absorbed by uranium-238
- control the rate of reaction by raising/lowering control rods which absorb neutrons (contain boron/cadmium)
what is background radiation
a low level of nuclear radiation always present because of radioactive materials in the environment (natural/man made)
what are sources of background radiation
natural:
soil, rocks, air, building materials, food, drink, cosmic rays
man-made:
nuclear power and weapons testing, medical
name 3 ways to detect radiation
Photographic film
GM tube
Cloud Chamber
describe how photographic film is used to detect radiation
- more radiation absorbed by film -> darker it gets (film initially white)
- worn as badges by people who work with radiation to check how much exposure they have had
describe how a GM tube is used to detect radiation
- ionising effect of radiation is used in the tube to detect radiation
- each time it absorbs radiation, it transmits an electrical pulse to the machine which makes a clicking sound
- greater frequency of clicks -> more radiation present
- *
- GM tube is a hollow cylinder filled with a gas at low pressure
- thin window made of mica at one end
- central electrode inside the GM tube
- high voltage supply is connected across the casing of the tube and the central electrode
describe how a cloud chamber is used to detect radiation
- small container full of water vapour
- alpha particles create short, broad tracks while beta particles produce long wispy tracks
discuss the random nature of radioactive emission
radioactive decay is spontaneous and random meaning:
- you cannot predict when a particular nucleus will disintegrate
- you cannot predict which direction a particle will be emitted
- process is unaffected by pressure, temperature, chemical change
some types of nucleus are more unstable and decay faster
