P7 Radioactivity Flashcards
Atomic structure
-building blocks of matter
-radius 1x10*-10 so are very small
-atoms have a tiny, dense nucleus at their centre, with electrons orbiting the nucleus
-radius of nucleus but contains all of the mass of the atom
Parts of atom:
Nucleus contains:
protons- positively charged particles with relative atomic mass of one unit
Neutrons-no charge, and also with a relative atomic mass of one unit
Almost all of the atom is empty space, but moving around nucleus there are:
Electrons-negative charge with almost no mass
Relative mass and relative charge of each particle
Proton:
Relative charge:+1
Relative mass:1
Neutron:
Relative charge:0
Relative mass:1
Electron:
Relative charge: -1
Relative mass: 1/2000
Electrons and protons
-atoms have no charge overall so charge of 0
-charge of protons is same as charge of electron but opposite
-this means the number of protons always equals the number of electrons in a neutral atom
-if an atom loses or gains electrons they become an ion
The absportion and element lf EM radiation
electrons in atoms are arranged in energy levels
-lower energy levels are closer to the nucleus, whilst higher energy levels are further away from the nucleus of atom
-these electrons can absorb energy
-this happens when waves of electromagnetic radiation (such as light and heat) hit them
-when electrons absorb electromagnetic radiation,they move to a higher energy level
-dark coloured objects are good absorbers of radiation
Emission of electromagnetic radiation
-when an electron has moved up an energy level, it will be unstable
-eventually, it will move back down to its original energy level, which will be closer to nucleus
-as it moves back down, it emits a wave of electromagnetic radiation
-all of the colours in the visible spectrum are produced in this way
-the light waves come from electrons moving down energy levels and emitting electromagnetic radiation
Atomic and mass number
Atomic number:
The number of protons in atom is called atomic number (also known as proton number)
-elements in periodic table are ordered by atomic number
-therefore,the number of protons determines which element an atom is
-the atomic number of a particular element is always the same
For example:
-hydrogen has an atomic number of 1. It always just has one proton
-sodium has an atomic number of 11. It has 11 protons
-the atomic number is also equal to the number of electrons in an atom- this is because the nucleus have an overall charge of 0
Mass number:
-the total number of particles in nucleus of an atom is called its mass number
-the mass number is the number of protons and neutrons in an atom
Number of neutrons= mass number-atomic number
Nuclear notation
-the top number is mass number- equal to total number of particles (protons and neutrons) in the nucleus
-the lower number is the atomic number- equal to the total number of protons in the nucleus
Isotopes and differences between isotopes
-isotopes are atoms of the same element that have an equal number of protons but a different number of neutrons
-isotopes occur naturally, but some are more rare than others. E.g about 2 in every 10,000 hydrogen atoms is deuterium
Differences between isotopes:
-the number of neutrons in an atom does not affect the chemical properties of an atom, such as its charge, but only its mass
-the mass of chlorine is given as 35.5 because it has 2 isotopes, one with a mass number of 35band other and other with mass number of 37
-chlorine-35 is about 3 times more abundant than chlorine-37,so the given mass number of chlorine is closer to 35 than 37.
-the number of electrons and protons in different isotopes remains the same
-some isotopes are unstable as they have an imbalance of protons and neutrons
Psoitive ions
-electrons can be knocked out of of the outer energy levels of atoms
-this can be done by:
Friction by objects getting rubbed together
When electrons absorb electromagnetic radiation, they can gain enough energy to leave the atom
From chemical resction
-when one or more electrons are removed from an atom, it becomes positively charged and atom becomes an ion
-ions are more chemically reactive than neutral atoms because of their positive charge
The plum pudding model
-the greek philosopher (Democritus) thought that although objects could be cut into smaller pieces, the smallest possible piece would be indivisible (atomos)
-therefore, atoms were initially though to be tiny spheres that could not be divided before the discovery of the atom
What is plum pudding model?
-Jon Thomson discovered existence of electrons-end of 19th century (1897)
-Thomson proposed plum pudding model-atom was though to consist of negatively charged electrons in a positively charged ‘dough’ or ‘pudding’
-it was known that electrons were much smaller than atoms, so it made sense that they should be embedded within a larger atom
- since electrons have a negative charge, it was reasoned that the rest of the atom would be positive, making atom neutral overall
Alpha scattering and nuclear model
In 1909 a group of scientists were investigating plum pudding model
-physicist Ernest Rutherford was instructing 2 students (Hans Geiger and Ernest Marsden) to carry experiment
-they were directing a beam of alpha particles at a thin gold foil
-they expected the alpha particles to travel through the gold foil, and maybe change direction a small amount
They discovered that:
-most of alpha particles passed straight through the foil
-some of the alpha particles changed direction to be continued through the foil
-a few of the alpha particles bounced back off the gold foil
Nuclear model:
A) Most of the alpha particles passed straight through gold foil-atoms are mostly empty space
B) a few particles were deflected from their path but continued through the gold foil-the nucleus of the atom has a strong positive charge
C) A small number of alpha particles rebounded- the atoms contains a small,heavy nucleus
Rutherford proposed the nuclear model of atom
In the nuclear model:
-nearly all of the mass of the atom is concentrated un the centre of the atom (nucleus)
-the nucleus is positively charged
-negatively charged electrons orbit the the nucleus at a distance
Bohr’s model of the atom
-in 1913, Neils Bohr came up with an improved model of the atom
-He used the nuclear model to create this model
In the Bohr model of atom:
-electrons orbit the nucleus at different distances
-the different orbit distances are called energy levels
-up to 2 electrons orbit first energy level
-up to 8 electrons orbit in 2nd,3rd 4th…..
The discovery of proton and neutron
Discovery of proton:
-nuclear model was improved when the particles within nucleus were discovered
-Ernest Rutherford discovered that the positively charged nucleus of all materials could be broken down into smaller positively charged particles
-he called the positively charged particles in the nucleus as protons
The discovery of neutron:
-it was about 20 years after models of the nucleus at the centre of atom, that the final particle was confirmed
-in 1932, James Chadwick proved the existence of neutrons in the nucleus, he discovered that!
A neutron has a similar mass to the proton
-a neutron has no charge-it is neutral
-the discovery of the neutron gave another,better model of atom
A positive, dense nucleus made from neutrons and protons
Negative electrons on different energy levels orbit the nucleus
Timeline of the atomic model
400 B.C-indivisible objects
1904-plum pudding
1909-nuclear model
1913-Bohr model
1932-neutrons and protons
Comparing model:
Plum pudding model vs nuclear model:
The atom is mostly positively charged ‘dough’- the atom is mostly empty space
Negative electrons are distributed throughout the atom-negative electrons orbit the nucleus at a distance
The mass of the atom is evenly distributed-almost all of the mass is concentrated in the positively charged nucleus
Radioactive decay-unstable nuclei and radiation
Unstable nucleu:
-some atomic nuclei is unstable
-this is because of an imbalance in the forces within the nucleus
-carbon-14 is an isotope lf carbon which is unstable as has 2 extra neutrons
-some isotopes are unstable because of their large size or have too many or too few neutrons
Radiation:
-unstable nuclei can emit radiation to become more stable
- as radiation moves away from nucleus, it takes some energy with it which reduces the overall energy of nucleus and makes it more stable
-the process is called radioactive decay which is a RANDOM process
Radioactive decay-activity
-objects containing radioactive nuclei are called sources of radiation
-sources of radiation decay at different rates which are defined by their activity
Definition: the rate at which the unstable nuclei from a source of radiation decay
Measured in becquerels (Bq)
Radioactive decay-detecting radiation
-a geiger-muller tube is a device used to detect radiation
It is connected to a geiger counter. This is a common way of detecting radiation and measuring count-rate
-Within a Geiger-muller tube, ions are created by radiation passing through it
- the geiger-muller tube can be connected to a geiger counter
-this counts the ions created in geiger-muller tube
- count-rate is the number of decays recorded each second by a detector
Types of radiation
Alpha particles:
- made out of 2 protons and 2 neutrons
Relative mass:4
Relative charge: +2
Range in air:few cm
Penetration: stopped by paper
Ionisation: high
Beta:
-electron (fast-moving)
-produced when a neutron changes into a proton and an electron
Relative mass: 0
Relative charge: -1
Penetration: stopped bythin aluminium
Range in air: few 10s lf cm
Ionisation: medium
Gamma:
-high frequency electromagnetic wave
-highest energy
Relative mass and charge:0
Penetration: reduced by thick lead
Range in air: infinite
Ionisation:low
Neutrons:
-neutrons are one of the two particles found in nucleus of atom
- neutrons are neutral, have no charge
Penetrating power
-alpha,beta and gamma penetrate in different ways
-alpha is least penetrating and gamma is most penetrating
-alpha is stopped by paper
-beta is stopped by a few mm of aluminium
-gamma rays are partially stopped by thick lead
Ionising power:
-all nuclear radiation is capable of ionising atoms that hits
-when an atom is ionised , the number of electrons vhnages
-this gives it a non-zero charge
-alpha radiation is most ionising firm of nuclear radiation as alpha particles have a charge of +2
-gamma is least ionising
Uses of radiation
Alpha particles-smoke detectors:
-alpha particles are used in smoke detectors
-the alpha radiation will normally ionise the air within the detector, creating a current
-the alpha emitter is blocked when smoke enters the detector
-the alarm id triggered by a microchip when the sensor no longer detects alphs
Automatic thickness monitoring in foil production-beta particles:
-the amount of beta radiation passing through the foil depends on the thickness of foil
- if foil is too thick, the detector reading drops and the detector sends a signal to increase pressure of the rollers on the metal sheet.
-this makes foil thinner again
Radiotherapy for treatment of cancer- gamma rays
Sterilisation of food as well
Random nature of radioactive decay
-it cannot be predicted when a particular unstable nucleus will decay
This is because radioactive decay is a random process, this means that:
-there is an equal probability of any nucleus decaying
- it cannot be known which particular nucleus will decay next
- it cannot be known at what time a particular nucleus will decay
-the rate lf decay is unaffected by the surrounding conditions
-it is inly possible to estimate the probability of a nuclei decaying in a given time period
Dice analogy:
-imagine rolling a dice and trying to get a ‘6’
-each time they roll, they do not know what the result will be
- but they know there is 1/6 probability that it will be a 6
- if roll dice 1000 times, ti wld be very unlikely that they wld role a 6 at least once
-the random nature of decay can be demonstrated by observing the count rate of a geiger-muller tube
When a Gam tube is placed near a radioactive source, the counts are found to be irregular and cannot be predicted
Each count represents a decay lf unstable nucleus
-these fluctuations in count rate on the GM tube provide evidence of randomness of radioactive decay
Hlaf-life and calculating half-life
Half life is defined as: the time it takes for the number of nuclei of a sample of radioactive isotopes to decrease by half
-in other word the time it takes for the activity lf a sample to fall to half of its original level
-different isotopes have different half-lives and can vary from a fraction of a second to billion of years in length
Using half life:
-uranium 235 has a half life of 704 million years which means that it would take 704 million years for the activity of uranium 235 to decrease to half its original amount
Carbon 14 has a half life of 5700 years
So after 5700 years, there would be 50% of its original amount remaining
After 2 half lives (11400 years) there would be 25% of the carbon 14 remaining
Which each half-life, the amount decreases by half and is constant
Calculating half-life:
-measure the initial activity of the sample
-determine the half-life of this original activity
-measure how the activity changes over time
-the time take for the activity to decrease to half of its original value is the half-life
The count rate after n half lives = the initial count rate/ 2*n
Calculating radioactive decay
-with each half life the activity of a sample decreases by half
Method 1:
-determine the number of half lives elapsed
-multiply the number 1 by half for each half-life elapsed
E.g if 4 half lives have elapsed: 1 x 0.5 x 0.5 x 0.5 x 0.5= 1/16
-this is the same as ration remaining 1 remaining: 16 nuclei (1:16)
Method 2 raising to a power:
-determine the half lives elapsed
- use your calculator to raise 1/2 to the number of half-lives
-e.g, if 4 half-lives have elapsed: (1/2)*4 = 1/16
Contamination
Defined as:
-the unwanted presence of materials containing radioactive atoms on other materials
-a substance is only radioactive if it contains radioactive atoms that emit radiation
-contamination occurs when a radioactive isotope gets onto a material where kt should not be
-as a result, small amounts of isotope in contaminated areas will emit radiation and the material becomes radioactive
-last for a long period of time
