P4 - Atomic structure Flashcards
Which elements are diatomic?
iodine, I2
bromine, Br2
chlorine, Cl2
fluorine, F2
oxygen, O2
nitrogen, N2
hydrogen, H2
A mnemonic can be used to remember these elements:
I Bring Clay For Our New House.
What is the Mass number or Relative Atomic Mass?
The total number of protons and neutrons is called the mass number
What is the Atomic number?
The number of protons is called the atomic number.
Atomic Symbols
Mass number and atomic number are two important pieces of information about an atom.
An atom can be represented using the symbol notation:
Where:
A is the mass number
Z is the atomic number
X is the symbol
For example, chlorine (Cl) can be shown as:
Chlorine atom with mass number 35 and atomic number 17.
This symbol shows that chlorine has 35 particles in the nucleus (protons and neutrons), 17 of which are protons. It also tells us that chlorine has 18 neutrons (35 - 17) and, as the number of electrons and protons are equal in a neutral atom, chlorine also has 17 electrons.
What are energy levels?
Different distances of electrons from the nucleus.
Energy levels which are further from the nucleus are at a higher energy than those which are closer to the nucleus.
Electrons can change energy levels if the atom absorbs electromagnetic radiation from a lower to a higher energy level.
Atoms can also emit EM radiation back and the electrons will return to a lower energy level.
What are Isotopes?
Isotopes are atoms of the same element that have different numbers of neutrons.
What does it mean if an electron has moved up an energy level?
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 the 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
How do you find the number of neutrons?
- The total number of particles in the nucleus of an atom is called its mass number
- The mass number is the number of protons and neutrons in the atom
- The number of neutrons can be found by subtracting the atomic number from the mass number
Number of Neutron = Mass Number - Atomic Number
- For example, if a sodium atom has a mass number of 23 and an atomic number of 11, then the number of neutrons would be 23 – 11 = 12
What does the number of neutrons affect in an atom?
- The number of neutrons in an atom does not affect the chemical properties of an atom, such as its charge, but only its mass
- This is because neutrons have no charge but do have mass
- In the periodic table, the mass number of Chlorine is often given as 35.5
- The mass number of Chlorine is given as 35.5 because it has roughly equal numbers of isotopes with a mass number of 35, and of 36
- The number of electrons and protons in different isotopes remains the same
- Isotopes tend to be more unstable due to the imbalance of protons and neutrons
How can electrons be knocked out of their energy levels?
- Electrons in the outer energy level can be knocked out from an atom
- This can happen in a number of ways:
- When objects are rubbed together, electrons can be removed by friction
- When electrons absorb electromagnetic radiation they can gain enough energy to leave the atom
- From chemical reactions
- When one or more electrons are removed from an atom, it becomes positively charged
- This is because an electron is negatively charged
- The atom becomes a positive ion
- An ion is an atom or particle with a non-zero charge
What is radioactive decay?
Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable. This is a random process called radioactive decay.
Why do Nuclei become unstable?
- Some atomic nuclei are unstable
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This is because of an imbalance in the forces within the nucleus
- Forces exist between the particles in the nucleus
- Carbon-14 is an isotope of carbon which is unstable
- It has two extra neutrons compared to stable carbon-12
- Some isotopes are unstable because of their large size or because they have too many or too few neutrons
What is radiation?
- Unstable nuclei can emit radiation to become more stable
- Radiation can be in the form of a high energy particle or wave
- As the radiation moves away from the nucleus, it takes some energy with it
- This reduces the overall energy of the nucleus
- This makes the nucleus more stable
- The process of emitting radiation is called radioactive decay
- The process of emitting particles or waves of energy from an unstable nucleus is called radioactive decay.
What is Activity and its unit?
- The rate at which the unstable nuclei from a source of radiation deccays(measured in Becquerels or Bq)
- 1 Becquerel is equal to 1 nucleus in the source decaying in 1 second
What is count rate
How do you detect Radiation?
Count-rate is the number of decays recorded each second by a detector
- A Geiger-Muller tube is a device used to detect radiation
- Within the tube, ions are created by radiation passing through it
- The Geiger-Muller tube can be connected to a Geiger counterThis counts the ions created in the Geiger-Muller tube
What are the four types of radiation?
- When an unstable nucleus decays it emits radiation, called nuclear radiation
- There are different types of radiation that can be emitted:
- Alpha
- Beta
- Gamma
- Neutrons
Properties of Alpha particles
- The symbol for alpha is α
- An alpha particle is the same as a helium NUCLEUS
- This is because they consist of two neutrons and two protons
- Alpha particles have a charge of +2
- This means they can be affected by an electric field
Properties of Beta particles
Beta Particles
- The symbol for beta is β
- Beta particles are fast-moving electrons
- They are produced in nuclei when a neutron changes into a proton and an electron
- Beta particles have a charge of -1
- This means they can be affected by an electric field
What is the penetrating power of Alpha, Beta and Gamma radiation/
Alpha, beta and gamma are different in how they penetrate materials. Alpha is the least penetrating, and gamma is the most penetrating
- Alpha is stopped by paper, whereas beta and gamma pass through it
- Beta is stopped by a few millimetres of aluminium
- Gamma can pass through aluminium
- Gamma rays are only partially stopped by few millimetres thick lead
What is the ionising Power of Alpha & Gamma radiation?
- All nuclear radiation is capable of ionising atoms that it hits
- When an atom is ionised, the number of electrons it has changes
- This gives it a non-zero charge
- Alpha radiation is the most ionising form of nuclear radiation
- This is because alpha particles have a charge of +2
- Gamma radiation is the least ionising form of nuclear radiation
How are Alpha Particles used in 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 is triggered by a microchip when the sensor no longer detects alpha
Explain why is alpha radiation used in smoke detectors, and not beta or gamma radiation.
- Consider the different properties of alpha, beta and gamma:
- Alpha is the most weakly penetrating and strongest ioniser
- Beta and gamma have stronger penetrating power and weaker ionising power
- If beta or gamma radiation were used in this situation then they would pass straight through the smoke and the alarm would not go off
- Therefore, since alpha is absorbed by smoke, and beta and gamma are not, this makes it most suitable for use in a smoke detector
What happens in Alpha Decay?
- An alpha particle is a helium nucleus
It is made of 2 protons and 2 neutrons - When the alpha particle is emitted from the unstable nucleus, the mass number and atomic number of the nucleus changes
- The mass number decreases by 4
- The atomic number decreases by 2
- The charge on the nucleus also decreases by 2
- This is because protons have a charge of +1 each
Decay Equations for Alpha particles
The mass number decreases by 4
The atomic number decreases by 2
- During decay equations, the sum of the mass and atomic numbers before the reaction must be the same as the sum of the mass and atomic numbers after the reaction
- The following equation shows Polonium-212 undergoing alpha decay
- It forms Lead-208 and an alpha particle
What happens during beta decay?
- During beta decay, a neutron changes into a proton and an electron
- The electron is emitted and the proton remains in the nuclei
- A completely new element is formed because the atomic number changes
- A beta particle is a high-speed electron
- It has a mass number of 0
- Therefore, the mass number of the decaying nuclei remains the same
- Electrons have an atomic number of -1
- This means that the new nuclei will increase its atomic number by 1 in order to maintain the overall atomic number before and after the decay
Decay equation for Beta Decay
- The following equation shows carbon-14 undergoing beta decay
- It forms nitrogen-14 and a beta particle
- It will form a new element
What happens during Gamma Decay?
- During gamma decay, a gamma ray is emitted from an unstable nucleus
- The process that makes the nucleus less energetic but does not change its structure
The emission of a gamma ray does not cause the mass or the charge of the nucleus to change
The gamma ray that is emitted has a lot of energy, but no mass or charge
Decay equation for Gamma rays
- The gamma ray that is emitted has a lot of energy, but no mass or charge
- Here is an example of Uranium-238 undergoing gamma decay
- Notice that the mass number and atomic number of the unstable nuclei remains the same during the decay
How do we measure Half-Life?
- Scientists can measure the half-lives of different isotopes accurately:
- Uranium-235 has a half-life of 704 million years
- This means it would take 704 million years for the activity of a uranium-235 sample 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 the original amount of carbon-14 remaining
- After two half-lives, or 11 400 years, there would be just 25% of the carbon-14 remaining
- With each half-life, the amount remaining decreases by half
- The time it takes for the activity of the sample to decrease from 100 % to 50 % is the half-life
- It is the same length of time as it would take to decrease from 50 % activity to 25 % activity
- The half-life is constant for a particular isotope
How do you calculate half life?
The radioisotope technetium is used extensively in medicine. The graph below shows how the activity of a sample varies with time.
Determine the half-life of this material.
Step 1: Draw lines on the graph to determine the time it takes for technetium to drop to half of its original activity
Step 2: Read the half-life from the graph
- In the diagram above the initial activity, A0, is 8 × 107 Bq
- The time taken to decrease to 4 × 107 Bq, or ½ A0, is 6 hours
- The time taken to decrease to 2 × 107 Bq is 6 more hours
- The time taken to decrease to 1 × 107 Bq is 6 more hours
- Therefore, the half-life of this isotope is 6 hours
How is Irradiation used in medicine?
Medical irradiation
Doctors also use radioactive sources for a number of reasons, eg:
- sterilisation of surgical instruments
- beams of gamma rays, called a gamma knife, can be used to kill cancerous tumours deep inside the body
These beams are aimed at the tumour from many different directions to maximise the dose on the tumour but to minimise the dose on the surrounding soft tissue. This technique can damage healthy tissue, so careful calculations are done to establish the best dose - enough to kill the tumour, but not so much so that the healthy tissue is damaged.
In medical applications that involve using radioactive sources, efforts are made to ensure that irradiation does not cause any long-term effects. This is done by considering:
- the nature of decay (alpha, beta or gamma)
- the half-life (long enough for the isotope to produce useful measurements, but short enough for the radioactive sources to decay to safe levels soon after use)
- toxicity
If the half-life chosen is too long, the damaging effects of the radiation would last for too long and the dose received would continue to rise.
How is contamination used to find leaks?
Contamination to check for leaks
The isotope used for this purpose must:
- be a gamma emitter
- have a half-life of at least several days to allow the emissions to build up in the soil
- not be poisonous to humans as it will form part of the water supply
Water supplies can be contaminated with a gamma-emitting radioactive isotope to find leaks in pipes . Where there is a leak, contaminated water seeps into the ground, causing a build-up of gamma emissions in that area. The build-up of gamma emissions can be found using a Geiger-Muller tube. This makes it easier to decide where to dig to find the leak.
Pros and Cons of contamination
Compare contamination and irradiation
Protecting against Irradiation and Contamination
- It is important to reduce the risk of exposure to radiation
- Radiation can mutate DNA in cells and cause cancer
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Shielding is used to absorb radiation
- Lead lined suits are used to reduce irradiation for people working with radioactive materials
- The lead absorbs most of the radiation that would otherwise hit the person
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To prevent contamination an airtight suit is used by people working in an area where there may have been a radiation leak
- This prevents radioactive atoms from getting inside the person
- keep radioactive sources like technetium-99 shielded (preferably in a lead-lined box) when not in use
- wear protective clothing to prevent the body becoming contaminated should radioactive isotopes leak out
- avoid contact with bare skin and do not attempt to taste the sources
- wear face masks to avoid breathing in materials
- limit exposure time - so less time is spent around radioactive materials
- handle radioactive materials with tongs in order to keep a safer distance from sources
- monitor exposure using detector badges, etc
