Topic 4: Atomic Structure Flashcards
Why are some atomic nuclei unstable?
They are unstable due to an imbalance in the forces within the nucleus, often caused by having too many or too few neutrons.
What is the difference between carbon-12 and carbon-14?
Carbon-12 is stable, while carbon-14 is unstable because it has two extra neutrons.
What is radioactive decay?
It is the process by which unstable nuclei emit radiation (particles or waves) to become more stable.
Is radioactive decay a predictable process?
No, it is random. Scientists cannot predict exactly when a particular nucleus will decay.
What is radiation?
Radiation refers to the high-energy particles or waves emitted from an unstable nucleus during radioactive decay.
What is activity in radioactive decay?
Activity is the rate at which unstable nuclei decay, measured in Becquerels (Bq).
How many nuclei decay per second in a source with an activity of 2000 Bq?
2000 nuclei decay per second.
How do you calculate the number of decays over a time period?
Multiply the activity (Bq) by the time period in seconds.
What is a Geiger-Muller tube used for?
It is used to detect radiation by counting the ions created when radiation passes through it.
What is count rate?
Count rate is the number of decays detected per second by a device like a Geiger-Muller tube.
How do you calculate count rate?
Divide the total number of decays by the time period in seconds.
What is the difference between activity and count rate?
Activity measures the rate of decay of unstable nuclei, while count rate measures the rate of detected radioactive emissions.
What happens to the energy of a nucleus during radioactive decay?
The energy of the nucleus decreases as radiation is emitted, making the nucleus more stable.
What is the unit of activity?
The unit of activity is the Becquerel (Bq), where 1 Bq = 1 decay per second.
How does photographic film detect radiation?
It changes color when exposed to radiation.
What is nuclear radiation?
When an unstable nuclei decays and emits radiation
What is an alpha particle?
- The symbol for alpha is α
- An alpha particle is the same as a helium nucleus
- Consist of two neutrons and two protons
- Alpha particles have a charge of +2
- Can be affected by an electric field
What is a beta particle?
- The symbol for beta is β
- Beta particles are fast-moving electrons
- Produced in nuclei when a neutron changes into a proton and an electron
- Beta particles have a charge of -1
- Can be affected by an electric field
What are gamma rays?
- The symbol for gamma is γ
- Gamma rays are electromagnetic waves
- Have the highest energy of the different types of electromagnetic waves
- Have no charge
What are neutrons?
- The symbol for a neutron is n
- Neutrons are one of the two particles found in the nucleus of atoms
- Neutral, Have no charge
What is the range in air for the radiation particles?
Alpha - Few cm
Beta - Few 10’s of cm
Gamma - Infinite
What is the penetration for the radiation particles?
Alpha - Stopped by paper
Beta - Stopped by few mm Aluminium
Gamma - reduced by few mm Lead
What is the ionisation for the radiation particles?
Alpha - High
Beta - Medium
Gamma - Low
What is radiation used for?
- Producing electricity through nuclear fission
- Medical procedures including diagnosis and treatment
- Testing material
- Determining the age of ancient artefacts
- Checking the thickness of materials
- Smoke detectors
How are alpha particles 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
What happens during alpha decay?
- Alpha particle is emitted from an unstable nucleus
- Completely new element is formed in process
What happens during beta decay?
- Neutron changes into a proton and an electron
- Electron is emitted, proton remains in nuclei
- Completely new element is formed as atomic number changes
What happens in gamma decay?
- Gamma ray is emitted from unstable nucleus
- Nucleus becomes less energetic but doesn’t change structure
Describe the gamma ray emitted during gamma decay
Ray emitted has a lot of energy, but no mass or charge
What can be inferred due to the random process of radioactive decay?
- 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 of decay is unaffected by the surrounding conditions
- It is only possible to estimate the probability of a nuclei decaying in a given time period
What is half-life?
The time it takes for the number of nuclei of a sample of radioactive isotopes to decrease by half
How do you calculate half-life?
- To calculate the half-life of a sample, the procedure is:
> Measure the initial activity, A0, of the sample
> Determine the half-life of this original activity
> Measure how the activity changes with time - The time taken for the activity to decrease to half its original value is the half-life
What is contamination?
The unwanted presence of materials containing radioactive atoms on other materials
How is a substance radioactive?
- Contains radioactive atoms that emit radiation
When does contamination occur?
When a radioactive isotope gets onto a material where it should not be
What is irradiation?
The process of exposing a material to alpha, beta or gamma radiation
What does irradiation do?
- Does not make material radioactive
- Can kill living cells
What is irradiation used for?
- Used as method of sterilisation
Why is contamination dangerous?
- Can get into human body
- Internal organs will be irradiated as source emits radiation as it moves through body
What causes more harm: irradiation or contamination?
Contamination
- Continuous exposure to radiation it will produce
What is used to absorb radiation?
Shielding
What type of shielding is used to prevent irradiation?
- 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
What is used to prevent contamination?
Airtight suit
- Prevents radioactive atoms getting inside person
- Used in areas with radiation leak
What is background radiation?
The radiation that exists around us all the time
What are the two types of background radiation?
- Natural Sources (e.g rocks, cosmic rays, foods)
- Man-made Sources (e.g exposure from medical testing)
What is the amount of radiation in a person received known as and measured in?
Dose , seiverts (Sv)
What is a big dose of radiation?
One Dose
- Cause acute radiation poisoning
How much radiation will people normally receive in one year?
3 mSv
What is used to measure amount of radiation in particular areas?
Dosemeter
What does a dosemeter measure?
Amount of radiation in particular areas
What affects level of background radiation and radiation dose?
Person’s occupation or location (living closer to a source)
Do radioactive isotopes have the same half-lives?
No, different radioactive isotopes can have very different half-lives
What happens if an isotope has a very short half-life?
- Decays very quickly
- Emits a lot of radiation in a short amount of time
What happens if an isotope has a very long half-life?
- Decays very slowly
- May not emit a lot of radiation, but remains radioactive for a long time
What is a tracer?
A radioactive isotope that can be used to track the movement of substances, like blood, around the body
Why are gamma rays used for tracers?
Gamma Rays are less ionising then other forms of radiation so harm caused to patient is minimised
What must you also choose when using a tracker other than a gamma emitter?
- The amount of isotope used is kept to a minimum to reduce people’s exposure to radiation
- Isotopes are chosen that have short half-lives of around a few hours: Long enough to carry out the procedure, but not so long that they cause long term harm
What is radiotherapy?
Treatment of cancer using radiation
What happens during external radiotherapy?
- Gamma Rays directed at cancerous tumour
- The machine rotates to target the tumour from different angles
- This minimises the exposure of healthy tissue to the gamma rays and minimises damage to healthy cells
What is the risk of using radiation in medicine?
- Can kill or damage living cells
- Cause cancer
- Cause mutations
Why is radiation used despite its risks in medicine?
Benefits of using radiation in medicine can out weigh the potential risks
What is nuclear fission?
The splitting of a large, unstable nucleus into two smaller nuclei
Name two isotopes which undergo fission and are used as fuels in power stations
Uranium and Plutonium
What happens during nuclear fission?
- When a neutron collides with an unstable nucleus, the nucleus splits into two smaller nuclei (called daughter nuclei) as well as two or three neutrons
- Gamma rays are also emitted
What happens to the products of fission?
- Move away very quickly
- Energy transferred from nuclear potential energy to kinetic energy
What is spontaneous fission?
Where nuclei undergoes fission without additional energy being put into nucleus
What is induced fission>
Where the nuclei undergoes fission by absorbing a neutron
Is fission a chain reaction?
Yes, the two or three neutrons produed can start another fission reaction, which again creates further excess neutrons
What happens in a nuclear reactor?
- A chain reaction is required to keep the reactor running
- When the reactor is producing energy at the correct rate, the number of free neutrons in the reactor needs to be kept constant
- This means some must be removed from the reactor
How are neutrons removed?
Control Rods
- Absorb neutrons without becoming dangerously unstable themselves
Why are uncontrolled chain reactions dangerous?
- Each new fission reaction releases energy
- The number of neutrons available increases quickly, so the number of reactions does too
What uses an uncontrolled chain reaction?
Nuclear Weapon
- Releases huge amount of energy in a short period of time as an explosion
What is nuclear fusion?
When two light nuclei join to form a heavier nucleus
Why is nuclear fusion hard to reproduce on Earth?
Process requires extremely high temperatures to maintain
Where does nuclear fusion normally take place?
- Centre of stars
- Hydrogen Nuclei fuse into helium nuclei and release large amounts of energy
What is the difference in size between the atom and the nucleus?
Radius of nucleus is over 10,000 times smaller than whole atom but contains almost all the mass of atom
What are the properties of the proton?
- In the nucleus
- +1 Charge
- Mass of 1
What are the properties of the neutron?
- In the nucleus
- Charge of 0
- Mass of 1
What are the properties of the electron?
- Orbiting the nucleus
- Charge of -1
- Mass of 1/2000 (Negligible)
Describe energy levels
- Electrons orbit nucleus at different levels
- Certain number of electrons can occupy each energy level
- Higher the energy level, the further the distance
Describe the number of electrons which can occupy in each level
1st Energy Level - 2 Electrons
2nd Energy Level - 8 Electrons
3rd Energy Level - 8 Electrons
What happens when EM Radiation is absorbed by an electron?
Move to a higher energy level
Why are dark coloured objects good absorbers of electrons?
Do not reflect energy that hits them
What happens when an electron moves down from a higher to lower energy level?
When an electron moves down to a lower energy level (closer to the nucleus), it emits a wave of electromagnetic radiation. This is how all colors in the visible spectrum are produced.
Why do electrons eventually move from higher to lower energy levels?
Electrons in higher energy levels are unstable. They will eventually return to their original, more stable energy level that is closer to the nucleus, emitting electromagnetic radiation in the process.
What is the atomic number?
The number of protons in an atom (also called the proton number). The atomic number determines which element an atom is.
How are elements arranged in the periodic table?
Elements in the periodic table are ordered by their atomic number.
What is the relationship between atomic number and electrons in a neutral atom?
The atomic number is equal to the number of electrons in an atom because atoms have the same number of electrons and protons to have no overall charge.
What is the mass number?
The total number of particles in the nucleus of an atom (protons + neutrons).
How do you calculate the number of neutrons in an atom?
Number of Neutrons = Mass Number - Atomic Number
Why aren’t electrons included in the mass number calculation?
Electrons have a tiny mass compared to neutrons and protons. Their mass is considered negligible when compared to the particles in the nucleus.
What is nuclear notation?
The way of showing the mass number and atomic number of an atom by writing them with the atomic symbol. The mass number is written as a superscript and the atomic number as a subscript to the left of the element symbol.
In nuclear notation, what does the top (superscript) number represent?
The mass number, which equals the total number of particles (protons and neutrons) in the nucleus.
In nuclear notation, what does the bottom (subscript) number represent?
The atomic number, which equals the total number of protons in the nucleus.
What is the atomic number of hydrogen?
1 (It has 1 proton)
What is the atomic number of sodium?
11 (It has 11 protons)
What is the atomic number of uranium?
92 (It has 92 protons)
If a sodium atom has a mass number of 23 and an atomic number of 11, how many neutrons does it have?
12 neutrons (23 - 11 = 12)
Front
Back
What are isotopes?
Atoms of the same element that have an equal number of protons but a different number of neutrons.
Name the three isotopes of hydrogen.
Hydrogen (1 proton, 0 neutrons), Deuterium (1 proton, 1 neutron), and Tritium (1 proton, 2 neutrons).
How common is Deuterium?
About 2 in every 10,000 Hydrogen atoms is Deuterium.
How common is Tritium?
About 1 in every billion billion hydrogen atoms is Tritium.
How do different isotopes of the same element affect chemical properties?
The number of neutrons does not affect the chemical properties of an atom, such as its charge, but only its mass.
Why do neutrons only affect the mass of an atom?
Because neutrons have no charge but do have mass.
Why is chlorine’s mass number on the periodic table listed as 35.5?
Because chlorine has 2 isotopes (chlorine-35 and chlorine-37), with chlorine-35 being about three times more abundant than chlorine-37.
What happens when electrons are knocked out from an atom?
When one or more electrons are removed from an atom, it becomes positively charged and is called a positive ion.
What is an ion?
An ion is an atom or particle with a non-zero charge.
What are three ways electrons can be removed from an atom?
1) Electrons can be removed by friction when objects are rubbed together 2) Electrons can absorb electromagnetic radiation and gain enough energy to leave the atom 3) From chemical reactions.
Why are ions more chemically reactive than atoms?
Because of their positive charge.
What is a model in scientific terms?
A model is a way of describing something in order to explain the way it behaves.
Who was the first to describe the idea of atoms?
Greek and Indian philosophers, including the Greek philosopher Democritus.
What does the Greek word ‘atomos’ mean?
Indivisible.
Who discovered electrons?
Physicist Joseph John Thomson at the end of the 19th Century.
Describe the Plum Pudding model of the atom.
The atom was thought to consist of negatively charged electrons (the ‘plums’) in a positively charged ‘dough’ or ‘pudding’.
Why was the Plum Pudding model replaced?
Experiments in 1909 (Rutherford’s scattering experiment) produced results that could not be explained by the Plum Pudding model.
Who conducted the alpha scattering experiment?
Physicist Ernest Rutherford instructed two of his students, Hans Geiger and Ernest Marsden, to carry out the experiment.
What were the three main observations from the alpha scattering experiment?
1) Most alpha particles passed straight through the gold foil 2) Some alpha particles changed direction but continued through the foil 3) A few alpha particles bounced back off the gold foil.
What conclusion was drawn from most alpha particles passing straight through the gold foil?
Most of the atom is empty space.
What conclusion was drawn from some alpha particles being deflected?
The nucleus has a positive charge which repels the positively charged alpha particles.
What conclusion was drawn from a few alpha particles bouncing back?
The nucleus contains most of the mass of the atom and it is very dense.
Describe the nuclear model of the atom proposed by Rutherford.
In the nuclear model: 1) Nearly all of the mass of the atom is concentrated in the center (the nucleus) 2) The nucleus is positively charged 3) Negatively charged electrons orbit the nucleus at a distance.
How did Niels Bohr improve the nuclear model in 1913?
He proposed that electrons orbit the nucleus at different specific distances called energy levels, with up to 2 electrons in the first level, and up to 8 in the second and third levels.
Why was the Bohr model accepted over the nuclear model?
1) It explained findings from different experiments better 2) It explained absorption and emission of electromagnetic radiation 3) Theoretical calculations matched experimental results.
Who discovered the proton and when?
Ernest Rutherford discovered that the nucleus contained positively charged particles he called protons.
What is the elementary charge?
The amount of charge in a proton, which is the same as the charge of a Hydrogen nucleus.
What is the origin of the word ‘proton’?
‘Proto’ means original - protons are the original unit of positive charge.
Who discovered the neutron and when?
James Chadwick proved the existence of neutrons in the nucleus in 1932.
What are the properties of a neutron?
A neutron has a mass similar to the proton but has no charge - it is neutral.
What charge does an alpha particle have?
An alpha particle has a charge of +2 (it has 2 protons and no electrons).
Why did the Plum Pudding model need to be replaced?
The experimental results of the gold foil experiment could not be explained using the plum pudding model.
Why is it important that experimental results match theoretical predictions?
If the predictions are correct, then this proves that the model being used is correct.
