Radioactivity Flashcards
How can we explain some of the properties of different elements?
By thinking about the particles that each element contains. We call these particles atoms. Chemical reactions occur when different atoms in a substance become joined in different ways
What did the experiments that JJ Thompson (1856-1940) carried out show? And what did he do to support his evidence?
They showed that atoms contain much smaller subatomic particles called electrons. These have a negative charge and hardly any mass. Thompson supported using a new model that could explain this new evidence. The plum pudding model. The new model described that atom as a ‘pudding’ made of positively charged material, with negatively charged electrons (the ‘plums’) scattered through the it.
Between 1909 and 1913 a team of scientists led by Ernest Rutherford (1871-1937) carried out a series of experiments called the gold foil experiment what did it involve? And what did they discover?
That involved studying what happens when positively charged subatomic particles called alpha particles passed through various substances, the scientists discovered that most of the alpha particles passed through the gold foil, a few bounced back. The plum pudding model couldn’t explain this. Rutherford suggested that atoms were mostly empty space, with most if their mass in a tiny central nucleus with a positive charge and electrons moving around the nucleus.
Go to page 90 and 91
And look at the diagrams
What is the radius of a nucleus and an atom?
Nucleus - 1 X 10 to the power of -15
Atom - 1 X 10 to the power of -10
So the atom itself is 100 000 times bigger than the nucleus inside it
Where is the mass of an atom concentrated?
In its nucleus
What is the nucleus itself made up of?
Smaller particles called nucleons. Nucleons can be protons or neutrons. All subatomic particles have very small masses so it is easier to describe their relative masses
How do we describe subatomic particles relative masses?
We give the proton a mass of 1 and we compare the masses of the other subatomic particles relative to this
Go to page 92
And look at the table carefully
What does the atomic number or the proton number represent?
Atoms of a particular element that have the same number of protons, this number is the atomic number. Atoms of different elects have different numbers of protons and so have different atomic numbers
What gives the nucleus its positive charge?
Neutrons have no charge so it’s the protons that give the needless its positive charge. Atoms have the same number of electrons as protons and so atoms are always electrically neutral (they have no overall charge)
What is the mass number or the nucleon number?
The total number of protons and neutrons in the nucleus
What are isotopes?
Two atoms of the same element will always have the same atomic number, but they can have different mass numbers if they contain different numbers of neutrons. Atoms of a single element that have different numbers of neutrons are called isotopes.
What can electrons only exist in?
Certain orbits around the nucleus called electron shells, each electron she’ll is at a different energy level
In a neon tube, what do the neon atoms do?
The neon atoms absorb energy transferred by the electricity because the electrons jump to higher shells. When the electrons fall back again they emit energy as electromagnetic radiation.
What is the emission spectrum (the diagram is on page 94)
When each colour is a wavelength of light. The emission spectrum is different for each element
Go to page 94 and 95
And look at the diagrams
Turn to page 94 for this question. Gases can also also be energy transferred by electromagnetic radiation, such a s visible light. What does the bottom part of photo c show?
It shows the parts of the visible spectrum that neon gas absorbs when light passes through it. This is the absorption spectrum for neon. The wavelengths of light that neon gas absorbs are the same wavelengths that is emits
How did Neils Bohr (1885-1962) amend rutherfordium model of the atom?
He amended it to explain observations by suggesting that electrons can only be in certain fixed orbits (electron shells) around the nucleus. They cannot be part-way between two orbits. This model could explain the lines in emission and absorption spectra
Explain ionisation
Sometimes an atom gain so much energy that one or more of the electrons can escape from the atom altogether. An atom that has lost or gained electrons is called an ion. Radiation that causes electrons to escape is called ionising radiation
What is a positive ion?
An atom has the same number of protons and electrons, so overall it has no charge. Is an atom loses an electron, it then has one more proton than it has electrons. It has an overall positive charge and is called a positive ion
What is background radiation?
We are constantly being exposed to ionising radiation at a low level, from space and from naturally radioactive substances in the the environment. This is background radiation
What is the main source of background radiation and how is it produced?
Radon gas. This radioactive gas is produced by rocks that contain small amounts of uranium. Radon diffuses into the air from rocks and soil and can build up in houses, especially where there is poor ventilation. The amount of radon in the air depends on the type of rock and its uranium content. Rock type and building stone vary around the country and so does the amount of radon
How do some foods contribute to your exposure to back ground radiation?
Because they naturally contain small amounts of radioactive substances
What do hospital treatments such as X-Ray’s, gamma-Ray scans and cancer treatment contribute to?
Peoples exposure to back ground radiation
high-energy, charged particles stream out if the sun and other stars. They are known as cosmic rays. What are they a form of?
Radiation, many cosmic rays are stopped in the upper atmosphere but some still reach the earths surface
How can radioactivity be detected?
Using photographic film, which becomes darker and darker as more radiation reaches it. However, the film has to be developed in order to measure the amount of radiation (the dose). People who work with radiation often wear film badges (called dosimeters) to check how much radiation they are exposed to. Newer dosimeters use materials that change colour without needing to be developed
How can the radioactivity of a source also be measured?
Using a Geiger-müller (GM) tube. Radiation passing through the tube ionises gas inside it and allows a short pulse of current to flow. A GM tube can be connected to a counter, to count the pulses of current, or the GM tube may give a click each time radiation is detected. The count rate is the number of clicks per second or minute
When scientists measure the radioactivity of a source, they need to measure the background radiation first by taking several readings and finding the mean. What happens next?
This mean is then subtracted from measurements
The nucleus of a radioactive substance is unstable, what does this mean? And what happens?
It can easily change or decay. When decay occurs radiation is emitted which can cause the nucleus to lose energy and become more stable. You cannot predict when a nucleus will decay - it’s a random process
What do alpha particles contain?
Two protons Two neutrons Relative mass of 4 No electrons Have a charge of +2 Can be written as alpha sign or helium sign
What are beta particles?
High-energy, high-speed electrons. They have a relative mass of 1/1835 and a charge of -1. They can be written as B-
What are positrons?
High-energy, high-speed particles with the same mass of electrons but a charge of +1. They can be written a B+
What are gamma rays?
High-frequency electromagnetic waves (they travel at the speed of light). They do not have an electric charge
What can also be emitted from an unstable nucleus?
Neutrons, they have a relative mass of 1 and no electric charge
What are the characteristics of alpha particles?
Will travel a few cm in air
Very ionising
Chen be stopped by a sheet of paper
Explain alpha particles
Alpha particles are emitted a high speeds. Due to this and their high routine mass, they transfer a lot of energy and so are good at ionising atoms they encounter. However, each time they ion side an atom they lose energy. Since they produce many ions in a short distance, they lose energy quickly and have a short penetration distance
What are the characteristics of beta particles?
Will travel a few metres in air
Moderately ionising
Can be stopped by 3mm thick aluminium
