Scientist used to believe that the atom was a neutral charged object which had negative charges evenly spaced
This was known as the plum pudding model
However, Scientists realised that the atom follows a different model and this was proven by Rutherford's alpha scattering experiment
Alpha particles where ejected towards a thin gold leaf and there were different observations
The observations of this experiment must be remembered.
This is because, the atom is mostly empty space and this also shows that the positive & negative charges are not evenly spaced.
This is because when the alpha particles passes near the nucleus they are deflected by a bit. This shows the nucleus is positively charged and so the alpha particles +2 experiences a repulsive force
This is around 1/20000 alpha particles and this shows that the nucleus of an atom is so tiny and densed at the center and positively charged. When alpha particle collide head on or sideways to the nucleus, they will deflect by an angle more that 90°
You will need to know how to draw some of these deflections
1. Most pass through without any deflection
2. Some pass with little deflection
3. Very few pass with a large deflections especially when the alpha particle collides with the tiny positive nucleus
Also compare the idea of the plum pudding model and Rutherford's experiment.
You will need to know some basic terms of an atom
Let us first see the structure of an atom
It contains both neutrons and protons of the atom and so it's positively charged. Why? This is because, neutrons are neutral and protons are positively charged. The nucleus is very small and highly densed and is at the center of the atom.
These are known to be fundamental particles which orbit around the nucleus of an atom in energy levels. If you need to know more go to the leptons section.
The proton number of an atom identifies the element. This is a fundamental fact and this proves that isotopes are of the same element
It is the same as the nucleon number
Particles in the nucleus of an atom are called nucleons. So if there are 7 protons and 8 neutrons, it has 15 nucleons.
The definition defines only atoms and not for ions
So really we usually compare atoms rather than ions so that's why isotopes usually have the same chemical properties because, they have the same number of electrons in the valence shell
However, the physical properties will be different such as the melting point and the density
This is because, the Physical properties depends on the nucleur properties of an atom and the number of electrons determine the chemical properties
So isotopes are always neutral and have the same proton number but, have different neutron or mass number
To calculate the mass of the nucleus, we must find the total number of neutrons and protons (nucleon number). We also need to remember that the mass of a proton is same as the mass of a neutron and it is 1.67*10-27kg
So we Multiply to find the total mass
To find the volume of the nucleus, we need to to know the diameter of a typical nucleus is 10-15m. Most of the time they will give it.
You will notice something. That the density is extremely high. This is because, the nucleus is strongly packed using strong nucleur force. But why is the real density way less than the nucleur density?
This is also because, the atom is mostly empty space and so combining the density of the empty space and the tiny densed nucleus will give an overall density which is alot less
These release radiations such as alpha beta and gamma radiation.
We will need to know some terms
The time taken for a particle to decay is unknown or random. However, as an overall picture, the average time can be found by using the Decay constant
The rate of the decay is not affected by external factors such as temperature
The ability to strip electrons from a gas molecule to form gaseous ions
Remember that a particle can ionise another atom if it has sufficient energy or charge
The ability for a radiation to pass through materials
So what laws are conserved during decay:
So at both sides the mass-energy is conserved. The products usually have less mass than the reactants but, the fast moving Beta particle(or any radiation) is created. The energy due to this kinetic energy compensates this loss in mass. This is called the mass-energy conservation.
Usually we don't see this in normal equations but, momentum is always conserved.
There are 3 types of radiation we must know and their characteristics
|Nature of radiation
|He - 4 or 42α
|It's a helium nucleus with 2 protons and 2 neutrons
|- e or 0-1β
|It is a fast moving electron
|It is a high frequency electromagnetic wave or a gamma photon
Now we will check their ionisation and penetration
Alpha has the lowest penetration and greatest ionisation as it has the highest charge of 2+ and it is also the slowest. So it has a greater ionisation effect and because it ionises more it loses more energy and thus can't penetrate far. Also alpha particles are stopped by a few cm of air or a sheet of paper or skin
Beta has medium penetration and ionisation as it is charged but, way faster. So it has greater penetration. It is stopped by few cm of aluminum
Gamma has the least ionisation and due to this, it has the greatest penetration. It is never truly stopped but, reduced almost completely by some cm of lead or few meters of concrete
We will see some more details
|Speed in terms of c
|More than β
|Range of KE as due to range of speeds
|High - depends on the frequency- usually a specific value. Whereas, beta always changes.
Always remember that during alpha decay, the nucleon number decreases by 4 and the proton number decreases by 2.
And a helium - 4 nucleus is formed
Remember that there are 2 types of beta decay so we will evaluate each:
This is usually the default decay whenever they talk about beta decay
Always in beta(-), an neutron becomes an electron and a proton and so The Nucleon number or mass number never changes but only the proton number increases by 1 and so it becomes the next element. Also an electron antineutrino is formed.
We need to know that a down quark is turned into an up quark due to the weak force during this process
Why? was an electron antineutrino formed rather than a neutrino?
In this a proton is converted to a neutron and a positron and a electron neutrino
Always the Mass number remains constant in any beta decay but in beta (+) the atomic number decreases to form the stable product.
Also you need to know that an up quark turns into a down quark during this process
There is no change in Mass number or proton number as no particles are released except energy. So this allows particles to release excess energy.
As gamma radiation is neutral, it is not deflected by an electric field or a magnetic field
Beta and alpha are deflected in an electric field
Always as alpha is +2 it deflects towards the negative plate, whereas the beta particle deflects towards the positive plate. However, as beta particle has a lower mass to charge ratio, the deflection is relatively higher than alpha particle.
So when we usually compare the deflection of the same charge to find which one has the greater deflection we use this equation
For example, a proton has a mass of 1 and a charge of +1
It has a mass to charge ratio of 1.
Another particle of charge +2 and a mass of 4 has a mass to charge ratio of 2
So relative to the first one the alpha particle has twice the mass-charge and so twice as less deflection or further apart
Alpha and beta rays are both deflected in a magnetic field but, gamma rays are not deflected.
Just imagine that alpha is current as current is the flow of positive charge.
Then use Fleming's left hand rule and find the direction of the motion:
1. The thumb is the deflection direction
2. The index is the direction of the magnetic field(in or out of the page).
3. The middle finger is the direction of the alpha particle.
So to find beta decay, always remember that beta deflects in the opposite direction of alpha particles and also by a larger angle
All things are made from the smallest indivisable particles known as elementary particles
You will only need to know these particles:
There are 6 types of quarks and you need to remember them all but, there is an easier way to remember there charges
|Charge = +2e/3
|Charge = -1e/3
So what this table shows us that the particles in the first row have the same charge of +2e/3 where as the 2nd row has charges of -1e/3
Also remember that a quark has another similar but, opposite name like top and bottom quarks
Remember this table!
Protons and neutrons are made out of elementary particles called quarks
Remember that neutrons has 2 down quarks and 1 up quark
Remember that protons have 2 up quarks and 1 down quarks
If you do forget, use this method below!
Charge of a proton is +1e
So we can find the number of up and down quarks
An electron is considered to be an elementary particle but, there are many types of electrons(actually 3 types):
They all have the same charge but, we only need to know electrons
And for each particle there is a corresponding neutrino
1. Muon Neutrino
2. Tau Neutrino
3. Electron Neutrino
Mostly, we talk about electrons and the electron neutrinos in beta decay only.
Also, Neutrinos are considered to be massless and chargeless so they don't affect the decay equation but, must be still added. This is because, it still carries a very little energy
These are known to be called gamma photons as they are responsible for gamma radiation. Actually, gamma radiation are not just waves. According to Heinsberg's uncertainty principle, waves could be particles at the same time.
This radiation is responsible for pair production and annihilation of antimatter and matter
So for each elementary particle, you have seen there is a corresponding antiparticle which has the same mass but, Opposite charge and spin. When representing them we put a dash above the original symbol
We will give some examples
|Charge = -1
|Charge = +1
We will also check the antiparticle of neutrinos
|Charge = 0
|In beta(+) decay
|Charge = 0
|In beta(-) decay
Also the antiparticle has the opposite properties
Particles can be classified into 2 main groups
These are large particles such as protons and neutrons and are made out of quarks
These particles are affected by strong nucleur force and the weak forces. Actually all types of forces.
Hadrons can be divided in two more groups
These are particles which contain 3 quarks together
For example the proton - uud
These contains 2 quarks together. Usually an antiquark and a quark
These includes phi+ meson - up quark and antidown quark
We will need to know one more thing. The baryon number in an equation is always conserved and antiparticles have the opposite baryon number. But more information is not necessary...
These are electrons and neutrinos and their antiparticle
These particles are affected by all other forces except the strong nucleur force
The below part is not necessary but, it can help you with identifying if a neutrino or antineutrino is produced
During decay, the lepton number is conserved. The antiparticles have the opposite properties, which means that an antineutrino and a positron has a lepton number of -1 whereas, neutrinos and electrons have a lepton number of 1
So on the left side, it only contains neutrons and protons. It has no leptons so it is 0. On the right side, it has a electron with a lepton number 1 and another lepton must be present which is a antineutrino of lepton number -1 to cancel the other lepton and equate to 0
There are 4 forces which govern the universe:
Forces between masses
This is the weakest force in the universe but, it also has the largest range. In fact, the range is said to be infinite!
Attraction forces which holds up the positive nucleus together. As the nucleus is made out of protons - positively charged. The strong nucleur force overcomes the repulsion forces between the protons.
You will need to know some details of this force:
1. This is the strongest force in the universe
2. This doesn't affect leptons but, only Hadrons and Quarks.
3. This force is highly short ranged and doesn't extend to the outer shell of an atom.
All you have to know is that this force is necessary for both types of beta decay as we have seen it changes neutrons to protons
To change the identity, this force is required. So this is not technically a force but, an interaction
These are forces between charged objects. Anything that has a charge, creates an electric field around it and causes other charges to experience a force
When a particle and the corresponding antiparticle meets, they annihilate and release energy in the form of Gamma radiation or two gamma photons
So Momentum, charge and mass-energy is conserved
This is the reverse of annihilation.
When a Gamma ray is passed through a nucleus, a particle and the antiparticle is formed. The nucleus is essential to conserve momentum.
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