Chapter: Atomic Physics
Everything is made of atoms. An atom is made up of smaller particles:
- There is a central nucleus made up of protons and neutrons. Around this, electrons orbit at high speed. The number of particles depends on the type of atom.
- Protons have a positive charge. Electrons have a negative charge. Generally, an atom has the same number of electrons as protons, so its total charge is zero.
- Protons and neutrons are called nucleons.
- Electrons are held in orbit by the force of attraction between opposite charges. Protons and neutrons are bound together by a force called the strong nuclear force.
Each element has a different number of protons in its atoms: it has different atomic number. The atoms of any one element are not all exactly alike. Some may have more neutrons than others. These different versions of the element are called isotopes. They have chemical properties although their atoms have different masses. Most elements are a mixture of two or more isotopes.
The total number of protons and neutrons in the nucleus is called the mass number. Isotopes have the same atomic number but different mass numbers.
Electrons orbit the nucleus at certain fixed levels only called shells. It is an atom’s outermost electrons which form the chemical bonds with other atoms.
The particles and waves radiate from the nucleus, so they are called nuclear radiation. Materials which emit nuclear radiation are known as radioactive materials. The disintegration of a nucleus is called radioactive decay.
Nuclear radiation can remove electrons from atoms in its path, so it has an ionizing effect.
There are three main types of nuclear radiation: alpha particles, beta particles, and gamma rays. Gamma rays are the most penetrating and alpha particles the least. Alpha particles are more ionizing than beta particles. They have a greater charge. Gamma rays are least ionizing because they are uncharged.
Nuclear radiation can damage or destroy living cells and stop organs in the body working properly. It can also upset the chemical instructions in cells so that these grow abnormally and cause cancer. The greater the intensity of the radiation, and the longer the exposure time, the greater the risk.
There is a small amount of radiation around us all the time because of radioactive materials in the environment. This is called background radiation. It mainly comes from natural resources such as soil, rocks, air, building materials, food and drink.
This can be used to detect alpha, beta, and gamma radiation. The GM tube can be connected to the following:
1. A rate meter:
This gives a reading in counts per seconds
This counts the total number of particles detected by the tube.
3. An amplifier and loudspeaker
The loud speaker makes a click when each particle or burst of gamma radiation is detected.
This is useful for studying alpha particles because it makes their tracks visible. At one time cloud chambers were widely used in nuclear research but they have since been replaced by other devices.
If an isotope is radioactive, it has an unstable arrangement of neutrons and protons in its nuclei. The emission of an alpha or beta particle makes the nucleus more stable, but alters the numbers of protons and neutrons in it. So it becomes the nucleus of a different element. The original nucleus is called the parent nucleus. The nucleus formed is the daughter nucleus. The daughter nucleus and any emitted particles are the decay products.
The half life of a radioactive isotope is the time taken for half the nuclei present in any given sample to decay.
In a radioactive sample, the average number of disintegrations per second is called the activity. The SI unit of activity is the Becquerel (Bq). An activity of 100 Bq means that 100 nuclei are disintegrating per second.
Stability of the nucleus:
It has the following features:
- Stable isotopes lie along the stability line
- Isotopes above the stability line have too many neutrons to be stable. They decay by beta-(electron) emission because this reduces the number of neutrons.
- Isotopes below the stability line have too few neutrons to be stable. They decay by beta+ (positron) emission because this increases the number of neutrons.
- The heaviest isotopes decay by alpha emission.
Whenever a particle penetrates and changes a nucleus, this is called a nuclear reaction.
The nucleus of a heavy atom bombards with low energy neutron and splits apart. In this process it produces tremendous amount of energy. This released energy is used to produce steam and thus electricity can be generated.
The splitting process is called fission and the fragments are thrown apart as energy is released. If the emitted neutrons go on to split other nuclei and so on the result is a chain reaction, and a huge and rapid release of energy.
In a nuclear reactor in a nuclear power station, a controlled chain reaction takes place and thermal energy is released at a steady state. The energy is used to make steam for the turbines. Enriched natural uranium is used as fuel and the fuel is in sealed cans. To maintain a chain reaction in a reactor, the neutrons have to be slowed down. A material called a moderator is used to slow down the neutrons. Graphite is used in some reactors. The rate of reaction is controlled by raising or lowering control rods. These contain boron or cadmium which absorb neutrons.
The fuel must be removed and replaced after a fuel can have been in a reactor for three or four years. Spent fuel cans are taken to a reprocessing plant where unused fuel and plutonium are removed. The remaining waste which is now a liquid is sealed off and stored with thick shielding around it.
Lighter nuclei are joined to make heavier nucleus and during this process huge amounts of energy is released and is used to generate electricity. Scientists and engineers are trying to design fusion reactors for use as an energy source on Earth. But there are huge problems to overcome. Hydrogen must be heated to at least 40 million degree Celsius and kept hot and compressed otherwise fusion stops. No ordinary container can hold a superhot gas like this.
Natural fusion occurs in the Sun.
Fusion reactors will produce more energy per kilogram of fuel.
Radioactive isotopes are called radioisotopes. Some are produced artificially in a nuclear reactor when nuclei absorb neutrons or gamma radiation.
Some of the practical uses of radioisotopes are:
Radioisotopes can be detected in very small quantities so they can be used as tracers. It means their movements can be tracked. Some examples include:
- Checking the function of body organs.
- Tracking a plant’s uptake of fertilizer from roots to leaves by adding a tracer to the soil water.
- Detecting leaks in underground pipes by adding a tracer to the fluid in the pipe.
Testing for cracks
Gamma rays have the same properties as short-wave length X-rays, so they can be used to photograph metals to reveal cracks.
In some production processes a steady thickness of material has to be maintained.
Carbon dating can be used to find the age of organic materials such as wood and cloth.
When rocks are formed, some radioisotopes become trapped in them.
Atoms and particles
Atoms are made up of even smaller particles. The following models tried to explain the atom and its structure.
1.Thomson’s plum pudding model
The electron was the first atomic particle to be discovered. It was identified by J. J. Thomson in 1897. Thomson suggested that an atom might be a sphere of positive charge with electrons dotted about inside it rather like raisins in a pudding. This became as the plum pudding model.
2.Rutherford’s nuclear model
Rutherford beamed alpha particles which are doubly charged helium ions through gold foil and detected them as flashes of light or scintillations on a screen. When alpha particles collide on the screen, it scintillates. The gold foil was only 0.0004 centimeter thick, meaning a few hundreds of atoms thick.
He predicted that the alpha particles to pass through with relatively little deflection and strike the fluorescent screen directly behind the foil.
This experiment showed that
i. Most of the mass of an atom (and all of its positive charge) is concentrated in a very small region in the center of an atom which is the nucleus.
ii. Most of the volume of an atom is empty space.
iii. The size of nucleus is very small as compared to the size of atom.
However, Rutherford also envisioned the electrons as moving around the nucleus similar to the way in which planets orbit the sun. Today we know that planetary motion is not a good model for the movement of electrons around the nucleus of an atom. Instead, we think of electrons as a cloud of charge surrounding the nucleus.
3.The Rutherford-Bohr model
In order to overcome the objections raised against Rutherford’s model of the atom, Neil Bohr made the following postulates about the model of an atom:
i. Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom
ii. The electrons do not radiate energy while revolving in discrete orbits
iii. If additional energy is supplied to an electron in a given energy level it can move up to a higher unfilled energy level. This transition requires the difference in energy between the energy levels
iv. When the electron drops down to a lower level it releases that energy
These special orbits or shells are called energy levels. Energy levels in an atom are shown in figure below.
In 1932, J. Chadwick discovered another subatomic particle which had no charge and a mass nearly equal to that of a proton. It was eventually named as neutron. Neutrons exist in the nucleus of all atoms, except hydrogen. In general, a neutron is represented as ‘n’. The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus.
How an atom gives off light
If an electron gains energy it jumps to a higher energy level. But the atom does not stay in this excited state for long. Soon, the electron loses energy by dropping back to a lower level. According to the quantum theory, the energy is radiated as a pulse of light called a photon. The greater the energy change, the shorter the wave length of the light.
A fundamental particle is one which is not made up of other particles. An atom is not fundamental because it is made up of electrons, protons and neutrons.
The present theory of particles is called the standard model. According to this model, electrons are fundamental particles. However, neutrons and protons are made up of other particles called quarks. There are two types of quark, called the up quark and the down quark for convenience. Each proton or neutron is made up of three quarks. Individual quarks have never been detected.