Describing Atoms Describing Atoms

When something is too small to se, finding out what it is like is bound to be difficult. In the space of just a few years at the beginning of the 20th century, more by imagination and instinct than anything else, a few men gave us a model of the model of the atom which is the basis of our understanding of much of modern science. There are three important stages in the evolution of the modes of the atom.

Sir Ernest Rutherford, who had great intuition about the structure of matter, gave us the idea and the evidence that atoms have a very small central core or nucleus.

One of the first atomic models was that proposed by J.J. Thomson after he discovered the existence of the electron as a result of his work with cathode rays.  Since each atom was electrically neutral, Thomson thought that it consisted of a relatively large, uniformly distributed, positive mass with negatively charged electrons  embedded in it like "raisins in a plum pudding."

Rutherford’s plan was to aim alpha particles at a thin gold foil and observe the scattering pattern on a zinc sulfide screen, which would show small bursts of light when struck by an alpha particle. Although the majority of the alpha particles passed through undeflected, some were strongly deflected into hyperbolic paths while others were scattered through almost 180°.   These dramatically recoiled alpha particles led Rutherford to use the analogy of firing bullets at a piece of tissue paper and discovering that some of the bullets bounced back!

Different atoms are distinguished by their numbers of protons and neutrons. We write the symbols using the following notation:

•A is called the nucleon number, or the mass number. It is the total number of nucleons.

•Z is the proton number or the atomic number, which is the number of protons. The number of protons determines the element.

•Be careful not to confuse atomic number with the symbol A. We will refer to A as the nucleon number in these notes and Z as the proton number.

nucleon number

or mass number=

Proton number or

Atomic number+

Neutron

number

A = Z + N

We can determine the number of neutrons simply by subtracting the proton number from the nucleon number. ( No of neutrons = A – Z) Atomic particles are always in whole numbers.

•Isotopes have the same numbers of protons, but different numbers of neutrons.

•Isotopes have the same physical and chemical properties.

•If the proton number is altered, the element changes.

•Some isotopes are radioactive, as the nuclei are unstable.

Chemical reactions involve the electrons of the outer shells. Nuclei are not involved in any way, and remain totally unaltered even in the fiercest chemical reactions.

Example:

Radioactivity

Radiation is the process by which an unstable parent nucleus becomes more stable by decay into a daughter nucleus by emitting particles and/or energy. The basic form can be summed up as:

The decay can consist of several steps. The unstable nucleus can decay to another nucleus of a different atom by a process called transmutation. If the new nucleus is unstable it will decay again. This is known as a decay chain. There may be several steps, some of which last a very long time indeed, or can be very short. Some elements have a decay time of thousands of millions of years. In others the decay time can be microseconds.

In 1899, Ernest Rutherford discovered that uranium compounds produce three different kinds of radiation. He separated the radiations according to their penetrating abilities and named them alpha, beta, and gamma radiation, after the first three letters of the Greek alphabet.

The radiation can be stopped by a sheet of paper. Rutherford later showed that an alpha particle is the nucleus of a He atom, 4He.

Beta particles were later identified as high speed electrons. Six millimeters of aluminum are needed to stop most b particles.

Several millimeters of lead are needed to stop rays , which proved to be high energy photons.

Alpha particles and rays are emitted with a specific energy that depends on the radioactive isotope. Beta particles, however, are emitted with a continuous range of energies from zero up to the maximum allowed for by the particular isotope.

decay     The emission of an particle, or 4He nucleus, is a process called a decay. Since a particles contain protons and neutrons, they must come from the nucleus of an atom. The nucleus that results from decay will have a mass and charge different from those of the original nucleus. A change in nuclear charge means that the element has been changed into a different element. Only through such radioactive decays or nuclear reactions can transmutation, the age-old dream of the alchemists, actually occur. The mass number, A, of an particle is four, so the mass number, A, of the decaying nucleus is reduced by four. The atomic number, Z, of 4He is two, and therefore the atomic number of the nucleus, the number of protons, is reduced by two. This can be written as an equation analogous to a chemical reaction. For example, for the decay of an isotope of the element seaborgium, 263Sg:

263Sg ----> 259Rf + 4He

The atomic number of the nucleus changes from 106 to 104, giving rutherfordium an atomic mass of 263-4=259. decay typically occurs in heavy nuclei where the electrostatic repulsion between the protons in the nucleus is large. Energy is released in the process of decay. Careful measurements show that the sum of the masses of the daughter nucleus and the particle is a bit less than the mass of the parent isotope.

β Decay     Beta particles are negatively charged electrons emitted by the nucleus. Since the mass of an electron is a tiny fraction of an atomic mass unit, the mass of a nucleus that undergoes b decay is changed by only a tiny amount. The mass number is unchanged. The nucleus contains no electrons. Rather, decay occurs when a neutron is changed into a proton within the nucleus. An unseen neutrino, accompanies each decay. The number of protons, and thus the atomic number, is increased by one. For example, the isotope 14C is unstable and emits a β particle, becoming the stable isotope 14N: 

14C ----> 14N + e- +   

In a stable nucleus, the neutron does not decay. A free neutron, or one bound in a nucleus that has an excess of neutrons, can decay by emitting a particle. Sharing the energy with the b particle is a neutrino. The neutrino has little or no mass and is uncharged, but, like the photon, it carries momentum and energy. The source of the energy released in decay is explained by the fact that the mass of the parent isotope is larger than the sum of the masses of the decay products. Mass is converted into energy just as Einstein predicted.

Decay     Gamma rays are a type of electromagnetic radiation that results from a redistribution of electric charge within a nucleus. A ray is a high energy photon. The only thing which distinguishes a ray from the visible photons emitted by a light bulb is its wavelength; the ray's wavelength is much shorter. For complex nuclei there are many different possible ways in which the neutrons and protons can be arranged within the nucleus. Gamma rays can be emitted when a nucleus undergoes a transition from one such configuration to another. For example, this can occur when the shape of the nucleus undergoes a change. Neither the mass number nor the atomic number is changed when a nucleus emits a g ray in the reaction 

152Dy* ----> 152Dy +

PROPERTIES OF THE RADIATION – Alpha Particle

Radiation Type of RadiationMass (AMU)

Charge Shielding material

Alpha Particle 4 +2 Paper, skin, clothes

Beta Particle 1/1836 ±1 Plastic, glass, light metals

Gamma Electromagnetic

Wave0 0

Dense metal, concrete, Earth

Neutrons Particle 1 0Water, concrete, polyethylene, oil

Radiation DetectionRadiation DetectionRadiation DetectionRadiation Detection

The most common type of instrument is a gas filled radiation detector. This instrument works on the principle that as radiation passes through air or a specific gas, ionization of the molecules in air occur. When a high voltage is placed between two areas of the gas filled space, the positive ions will be attracted to the negative side of the detector ( cathode ) and the free electrons will travel to the positive side ( anode ). These charges are collected by the anode which then form a very small current in the wires going to the detector. By placing a very sensitive current in the wires from the cathode and anode, the small current measured and displayed as a signal. The more radiation which enters the chamber, the more current displayed by the instrument.Many types of gas filled detectors exist, but the two most common are the ion chamber used for measuring large amounts of radiation and the Geiger-Mueller or GM detector used to measure very small amounts of radiation.

Background radioactivityis mainly natural radioactivity, all around us. As you can see from the pie chart, the vast majority of our annual dose comes from radon gas, food & drink, the ground, and cosmic rays (which are gamma rays coming in from space).Unless you are having radiotherapy, your dose from medical sources is quite low. The chart also shows that the nuclear industry adds very little to the level of background radioactivity. Many people don't realise that your radiation dose from cosmic rays is increased considerably if you fly a great deal. This is because our atmosphere provides some protection against cosmic rays, so the higher you fly the more you get. However, don't worry - this only tends to be a problem if you're an airline pilot or an astronaut.

More on radioactivity

ReviewReview

• Background RadiationEvery day of our lives we are exposed to radiation that is all around us. Most of the time this is completely harmless. So called background radiation comes from many sources: rocks in the ground, bricks in buildings, nuclear waste from power stations and bomb tests, hospitals even outer space!

When doing experiments with radioactivity, we have to take into account that some of the radiation we measure will have come from all around us.

We can account for background radiation by measuring how much of it there normally is, then subtracting this off from any other measurements we might take.

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