The nuclear atom
All matter is made up of atoms which consist of a central nucleus, surrounded by electrons. Inside the nucleus are protons and neutrons. Particle
Proton or atomic number (Z)
This is the total number of protons in the nucleus of an atom. Nucleon or mass number (A)
This is the total number of protons and neutrons in the nucleus of an atom. Note: a nucleon is a particle found in the nucleus i.e. a proton or a neutron. In general, if N is the neutron number of an element, then
In an atom the number of protons is equal to the number of electrons. Nuclide notation
A nuclide is a form of an element.
If X is a given nuclide with proton number Z and nucleon number A, then its nuclide notation will be: (_Z^A)X
The nuclide notation for some atoms is as indicated below:
Write down the number of protons, electrons and neutrons in each of the atoms in example1 above:
Protons = 1, electrons = 1, neutrons = 1 – 1= 0
Protons = 6, electrons = 6, neutrons = 12 – 6= 6
Protons = 11, electrons = 11, neutrons = 23 – 11= 12
Protons = 92, electrons = 92, neutrons = 238 – 92= 146
Isotopes are atoms of the same element which have the same number of protons but different numbers of neutrons. That is, their proton numbers are the same but their nucleon numbers are different. Examples
Hydrogen: (_1^1)H: (_1^2)H: (_1^3)H
Carbon: (_6^12)C: (_6^14)C
Chlorine: (_17^35)Cl: (_17^37)Cl
Uranium : (_92^235)U: (_92^238)U
Isotopes have the same chemical properties because they have same number of protons (or electrons). They have different physical properties since they have different mass numbers. Radioactivity
Radioactivity is the spontaneous disintegration of the nucleus of an atom by emitting alpha (α), beta (β) or gamma (ϒ) rays. Characteristics of the three types of radiation
Alpha particles ((_2^4)α or (_2^4)He) are helium nuclei in nature. They consist of two protons and two neutrons.
Beta particles((_-1^0)β or (_-1^0)e) are electrons in nature
Gamma rays((_0^0)ϒ) are electromagnetic waves in nature
Alpha particles have a double positive charge
Beta particles have a single negative charge
Gamma rays have no charge
Their relative ionizing effects
Alpha particles cause the most intense ionization, followed by beta and then gamma rays.
Their relative penetrating powers
Alpha particles have a range of a few centimetres in air. They can be stopped by a few sheets paper.
Beta particles have a range of several metres in air. They can be stopped by a few millimetres of aluminium.
Gamma rays are the most penetrating. They can only be stopped by thick lead.
Detection of radiation
Most detectors of radiation make use of the ionizing effects of radiation.
Geiger-Muller (GM) tube
When radiation enters the GM tube, it creates argon ions and electrons which are accelerated towards the electrodes. On reaching the electrodes, the ions produce a current pulse which is amplified and fed either to a scalar or ratemeter.
A scalar counts the pulses and shows the total received in a certain time.
A ratemeter gives the counts per second (or minute), or count-rate, directly. The following are some other examples of detectors:
Diffusion cloud chamber
The emission of an alpha or beta particle from an unstable nucleus produces an atom of a different element.
When an atom decays by alpha emission, its nucleon number decreases by 4 and the proton number decreases by 2. A completely different atom is formed....
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