Every atom contains a positively charged nucleus composed of protons and neutrons and electrons surrounding the nucleus. The word nucleon is used for a proton or a neutron in the nucleus. The table below shows the charges and mass of protons, neutrons and electrons.
Isotopes are atoms of the same elements with the same number of protons but different number of neutrons.
The specific charge of a charged particle is defined as its charge divided by its mass.
For example, a charged particle has a charge of 1.60 x 10-19
C and a mass of 1.67 x 10-27 kg.
Therefore, the specific charge is : 1.60 x 10-19 C /1.67 x 10-27 kg = 1.76 x 10^11
Ckg-1
Stable and unstable nuclei
A force that holds a stable nuclei is called the strong nuclear force, as it overcomes the electrostatic force of repulsion between the protons in the nucleus and keeps the protons & neutrons together.
Alpha radiation consists of alpha particles which comprise of 2 protons and 2 neutrons. Therefore its mass number is 4. As the nucleus emits an alpha particle, its nucleon number decreases by 4 and its atomic number decreases by 2.
For example:
Gamma radiation is electromagnetic radiation emitted by an unstable nucleus. It can pass through thick metal plates. It has no mass and no charge.
Photons
All electromagnetic waves travel at the speed of light (c), which is 3.0 x 10^8
.
Electromagnetic waves are emitted by a charged particle when it loses energy. They are emitted as short 'bursts' of waves, each burst leaving the source in a different direction. Each burst is a packet of electromagnetic waves and is referred to a photon. This is the emission of electrons from a metal surface when light is directed at the surface.
photon energy E = hf
h=Planck's constant (6.63 x 10^-34 Js)
Example: Calculate the frequency and the energy of a photon of wavelength 650nm
h = 6.63 x 10^-34 Js
c = 3.00 x 10^-8 ms^-1
f = (3.00 x 10^-8 ms^-1 ) / (650 x 10^-9
)
= 4.62 x 10^5
E = hf= 6.63 x 10^-34 x 4.62 x 10^5
= 3.06 x 10^-33
J
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