Radioactivity and other stuff

History
Definition
Representing the Atom
The Atomic Mass Unit (u)
Avagadros number
Einstein's Mass - Energy Equation
The Electron-Volt
Binding Energy and Mass Defect
Fission and Fusion

Some History

• 1897 Bequerel discovers radioactivity. Marie Curie and Rutherford isolate three main types of ionising radiation. To this day Marie Curie's domestic cookery book is still too radioactive to touch without protective clothing
• 1897 J.J. Thomson discovers the electron. The neutron is still unknown, only the proton and the electron 'exist'.
• 1899 Rutherford names a, ß and ? as the three types of naturally occurring radiation
• 1908 Rutherford identifies alpha particles as nuclei of Helium atoms.
• 1909 Alpha particle scattering experiment carried out by Geiger and Marsden under Rutherford's instruction. Orbital model (The Rutherford Model) for atomic nucleus put forward
• 1932 Chadwick discovers the neutron.

Some Definitions:

Proton A positively charged particle found in the nucleus of an atom.
The proton has a mass of approximately 1.7 x 10 ^_-27 Kg and a charge of +1.6 x 10 ^_-19 C

Neutron A neutrally charged particle found in the nucleus of an atom.
The Neutron has a mass of approximately 1.7 x 10 ^_-27 Kg

Nuclide the nucleus of an atom of particular specification

Nucleon a proton or a neutron

Isotopes two atoms of the same element having the same number of protons but different number of neutrons

Electron A small negatively charged particle not found in the nucleus. May be thought of as orbiting the nucleus.
The Electron has a mass of approximately 9.1 x 10 ^_-31 1 Kg and a charge of -1.6x10 ^_-19 C.

Ionisation Occurs when electrons are removed from the outer orbitals of atoms leaving the a positively charged atom called an ION.

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Representing the atom

A: Mass or nucleon number [p +n]
X: Chemical symbol for atom
Z: Atomic or Proton Number [p {and e for a neutral atom}]

Avagadros number
the number of atoms in 12g of carbon-12. [6.02 x 10 ^₂₃ ]
{and no he didn't count them by hand!}

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E = mc ^₂

E = Energy in Joules
[this is important as eV are sometimes used as a unit of energy and so things can get confused]
m = mass in kilograms
[this is important as u are sometimes used as a unit of mas and so things can get confused]
c = is the speed of light

Because the value of c is so large [ 3 x 10 ^₈ ms ^_-1 ] even a small amount of energy can be converted into a great deal of energy.

!!If a small infant has a mass of 20kg = how much pure energy is there!!
!!20 x [3 x 10 ^₈ ]2 = 1.8 x 10 ^₁₈ Joules!!

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This is defined as the amount of work [energy] needed to accelerate an electron through a p.d. of 1V

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Binding Energy and Mass Defect
If you take the constituent parts of an atom [p, n] and weigh them and them weigh the atom in its combined state you discover that the combined mass is less. Some of the mass of the constituent parts has been converted into the energy that holds the atom together. This energy is called the binding energy for reasons that I really hope are obvious.

When an atom disintegrates in any way then some of this energy is released. This principle is essentially the basis of what is happening in radioactive decay. An atom may split up, fine, but an alpha particle speeds of at 10% the speed of light - where does it get that energy?

Here and in many cases we ignore the mass of the electrons. In any question you come across assume that this is the case [but state it as an assumption if prompted].

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Fission and Fusion
Stick small atoms together and get energy: Fusion
Break big atoms up and get energy: Fission

Well, it is a bit more complicated than that but the principle holds true. Of the two fusion is the more efficient and this is why a great deal of time, money and effort is expended in trying to harness it as a viable energy source.

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