# Chapter 1 - Atomic Structure (F321)

OCR unit 1 chem - Chapter 1 Atomic Structure

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Revision Notes: Chapter 1 ­ Atomic Structure
Discovering the Electron:
Electrolysis :
When electricity flows through a solution of metal salt, the metal appears at the
cathode (negative electrode).
This is because the metal exists in the solution as positive charged ions
One metal ion plus one unit of electricity gives the metal atom.
George Johnstone Stoney give this unit of electricity the name `electron'.
Study of cathode rays:
Gases conduct electricity better at low
pressures.
Cathode rays cast a shadow
They can be deflected by a magnetic
field, proving they are electrically
charged particles
J.J. Thompson measured the deflection
of a narrow beam of cathode rays in
both electric and magnetic fields. He
calculated that the charge to mass ratio
was exactly the same, whatever the
type of gas or electrode used. The particles had a tiny mass, about 1/2000th of a
hydrogen atom. He called them `electrons' as Stoney suggested.
Millikan's `oil-drop' experiment:
The electron charge was first measured
accurately by Robert Millikan.
Oil drops which were sprayed into the
container acquired a negative charge. The
drops remained stationary when the
upward force of attraction to the positive
plates equalled the force caused by
gravity.
He calculated the charge to be 1.602 x
10-19 C
The mass of an electron was calculated to be 9.109 x10-31 kg
Discovering Protons and Neutrons:
Since ancient Greek times, scientists assumed that atoms were soling spheres. The
discovery of the electron disproved this theory as now negatively charged particles exist
in what was believed to be a neutral particle.
J.J. Thompson put forward his `plum-pudding' model. He believed that electrons were
embedded in a sphere of positive charge.
Two of Ernest Rutherford's students conducted an experiment that proved the
plum-pudding model wrong.
­ particles (helium nuclei), which are positively charged, were fired at a thin sheet of
gold. Any particles that made it through were detected by fluorescent screen.
The plum-pudding model would suggest that most of the particles would be deflected
very slightly be the positive `pudding' that made up most of the atom. In fact most

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º.
Rutherford came up with the nuclear model: There is a tiny, positively charged nucleus at
the centre of the atom where most of its mass is concentrated, surrounded by a `cloud'
of negatively charged electrons. The atom is mostly empty space.
Nuclear charge and atomic number:
Henry Moseley, a member of Rutherford's team compared the positive charges of the
nuclei of different elements. He found that the charge increases by one unit from
element to element in the periodic table.…read more

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This is what is shown on the periodic table and is not usually a whole number e.g. 35.5Cl
Ar can be worked out from isotopic abundances. For example, 76% chlorine atoms
have relative isotopic mass of 35, while 24% have relative isotopic mass of 37.
(76 x 35) + (24 x 37) = 2660 + 888 = 3548
3548 ÷ 100 = 35.5 (to 1d.p.)
Relative Isotopic Mass is the mass of an atom of an isotope of an element relative to
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