ATOMIC THEORIES AND MODELS
1. Dalton’s Particle Theory
a) Matter is made up of tiny, indivisible particles
called atoms. These atoms are indestructible.
b) Atoms of the same elements are IDENTICAL in
mass and size.
c) Chemical compounds are formed by the union
of 2 or more atoms of different elements.
d) Atoms of different elements combine in simple
whole number ratios.
e) Atoms of 2 or more elements may combine in
different ratiosto form more than one
compound.
Eg. C3Hs CoHe C3HgO C2H20 H20 4H202
a? A mag x2 = \O mass
Mass units Yate uaits
~ with the acquisitio
n of greater knowle
the above th dge, two of
eories were modified.
These are:
i) “Atoms are indivisi
ble and indestructib
Modified due to the le” were
discovery of Subatom
Particles and the de ic
velopment of the nu
clear bomb.
° = James Chadwick
p” = Ernest Rutherfo
rd
€°= Joseph John Tho
mson
number of neutrons
.
~ Isotopes have the
same atomic numbe
atomi r and
c mass but differ
ent mass number.
co iS “fuimbers
Eg. Isotopes of car —
bon 120 3C
ee
on
14C
Atomic number = 6
Atomic mass = 12
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2. Thomson Model
~ this model is based on the results of J.J.
Thomson’s cathode ray experiment.
~ his experiments with cathode ray tubes showed
that a// atoms contain tiny negatively charged
subatomic particles or electrons.
vA ~ he proposed that the atom is comprised of a solid
=
positive mass with € embedded in it.
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te re
sis re, eh ofbletetetetoteteteteteteteteteteteratetetetetetetetetetetetetetetetetetetetetetetete
Plum Pudding Model
3. Rutherford Model
~ Rutherford’s experiments were conducted to
prove if Thomson’s model was correct
~ his experiment comprised of alpha (c) particles
which are (+) particles being shot at gold foil
(note: the a particles used were He ions or nuclei)
~ the experimental set up had the following:
Sy A ey Gold. fol
- Ocqeriment
Results: - some o particles went straight through
- some came back directly
- some were deflected to the sides
~ based on these results, Rutherford proposed
these conclusions:
\- - the atom is comprised of a small dense
positively charged ‘nucleus’ surrounded
by predominantly empty space.
- the e- exist in this empty space where
they orbit the nucleus like the planets
around the Sun
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sali Me
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~ “mam a
Problems with the Rutherford model:
~ Any moving body as it moves loses energy and
slows down. This model does not explain why e- do
not crash into the nucleus as they lose energy and
slow down.
~ the model does not provide any info on e-
arrangement around the nucleus
4. Bohr Model
~ Niels Bohr studied the e- of hydrogen
~ the Bohr model suggested that electrons revolve
around the positively charged nucleus in a definite
circular path called orbits or shells
~ each orbit or shell has a fixed amount of energy
known as quantized energy; this means that energy
comes in fixed amounts or in discrete bundles of
specific amount
~ Bohr was not the first to introduce the concept of
quantized energy. This was done by Max Planck.
Bohr used the concept to explain the energy of e- in
his atomic model.
~ the shell or energy levels are represented by
integers (n=1, 2, 3, ...) where n=1 is closest to the
nucleus; they were also designated K, L, M, N....
= e- are permitted to move from orbit to orbit; e- in
an atom will move from lower energy orbit to
higher energy orbit by gaining or absorbing the
required energy and will move from higher energy
level to lower energy level by losing or releasing
energy
~ when energy is released by e- it is in the form of
light
= the light appears in different colours specific to
the amount of energy released
3rd orbit se, *
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Problems with Bohr Model
1) It attempts to map the path of an e- and this
cannot be done.
2) It works for the hydrogen e- but it cannot
explain the light spectrum in other atoms.
5. Quantum Mechanical Model
~ This atomic structure model proposes that orbits
are REGIONS around the nucleus of the atom where
there is a high probability of finding e-
= Erwin Schrodinger took the Bohr model a step
further and used mathematical equations to
describe the likelihood of finding an electron ina
certain position.
= In this model, the orbits/shells are called principal
energy levels or quantum levels.
~ the model also suggests that there are subshells
within the energy levels called ORBITALS; they are
designated s, p, d, and f depending on their shape
dh wir fis — —— Fee Pe wep
2s orbite} Nucleus
orbitals
ts orbitat
3s orbital
Cloud Model of Ye hrom
Dol Structure Su o\tUM Stu Ae
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S= de7 p= be d=\0e Ys \4e"
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orbit cls
Other Scientists to Note
1. Michael Faraday (1791 — 1867) — discovered
that certain substances can conduct electricity
when dissolved in water; also discovered that
certain: decompose into its elements when an
electric current is passed through them. He
called these electrically charged atoms, IONS.
Svante Arrhenius (1859 — 1927) — extended
Faraday’s work and discovered that these ions
are atoms or group of atoms -carrya+or-—
charge. He reasoned that they do not need to
be in water to carry a charge (eg. melted NaCl
breaks into Na* and CI). + ions are called
CATIONS and — ions are called ANIONS.
Robert Millikan (1868 - 1953) - credited with
discovering the value for electron charge, e,
through the famous oil drop experiment, as well
as achievements related to the photoelectric
effect and cosmic radiation.
