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Inside the Atom
So far in our chemistry unit we have looked at some of the basic properties of matter,
and have started to look at elements, compounds, and the periodic table. But before
we go any further, it is important for us to understand exactly what an atom is. To do
this, we are going to briefly examine the development of our current model of the
atom.
John Dalton
The earliest model of the atom was that of John Dalton in 1803. He proposed that
atoms were tiny, indivisible spheres, like a billiard ball. However, experiments soon
showed that this was not the case.
Heinrich Geisier -- Gas Discharge Tube
Near the end of the 1700’s scientists had discovered electricity. They had made
hydroelectric power, light bulbs, batteries, electric motors, and many more inventions.
However, they still did not know what electricity actually WAS!
One of the inventions that was being used to investigate electricity was a gas discharge
tube. It consisted of a hollow glass tube with electrodes (metal) embedded at each
end. When a source of electricity was attached to the electrodes, the gas inside the
tube would glow. It was very similar to the old television tubes that we still see today.
In 1855, a German glass blower named Heinrich Geissler improved a gas discharge tube
by including a vacuum pump (to remove the gas inside the tube). He discovered that if
you remove the gas from inside the tube, the tube would glow green.
J.J Thomson - The Electron and the Plum Pudding Model
In 1987, the scientist J.J. Thomson was doing more experiments with vacuum tubes.
The conclusion that he made was that a very tiny particle was passing through the tube
and this was what made the tube glow green. Since the particles seemed to be
coming from the negative side of the electricity source, it was concluded that the
particles must be negative. We call these particles electrons.
While thinking about his experiments and those of other scientists, Thomson came to
some important conclusions:
• The electron seems to come from inside atoms
• Electrons are negative but atoms themselves are neutral
• Electrons have very little mass compared to atoms
• There must be something else that is positive and with lots of mass in the
atom.
To account for the neutral charge, Thompson proposed the first model of atomic
structure, the “plum pudding” model or “raisin bun” model. The electrons were like
raisins in a sea of positive dough.
Positive Dough
Rutherford - Discovers the Nucleus
The discovery of radioactive elements (elements that release small particles of matter
and energy) such as radium allowed a scientist named Ernest Rutherford to conduct
experiments which completely changed our understanding of the atom.
While working at McGill University in Montreal in 1909, he carried out the famous gold
foil experiment to confirm the plum pudding model. Rutherford’s students placed a
sample of radium inside a lead block so that the positively charged particles could only
escape in one direction. They aimed the particle beam at a very thin piece of gold foil,
so thin it was only a few of atoms thick.
Rutherford’s hypothesis was that the radiation should pass right through the foil since
the positive “dough” would be too weak to stop it.
At first, the results were as he
expected. He found that although 9999 out of every 10 000 particles when straight
through. However, an interesting anomaly occurred. The ones that did not go
straight through were deflected at many different angles. A few even bounced straight
back!
Rutherford described the result as “firing a 15 inch [artillery] shell at a piece of tissue
paper, expecting it to go straight through, and having it bounce back.”
Based on his results, Rutherford proposed a new model of the atom. In it, the atoms
was mostly made of empty space, with a very dense core of positively charged matter
he called the nucleus.
Later experiments showed that the nucleus itself was made up of positively charged
particles called protons, and the electrons were outside the nucleus. Rutherford
discovered that the proton was 2000 times heavier than the electron.
This model was known as the planetary model of the atom.
James Chadwick – Discovers the Neutron
Hydrogen only has one proton and helium has two protons. But helium has four times
the mass of hydrogen. In 1932, James Chadwick solved the mystery by identifying an
uncharged particle in the nucleus with a mass almost identical to a proton. It was
called the neutron.
Controversy
Despite all of the evidence in favor of the Rutherford model, when it was
published it was controversial. Many scientists were unwilling to admit they were
wrong in agreeing with the plum pudding model. Others argued that if the
(negative) electrons were outside the (positive) nucleus, they should be drawn
together because they had opposite charges. This would make an atom unstable.
Neils Bohr - The Bohr-Rutherford Atom
A scientist named Bohr became interested in the controversy, and proposed a
modification to the Rutherford model that solved many of the problems. He
proposed that electrons can only circle the nucleus at very specific distances he
called orbits or energy levels. Bohr proposed that the first orbit could contain
two electrons and the second and third could hold eight. The Bohr-Rutherford
model of a beryllium atom, therefore, looked like this:
Atomic Number = _____
(from periodic table)
Atomic Mass = _____ (from periodic table )
# proton = ________
# electrons = _______
# neutrons = _____
Summary
Particle
Location
Charge
Mass (amu)
1.0073
1.0087
0.00055
1. The number of protons is ALWAYS equal to the atomic number.
2. The number of electrons for a neutral atom is equal to the number of
protons.
#e =#p
3. Since the protons and the neutron are the only heavy particles:
Atomic Mass = protons + neutrons
Therefore:
number of neutrons = atomic mass – atomic number
Complete this table.
Atomic
Number
Element
Symbol
1
2
6
8
16
hydrogen
helium
carbon
oxygen
H
He
C
Atomic
Mass
1
12
16
32
K
Protons
Electrons
1
2
6
1
2
19
79
gold
Neutrons
0
2
20
How many electrons are in each orbit? Remember:
1. The number of total electrons is equal to the number of protons, which is
equal to the atomic number.
2. The first level gets 2 electrons, the second level 8, and the third (well, you’ll
see)
Number of Electrons in the Level
Element
hydrogen
carbon
oxygen
magnesiu
m
chlorine
argon
Atomic
Number
1
6
1st
Level
1
2nd
Level
0
3rd
Level
0
Summary: Parts of the Atom
Part of
the Atom
Nucleus
Proton
Neutron
Electron
Location
center
of the
atom
inside
the
nucleus
inside
the
nucleus
orbiting
the
nucleus
Charge
Mass
Depends on the
atom
Extra Information
Consists of protons and neutrons.
Contains most of the mass of the
atom
They NEVER leave the nucleus. The
number of protons determines the
identity of the atom. They are equal
to the atomic number.
They never leave the nucleus. The
number of neutrons is the mass
minus the atomic number.
They circle the nucleus. They can be
made to leave an atom without
changing the identity of the atom.
There are usually as many electrons
as there are protons.