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Unit 3: Atoms and Elements – Let’s Get Atomic!
name:_______________________
period: ______________
Unit 3: Atoms and Elements – Let’s Get Atomic!
name:_______________________
period: ______________
At the beginning…
Democritus
460-370 BCE
Atomic Theory:
Atomic Model:
Leucippus
Unit 3: Atoms and Elements – Let’s Get Atomic!
name:_______________________
period: ______________
In the Common Era…
Our understanding of the physical world has grown at an incredible rate in the last 200 years. The key to the
advances made in chemistry has been our growing knowledge about atoms. In this unit we will look at some of
the early historical discoveries that helped us build up a useful 'working model' of the atom.
The ancient Greeks had ideas about particles and atoms,
but it wasn't until the start of the nineteenth century that a
theory of atoms became linked to strong experimental
evidence. It was then that an English scientist called John
Dalton put forward his ideas about atoms.
John Dalton
1766 – 1844
From his experiments and observations, he suggested that
atoms were like tiny, hard balls. Each chemical element
had its own atoms that differed from others in mass. An
element is a substance made from only one type of atom.
An element cannot be broken down into any simpler
substances. Dalton believed that atoms were the
fundamental building blocks of nature and could not be
split. In chemical reactions, the atoms would rearrange
themselves and combine with other atoms in new ways.
In many ways, Dalton's ideas are still useful today. For
example, they help us to understand elements,
compounds, and molecules.
Atomic Theory:
Atomic Model:
Absorb Chemistry for GCSE by Lawrie Ryan
http://www.broadeducation.com/htmlDemos/AbsorbChem/HistoryAtom/page.htm
Unit 3: Atoms and Elements – Let’s Get Atomic!
name:_______________________
period: ______________
At the end of the nineteenth century, a scientist called J.J. Thomson discovered the
electron. This is a tiny negatively charged particle that is much, much smaller than
any atom. An atom is the smallest particle of an element that can still be defined as
that element.
When he discovered the electron, Thomson was experimenting by applying high
voltages to gases at low pressure. He noticed an interesting effect.
Draw his experiment and list his major findings below:

J.J. Thomson
1856 - 1940



Thomson did experiments on the beams of particles in his tube. They were attracted to a positive charge, so
Thomson correctly concluded that they must be negatively charged themselves. Other experiments showed that it
would take about 2000 electrons to weigh the same as the lightest atom, hydrogen. He called the tiny, negatively
charged particles electrons.
But where had these tiny particles come from? Since they were so small, Thomson suggested that they could only
have come from inside atoms. So Dalton's idea of the indestructible atom had to be revised.
Thomson proposed a different model for the atom. He said that the tiny negatively charged electrons must be
embedded in a cloud of positive charge (after all, atoms themselves carry no overall charge, so the charges must
balance out). Thomson imagined the electrons as the bits of plum in a plum pudding (rather like currants spread
through a Christmas pudding – but with lots more space in between).
Atomic Theory:
Describe the new evidence Thomson used to
develop his atomic theory:
Atomic Model:
Unit 3: Atoms and Elements – Let’s Get Atomic!
name:_______________________
period: ______________
The next development came about 10 years later. Two of Ernest Rutherford's
students, Hans Geiger and Ernest Marsden, were doing an experiment at Manchester
University with radiation. They were using the dense, positively charged particles
(called alpha particles) as 'bullets' to fire at a very thin piece of gold foil. They
expected the particles to barge their way straight through the gold atoms
unimpeded by the diffuse positive charge spread throughout the atom that
Thomson's model described. However, they got a big surprise.
Draw their experiment and list major findings below:


Ernest Rutherford
1837 - 1937


In 1911, Ernest Rutherford interpreted these results and suggested a new model for the atom. He said that
Thomson's model could not be right. The positive charge must be concentrated in a tiny volume at the centre of
the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source.
On this model, the electrons orbited around the dense nucleus (center of the atom).
Atomic Theory:
Describe the new evidence Rutherford used to
develop his atomic theory:
Atomic Model:
Unit 3: Atoms and Elements – Let’s Get Atomic!
name:_______________________
period: ______________
The next important development came in 1914 when Danish physicist
Niels Bohr revised the model again. It had been known for some time
that the light given out when atoms were heated always had specific
amounts of energy, but no one had been able to explain this. Bohr
suggested that the electrons must be orbiting the nucleus in certain
fixed energy levels (or shells). The energy must be given out when
'excited' electrons fall from a high energy level to a low one.
Niels Bohr 1885 – 1962
Atomic Theory:
Describe the new evidence used to develop this atomic theory:
Atomic Model:
Absorb Chemistry for GCSE by Lawrie Ryan
http://www.broadeducation.com/htmlDemos/AbsorbChem/HistoryAtom/page.htm
Unit 3: Atoms and Elements – Let’s Get Atomic!
name:_______________________
period: ______________
Werner Heisenberg 1901-1976
Erwin Schrodinger
1887 - 1961
Heisenberg pointed out that it is impossible to know both the exact position and the exact momentum of an object
at the same time. Applying this concept to the electron we realize that in order to get a fix on an electron's
position at any time, we would alter its momentum. Any attempt to study the velocity of an electron will alter its
position. This concept, called the Heisenberg Uncertainty principle, effectively destroys the idea of electrons
traveling around in neat orbits. Any electron that is subjected to photons will have its momentum and position
affected.
Experiments conducted in the 1920's, 1930's and 1940's continued to point out problems with the planetary model
of the atom. These experiments lead to the development of the charge-cloud model. The charge-cloud model,
which is also called the quantum-mechanical model, does not attempt to describe the path of each electron in a
fixed orbit. Scientists now describe the possible positions of electrons in terms of probability. Computers can
calculate the points in space that an electron has the highest probability of occupying. These points can be
connected to form a three-dimensional shape. Electrons are characterized in terms of the three-dimensional
shapes that their probability fields define. The sum total of the various paths of electrons, traveling at very high
speeds, is described as the electron cloud.
Atomic Theory:
Describe the new evidence used to develop
this atomic theory:
Atomic Model:
Unit 3: Atoms and Elements – Let’s Get Atomic!
name:_______________________
period: ______________
Summary
Our ideas about the nature of atoms have progressed over the last two centuries (and continue to develop
today).
John Dalton introduced a new form of the ancient Greek idea of atoms at the beginning of the nineteenth
century.
In 1897, J.J. Thomson discovered the electron and suggested the 'plum pudding' model of the atom.
In 1911, Rutherford suggested that electrons orbit the atomic nucleus like planets round the Sun.
In 1914, Bohr modified Rutherford's model by introducing the idea of energy levels.
We can think of the atom as a positively charged nucleus with negatively charged electrons orbiting the nucleus in
energy levels (or shells).
In the 1920s, 30s and 40s, experiments show the Bohr model is limited in its ability to predict the actual behavior of
electrons. The electron cloud model and the age of quantum physics is born.
Quiz
1. Which of Dalton's ideas below do we no longer believe?
Elements contain only one type of atom.
Atoms rearrange in chemical reactions.
Atoms are solid masses that can't be split into smaller particles.
2. Which of the following ideas of John Dalton are still believed to be correct?
Atoms are solid.
Atoms in an element are all the same type of atom.
Atoms cannot be split into smaller particles.
Compounds contain different types of atom.
3. Which statement below is correct?
Most of the volume of an atom is taken up by a large nucleus.
Atoms are mostly space.
Unit 3: Atoms and Elements – Let’s Get Atomic!
name:_______________________
period: ______________
4. Who proposed the 'Plum Pudding' model of the atom?
Dalton
Thomson
Rutherford
Bohr
5. What is the charge on an electron?
Neutral
Positive
Negative
6. Who performed the famous alpha particle scattering experiment as part of Rutherford's team?
Dalton
Thomson
Geiger and Marsden
Bohr
7. In the experiment carried out by Rutherford's team, what type of radiation was fired at gold foil?
Gamma radiation
Beta radiation
Alpha radiation
Unit 3: Atoms and Elements – Let’s Get Atomic!
name:_______________________
period: ______________
8. Which statement below is correct?
Most of the volume of an atom is taken up by a large nucleus.
Atoms are mostly space.
9. How did Rutherford revise ideas about the distribution of positive charge in an atom compared with the 'plum
pudding' model?
Rutherford suggested that the charge was evenly spread throughout the atom.
Rutherford suggested there was no positive charge in the atom.
Rutherford suggested that the positive charge was concentrated in the centre of the atom.
10. Who first suggested that the electrons orbited the nucleus in fixed energy levels (shells)?
Dalton
Thomson
Rutherford
Bohr
11. Which was the last of these 3 sub-atomic particles to be discovered?
Electrons
Protons
Neutrons
12. Which statement about Thomson's model of the atom is true?
Unit 3: Atoms and Elements – Let’s Get Atomic!
name:_______________________
period: ______________
The charge on the electrons is far greater than the positive charge in the
atom.
The total charge carried by the electrons equals the positive charge in the
atom.
13. In which two ways did Rutherford change Thomson's model of the atom?
He said that the electrons were concentrated in the centre of the atom
(nucleus).
He said that the positive charge was concentrated in the centre of the atom
(nucleus).
14. Dalton made a list of substances that he believed were elements. Decide whether the following substances from
his list are elements or compounds.
Soda
Oxygen
Carbon
Gold
Lime
He said that the electrons were orbiting centre of the atom (nucleus).
15. How did Bohr change Rutherford's model of the atom?
Unit 3: Atoms and Elements – Let’s Get Atomic!
name:_______________________
period: ______________
He said that the electrons were concentrated in the centre of
the atom (nucleus).
He said that electrons could only occupy specific energy
levels as they orbit the nucleus.
He said that the electrons could orbit the nucleus in a
completely random way.