Wednesday, 08 June 2022
Bell Task
What is the definition of the word: Theory ?
General Definition: an untested hunch, or a guess without supporting
evidence.
But for scientists, a theory has nearly the opposite meaning…
Science Definition: A theory is a well-substantiated explanation of an
aspect of the natural world that can incorporate laws, hypotheses and
facts. It is robustly tested and is the consensus best explanation.
Examples include:
• Astronomy: Big Bang Theory.
• Biology: Cell Theory; Theory of Evolution; Germ Theory of Disease.
• Chemistry: Atomic Theory; Kinetic Theory of Gases.
• Physics: General Relativity; Special Relativity; Quantum Field Theory.
Wednesday, 08 June 2022
Lesson Objectives
Knowledge
• Name some of the scientists involved in the stages of atomic
structure evolution, and state what they found.
Skills
• Read and interpret a timeline.
Understanding
• Explain how the alpha particle scattering experiment led to
a change in the model of the atom from the plum pudding
model.
• Describe the differences between the Plum Pudding and the
Nuclear models
Wednesday, 08 June 2022
Early Theories
(pre 1890)
Early investigations were limited due to technology however
some philosophers and scientists came up with some theories
Wednesday, 08 June 2022
Atomic Theory Timeline
450BC
Democritus – a Greek natural philosopher
1.
2.
3.
4.
Atoms are indestructible.
Atoms are solid but invisible.
Atoms are homogenous.
Atoms differ in size, shape, mass,
position, and arrangement.
1803
Dalton – English chemist, physicist and
meteorologist
1.
2.
All matter is made up of atoms .
All atoms of the same element are
identical. All atoms of different
elements are different.
In Summary: Before the discovery of the electron, atoms were
thought to be tiny spheres that could not be divided.
Wednesday, 08 June 2022
Post 1890 discoveries
1890
1900
1910
1920
1930
1940
1897 – Electrons
discovered
1897 – JJ Thompson
developed the Plum
Pudding model
1. The discovery of electrons and
the plum pudding model
Wednesday, 08 June 2022
The Plum Pudding model
In 1897, whilst studying cathode rays, JJ Thomson discovered
tiny particles with a negative charge (electrons).
These negative particles
were given out by atoms
and were much smaller
than atoms.
Thomson had
discovered the existence
of electrons.
Wednesday, 08 June 2022
The Plum Pudding model
Based on his discovery, Thomson adapted
Dalton’s model of the atom.
He suggested that an atom is a positivelycharged sphere with negative electrons
distributed throughout it.
Thomson’s model became known as the
plum pudding model, because the
electrons in the atom were thought to
be like raisins in a plum pudding.
Wednesday, 08 June 2022
Post 1890 discoveries
1890
1900
1910
1920
1930
1940
1897 – Electrons
discovered
1. The discovery of electrons and
the plum pudding model
1897 – JJ Thompson 1909 – E Rutherford
2. the Nuclear model using alpha
developed the Plum
developed the
particle scattering
Pudding model
Nuclear model
using the alpha
particle scattering
experiment
Wednesday, 08 June 2022
The Nuclear model
In 1909 Ernest Rutherford designed an experiment to test the plum pudding
model. Positively charged alpha particles were fired at thin gold foil.
Copy and complete the following:
Observation
Conclusion
most alpha particles passed
straight through the gold foil
the mass of the atom is in the nucleus or
most of the atom is empty space
some alpha particles were
deflected / reflected
the atom has a positively charged nucleus
Wednesday, 08 June 2022
Comparing Models
1. Create a table that compares the Plum Pudding and Nuclear models
(Level (4), 3)
Ball of positive charge spread throughout
Atom solid sphere with no empty space
mass concentrated at the centre
Mass spread throughout
Atom mostly empty space
Electrons outside the nucleus
Positive charge concentrated at the centre
Electrons embedded in the ball of positive charge
2. What similarities do the models have? (Level (5), 3)
3. Evidence from the alpha particle scattering experiment led to a change
in the model of the atom from the plum pudding model. Explain how.
(Level (7), 4)
Wednesday, 08 June 2022
Comparing Models
1. Create a table that compares the Plum Pudding and Nuclear models
(Level (4), 3)
Plum Pudding
Ball of positive charge spread
throughout
Nuclear Model
Positive charge concentrated at the
centre
Atom solid sphere with no empty space Atom mostly empty space
Electrons embedded in the ball of
positive charge
Electrons outside the nucleus
Mass spread throughout
Mass concentrated at the centre
2. Both have positive charges (1), both have (negative) electrons (1),
neither has neutrons (1) – Don’t like Q as Protons and Neutrons are not
discovered yet
3. most alpha particles passed straight through the gold foil (1), the mass
of the atom is in the nucleus or most of the atom is empty space (1).
Some alpha particles were deflected / reflected (1), the atom has a
positively charged nucleus (1)
Wednesday, 08 June 2022
Post 1890 discoveries
1890
1900
1897 – Electrons
discovered
1910
1920
1930
1940
1913 – Bohr Model
1. The discovery of electrons and
the plum pudding model
1897 – JJ Thompson 1909 – E Rutherford
2. the Nuclear model using alpha
developed the Plum
developed the
particle scattering
Pudding model
Nuclear model
3. Bohr’s revision of the nuclear
using the alpha
particle scattering
model
experiment
Wednesday, 08 June 2022
Bohr’s Changes
Ernest Rutherford’s back-scattering experiment resulted in the acceptance
of his Nuclear Model
Nuclear Model
+
Revised Nuclear (Bohr) Model
In 1913, Niels Bohr
revised Rutherford's
model
+
Electrons are randomly distributed
What 2 changes did Bohr make to the Nuclear Model? (Level (5), 2)
1. Electrons orbit the nucleus
2. Electrons are at specific distances from the nucleus
Wednesday, 08 June 2022
Post 1890 discoveries
1890
1900
1910
1920
1918 – Protons
discovered
1897 – Electrons
discovered
1930
1940
1932 – Neutrons
discovered
(James Chadwick)
1913 – Bohr Model
1. The discovery of electrons and
the plum pudding model
1897 – JJ Thompson 1909 – E Rutherford
2. the Nuclear model using alpha
developed the Plum
developed the
particle scattering
Pudding model
Nuclear model
3. Bohr’s revision of the nuclear
using the alpha
particle scattering
model
experiment
4. Protons and Neutrons
Wednesday, 08 June 2022
Post 1890 discoveries
The Plum Pudding Model
1890
1900
The Nuclear model
1910
1920
1918 – Protons
discovered
1897 – Electrons
discovered
1930
1940
1932 – Neutrons
discovered
(James Chadwick)
1913 – Bohr Model
1. The discovery of electrons and
the plum pudding model
1897 – JJ Thompson 1909 – E Rutherford
2. the Nuclear model using alpha
developed the Plum
developed the
particle scattering
Pudding model
Nuclear model
3. Bohr’s revision of the nuclear
using the alpha
particle scattering
model
experiment
4. Protons and Neutrons
Timeline Review
Wednesday, 08 June 2022
1803
1897
1907
1913
John Dalton
JJ Thompson
Ernest Rutherford
Niels Bohr
XX
−
−
−
X
XX
atoms are positivelycharged spheres with
negative electrons
embedded
throughout.
Date
Particle
1897
Electron
1918
Proton
1932
Neutron
+
XX
atoms are tiny
spheres that
could not be
divided
+
+
XX
−
X
−
the atom has a
positively charged
nucleus, most of the
atom is empty space
electrons orbit at
specific distances
from the nucleus
Additional Notes
Discovered by James Chadwick
Wednesday, 08 June 2022
On mini whiteboards:
Plenary
B
1.
Which diagram shows the plum pudding model of the atom?
2.
Which diagram shows the model of the atom developed from the alpha
particle scattering experiment?
3.
Which diagram shows the model of the atom resulting from Bohr’s work?
Extension In your book
Chadwick’s experimental work on the atom led to a better understanding of
isotopes. Explain how his work led to this understanding (Level (6), 3).
Chadwick discovered neutrons (1)
isotopes have the same number of protons (atomic number) (1)
but with different numbers of neutrons (or different mass numbers) (1)
C
A