Model of the atom
Friday, November 10, 2023 5:24 PM
Did you know: The word 'atom' comes from the Greek word 'atomos' which means uncuttable
The nucleus of an atom is 10,000 times smaller than the atom itself
1) The plum pudding model:
• The plum pudding model was introduced in 1897 after the discovery of electrons by JJ Thompson
Properties:
• JJ Thompson described the atom as a ball of positive 'dough' with negative electrons embedded in it
• In other words, it was made of negatively charged electrons surrounded by positively charged matter
• Solid Sphere of positive charge with electrons in it
• Overall, the atom had a neutral charge
2) The Nuclear model:
Alpha particle scattering experiment:
In 1905, Rutherford asked two of his students, Geiger and Marsden to investigate The plum pudding model.
They did this by firing alpha particles at a very thin piece of gold (Gold was the only metal at that time which could be rolled out very
thin without cracking) foil, suspended in a vacuum (So particles deflected are only because of the gold foil and not by anything else)
(Alpha particles are a form of nuclear radiation with a large positive charge)
If the plum pudding model was correct, then all of the alpha particles would just fly through the golf foil undeflected since the
charges are evenly distributed. However, this was not the case. Geiger and Marsden made 3 observations:
1. Most particles did pass straight through the foil. This proves that most of the atom is empty space
2. A very small number of particles came straight back. This suggests that the positive charge and mass of an atom is concentrated
in one tiny volume , known as the nucleus. This is because the chances of being on this collision course where the alpha particle
comes straight back is also very small, meaning the positive charge the alpha particle repelled off of must also be equally small.
This observation occurred when the alpha particle hit the gold nuclei.
3. A small number of particles deflected at large angles, showing there is a concentration of positive charge in the atom. The
repulsion that occurred could only occur if the concentration it was being repelled off was also positive, therefore positive
charge is not distributed all over the atom.
• The Nuclear model was introduced in 1909 by Ernest Rutherford
Properties:
• Most of the atom is empty space
• Electrons orbit the nucleus, close enough to maintain the neutral charge
• Nucleus is positively charged and is very small compared to the rest of the atom
• Positive charge is concentrated in the centre of an atom
• Has an overall neutral charge
3) The Bohr Model:
• Introduced in 1913 by Neils Bohr
Properties:
Atomic Structure Page 1
,Properties:
• Electrons orbit the nucleus at fixed distances
• Atoms have energy levels (electron shells)
4) James Chadwick – formation of the final nuclear model
In 1932, Chadwick proved the existence of neutrons in the nucleus
Similarities & Differences between the Plum pudding model and the nuclear model
Similarities Differences
• Both show atoms having an • Most of the atom is empty space in
overall neutral charge the nuclear model
• Both have spherical shapes • Electrons are embedded rather than
• Both have positive and negative orbiting the nucleus in the plum
charges pudding model
• The positive charge is concentrated
into a very tiny volume in
Rutherford's model
Atomic Structure Page 2
,Atoms, Isotopes, and ions
Sunday, November 12, 2023 12:27 PM
Because protons and neutrons
Charges: are the heaviest parts of the
Proton +1 1 atom, they make up the mass
Neutron 0 1 number. The mass of an
electron is not considered to be
Electron -1 1/2000
significant
The number of protons in an atom defines it.
• For example, an atom with 6 protons will always be Carbon.
• Generally, the atomic number is also the total number of electrons, as atoms are usually neutrally charged – however, it is
worth remembering that when the atom becomes ionised, the number of electrons will change.
Mass number
Chemical symbol
Atomic number
Chlorine has 35 particles in its nucleus. It has 17 protons, 17 electrons, and 18 neutrons. (35-17)
So, an elements atomic number defines it – however an elements mass number can vary. This means the number of neutrons it has
can differ.
Isotopes are elements with the same number of protons, but different numbers of neutrons
For example, Chlorine has a mass number of 35, and 18 neutrons, however an isotope of chlorine may have a mass number of 37
which means it has 20 neutrons
Ions:
Atoms are not always neutrally charged. They can lose or gain electrons due to collisions or other interactions. When they do, they
form charged particles called ions.
An ion is a charged particle formed when an atom or molecule gains or loses electrons.
If an atom loses an electron, it becomes Positively charged
If an atom gains electrons, it becomes negatively charged
Atomic Structure Page 3
, Radioactive decay
Sunday, November 12, 2023 12:50 PM
An atoms nucleus can only be stable if it has a certain number of neutrons for the number of protons it has. For
example, Carbon 12 is a very stable element as it has 6 protons and 6 neutrons.
However, as the number of protons increases, as you go up the periodic table, the number of neutrons needed to keep
it stable increases too.
Atoms with too many, or too few neutrons are unstable will decay by emitting radiation.
Radioactive decay is a random process by which unstable atomic nuclei break apart or change to become more stable
Radiation is the energy carried out by particles from a radioactive substance
What is background radiation?
Background radiation is when naturally occurring isotopes which decay over time release radioactive particles or
waves into the surroundings.
This background radiation can come from natural sources from the environment, such as rocks and cosmic rays from
space, but also sources created by us such as the fallout from nuclear weapons testing.
Natural From us
• Cosmic rays (from space) TV sets
• Food and drink e.g. bananas Fallout from nuclear weapons
• Ground and buildings Nuclear power discharge
• Radon in atmosphere (51% of the background Medical usage
radiation we receive. It comes from the earth Non-medical industry
produced in rocks)
The actual amount of radiation that a person is exposed to depends on where they live, what job they do and many
other things. The level of background radiation and radiation dose is measured in Sieverts, Sv or mSv
• Back to radioactive decay:
An unstable nucleus decays by releasing radiation. There are 3 main types of radiation this unstable nucleus can emit.
Alpha α, Beta β, and Gamma γ.
1) Alpha radiation
If the nucleus has too few neutrons, it will release a package of 2 neutrons and 2 protons called an alpha particle.
Alpha particle:
Atomic Structure Page 4
Friday, November 10, 2023 5:24 PM
Did you know: The word 'atom' comes from the Greek word 'atomos' which means uncuttable
The nucleus of an atom is 10,000 times smaller than the atom itself
1) The plum pudding model:
• The plum pudding model was introduced in 1897 after the discovery of electrons by JJ Thompson
Properties:
• JJ Thompson described the atom as a ball of positive 'dough' with negative electrons embedded in it
• In other words, it was made of negatively charged electrons surrounded by positively charged matter
• Solid Sphere of positive charge with electrons in it
• Overall, the atom had a neutral charge
2) The Nuclear model:
Alpha particle scattering experiment:
In 1905, Rutherford asked two of his students, Geiger and Marsden to investigate The plum pudding model.
They did this by firing alpha particles at a very thin piece of gold (Gold was the only metal at that time which could be rolled out very
thin without cracking) foil, suspended in a vacuum (So particles deflected are only because of the gold foil and not by anything else)
(Alpha particles are a form of nuclear radiation with a large positive charge)
If the plum pudding model was correct, then all of the alpha particles would just fly through the golf foil undeflected since the
charges are evenly distributed. However, this was not the case. Geiger and Marsden made 3 observations:
1. Most particles did pass straight through the foil. This proves that most of the atom is empty space
2. A very small number of particles came straight back. This suggests that the positive charge and mass of an atom is concentrated
in one tiny volume , known as the nucleus. This is because the chances of being on this collision course where the alpha particle
comes straight back is also very small, meaning the positive charge the alpha particle repelled off of must also be equally small.
This observation occurred when the alpha particle hit the gold nuclei.
3. A small number of particles deflected at large angles, showing there is a concentration of positive charge in the atom. The
repulsion that occurred could only occur if the concentration it was being repelled off was also positive, therefore positive
charge is not distributed all over the atom.
• The Nuclear model was introduced in 1909 by Ernest Rutherford
Properties:
• Most of the atom is empty space
• Electrons orbit the nucleus, close enough to maintain the neutral charge
• Nucleus is positively charged and is very small compared to the rest of the atom
• Positive charge is concentrated in the centre of an atom
• Has an overall neutral charge
3) The Bohr Model:
• Introduced in 1913 by Neils Bohr
Properties:
Atomic Structure Page 1
,Properties:
• Electrons orbit the nucleus at fixed distances
• Atoms have energy levels (electron shells)
4) James Chadwick – formation of the final nuclear model
In 1932, Chadwick proved the existence of neutrons in the nucleus
Similarities & Differences between the Plum pudding model and the nuclear model
Similarities Differences
• Both show atoms having an • Most of the atom is empty space in
overall neutral charge the nuclear model
• Both have spherical shapes • Electrons are embedded rather than
• Both have positive and negative orbiting the nucleus in the plum
charges pudding model
• The positive charge is concentrated
into a very tiny volume in
Rutherford's model
Atomic Structure Page 2
,Atoms, Isotopes, and ions
Sunday, November 12, 2023 12:27 PM
Because protons and neutrons
Charges: are the heaviest parts of the
Proton +1 1 atom, they make up the mass
Neutron 0 1 number. The mass of an
electron is not considered to be
Electron -1 1/2000
significant
The number of protons in an atom defines it.
• For example, an atom with 6 protons will always be Carbon.
• Generally, the atomic number is also the total number of electrons, as atoms are usually neutrally charged – however, it is
worth remembering that when the atom becomes ionised, the number of electrons will change.
Mass number
Chemical symbol
Atomic number
Chlorine has 35 particles in its nucleus. It has 17 protons, 17 electrons, and 18 neutrons. (35-17)
So, an elements atomic number defines it – however an elements mass number can vary. This means the number of neutrons it has
can differ.
Isotopes are elements with the same number of protons, but different numbers of neutrons
For example, Chlorine has a mass number of 35, and 18 neutrons, however an isotope of chlorine may have a mass number of 37
which means it has 20 neutrons
Ions:
Atoms are not always neutrally charged. They can lose or gain electrons due to collisions or other interactions. When they do, they
form charged particles called ions.
An ion is a charged particle formed when an atom or molecule gains or loses electrons.
If an atom loses an electron, it becomes Positively charged
If an atom gains electrons, it becomes negatively charged
Atomic Structure Page 3
, Radioactive decay
Sunday, November 12, 2023 12:50 PM
An atoms nucleus can only be stable if it has a certain number of neutrons for the number of protons it has. For
example, Carbon 12 is a very stable element as it has 6 protons and 6 neutrons.
However, as the number of protons increases, as you go up the periodic table, the number of neutrons needed to keep
it stable increases too.
Atoms with too many, or too few neutrons are unstable will decay by emitting radiation.
Radioactive decay is a random process by which unstable atomic nuclei break apart or change to become more stable
Radiation is the energy carried out by particles from a radioactive substance
What is background radiation?
Background radiation is when naturally occurring isotopes which decay over time release radioactive particles or
waves into the surroundings.
This background radiation can come from natural sources from the environment, such as rocks and cosmic rays from
space, but also sources created by us such as the fallout from nuclear weapons testing.
Natural From us
• Cosmic rays (from space) TV sets
• Food and drink e.g. bananas Fallout from nuclear weapons
• Ground and buildings Nuclear power discharge
• Radon in atmosphere (51% of the background Medical usage
radiation we receive. It comes from the earth Non-medical industry
produced in rocks)
The actual amount of radiation that a person is exposed to depends on where they live, what job they do and many
other things. The level of background radiation and radiation dose is measured in Sieverts, Sv or mSv
• Back to radioactive decay:
An unstable nucleus decays by releasing radiation. There are 3 main types of radiation this unstable nucleus can emit.
Alpha α, Beta β, and Gamma γ.
1) Alpha radiation
If the nucleus has too few neutrons, it will release a package of 2 neutrons and 2 protons called an alpha particle.
Alpha particle:
Atomic Structure Page 4
