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John Dalton 1766 - 1844
In 1806, the elements were defined by Dalton as being substances composed of only one type of atom.
He also introduced the idea that atoms had a ‘solid ball’ structure.
We now define an element as a substance composed of atoms, each of which has the same number of protons.

In 1897, J. J. Thompson showed that an atom contained small negatively charged particles.
He called these particles electrons.
Thompson knew that the atom was neutral (had no electrical charge).
J. J. Thompson 1856 - 1940
He proposed that the atom must be made of a positively charged sphere which contained the negatively charged electrons.
Positively charged sphere
Negatively charged electron
This became known as the
‘plum pudding’ model.

In 1910, Earnest Rutherford’s team carried out experiments in which particles were fired at a thin sheet of gold.
E. Rutherford 1871 - 1937
Rutherford expected all of the particles to pass straight through...

In 1910, Earnest Rutherford’s team carried out experiments in which particles were fired at a thin sheet of gold.
E. Rutherford 1871 - 1937
Rutherford expected all of the particles to pass straight through...

Press ‘Start’ to take a closer look at Rutherford’s experiment...
Rutherford also predicted the existence of neutrons in the nucleus, and named the three types of nuclear radiation alpha (

), beta (

) and gamma (

).

An atom contains a central nucleus around which electrons orbit in different levels.
The nucleus is composed of neutrons and protons.

The nucleus is often shown as a static body, whose sub-atomic particles fill the space of the nucleus.
In fact, the nuclear particles are in constant motion.
The nucleus of an atom is tiny compared to the volume occupied by the electrons of the atom.
If an atom could be enlarged to the size of a football stadium, the nucleus of the atom would be about the size of a golf ball, somewhere above the centre spot.

An electron has an incredibly small mass, which is about 1/2000 the mass of a neutron or a proton.
Therefore, electrons are not considered to contribute anything to the mass of an atom.
Hence, the vast majority of the mass of an atom is found in the nucleus.

There are many different models used to describe the structure of an atom.
In this model the electrons are shown orbiting around a central nucleus.
However, electrons also possess properties typical of waves.

Electrons orbit so fast that they effectively form shells arranged around the nucleus.
The electrons fill the shells starting from the one nearest to the nucleus.
Each shell has a maximum number of electrons that it can hold.

Protons, neutrons, and electrons are types of sub-atomic particle.
Protons and electrons are electrically charged, which affects the structure and stability of the atom.

Protons are positively charged
 Electrons are negatively charged

Neutrons have no charge
The charges on the proton and electron are opposite in sign but equal in magnitude, so they cancel each other out exactly.

All substances on Earth are built from atoms containing these three sub-atomic particles.
Elements differ from each other in that their atoms contain different numbers of sub-atomic particles.
The number of protons in an atom is called the atomic number.
This is the factor which decides the identity of an atom - which element it belongs to, e.g.

All lithium atoms have three protons

All carbon atoms have six protons

All uranium atoms have ninety two protons
All atoms are electrically neutral, so they always have an equal number of protons and electrons.

The mass of an atom is mainly in the nucleus.
Protons and neutrons have an approximately equal mass of one atomic mass unit.
Electrons have negligible mass. Therefore, the mass of an atom is equal to the sum of the number of protons and neutrons.
This number is called the atomic mass number of the atom.
Example:
Atomic
Mass Number
12
Carbon atoms that have six protons and six neutrons, have an atomic mass of twelve atomic mass units.
Atomic Number
6
Atomic
Symbol

All atoms of the same element have the same number of protons.
However, atoms may have different numbers of neutrons.
Atoms of the same element with different numbers of neutrons are called isotopes of that element.
Within a sample of oxygen, some atoms can have 8, 9 or 10 neutrons - these are the different isotopes of the oxygen atom...
16 17 18
8
8 neutrons
8 protons
8
9 neutrons
8 protons
8
10 neutrons
8 protons

The most common isotope of carbon ( 12 C) - “carbon-12” can be represented as follows:
Atomic
Mass Number
12
Atomic
Symbol
Atomic Number
6
A carbon-12 atom has:

6 protons

6 neutrons

6 electrons

However, another isotope of carbon, 14 C can be represented as follows:
Atomic
Mass Number
14
Atomic
Symbol
Atomic Number
6
A carbon-14 atom has:

6 protons

8 neutrons

6 electrons

H. Becquerel
1852 - 1908
In 1896, Henri Becquerel discovered radioactivity by accident.
Becquerel was investigating the phosphorescent properties of uranium. As he was unable to continue his experiments, he placed the photographic paper and the uranium salt, with a key in between them, in a drawer.
Four days later Becquerel looked at his photographic paper and found it had an image of the key on it.
Following further experiments, Becquerel discovered that uranium produced radioactivity, for which he was awarded a Nobel Prize.
Image formed by Becquerel on photographic paper

Marie Curie and her husband Pierre, spent many years studying the radioactive uranium ore, pitchblende.
Pierre Curie 1859 - 1906 Marie Curie 1867 - 1934
Pitchblende had the unusual property of being more radioactive than uranium. The Curies deduced that pitchblende must contain another radioactive element.
Eventually after refining tonnes of pitchblende, Marie and Pierre identified two new elements, which they called polonium and radium.

In 1903 Marie Curie,
Pierre Curie and
Henri Becquerel were all jointly awarded the
Nobel Prize for Physics.
In 1911 Marie Curie was also awarded the Nobel
Prize for Chemistry and thus became one of only two people to be awarded a Nobel Prize in two different subjects.
Marie Curie working in her laboratory

Marie died in 1934 of a medical condition which was almost certainly caused by working with pitchblende.
The medical condition was aplastic anaemia which is known to be caused by exposure to radiation.
Marie Curie’s radioactive sources

 Isotopes such as 14 C are not stable and will emit atomic particles and/or energy - this is radioactivity.

The emission of a particle and/or energy is called radioactive decay, and the change that occurs is known as a decay reaction.
 A decay reaction will leave behind a more stable nucleus.

Heat is always given out.

 For an individual atom, the decay process is spontaneous and totally random. However, for a mass of radioactive material, the rate of decay is predictable.

Press the buttons to show the three types of radioactivity...

An unstable isotope is called a radioisotope.
When a radioactive element decays, it may become...

A different isotope of the same element

A new element
The original radioisotope is always referred to as the parent nuclide (the unstable nucleus), and the resulting isotope as the daughter nuclide.
Many radioactive elements decay as part of a decay chain, in which the original parent nuclide forms an unstable daughter nuclide, which also decays...

238
U
 234
Th
 234
Pa
 234
U
There are another 11 stages to this chain, ending in...
206
Pb
Click to see the complete decay chain

A transmutation is where one element changes into another, due to a nuclear reaction.
Example:
When alpha particles are fired at nitrogen-atom nuclei, the nitrogen atoms are changed into oxygen and hydrogen atoms.
14
7
N +
4
2
He

Checking the atomic mass figures...
17
O
8
+
1
1
H
14 + 4 = 17 + 1
Checking the atomic number figures...
7 + 2 = 8 + 1

Half-life is the time it takes for half the number of radioactive nuclei within a sample to decay.
Another way of saying this is...
Half-life is the time it takes for the count rate from a radioactive sample to fall to half its level.

Half-life is the time it takes for half the number of radioactive nuclei within a sample to decay.
Another way of saying this is...
Half-life is the time it takes for the count rate from a radioactive sample to fall to half its level.
What is the half-life of the above radioactive source?
10 years