The structure of the atom
Atom
so small that you cannot see them
without using the most powerful
microscopes yet invented
comes from a Greek word that
means ‘cannot be split’
All the atoms in a particular element are the same.
Different elements have different atoms.
For example the atoms in sodium are all the same as one
another. They are different from the atoms in potassium.
Different kinds of atoms
o There are many different types of atoms.
o Scientists have discovered 94 different kinds of atoms that occur naturally in
the Universe.
o Another 24 kinds of atoms have been made in laboratories.
o Some substances are made up of just a single kind of atom.
o A substance that is made of just one kind of atom is called an element.
o For example, carbon is made only of carbon atoms.
o Gold is made only of gold atoms.
o Silver is made only of silver atoms.
o Carbon, gold and silver are examples of elements.
o Each type of atom has different properties. This is why different elements
have different properties.
Atoms joining together
Some substances are made up of individual atoms.
For example, a piece of gold is made up of millions of individual gold atoms.
Neon – which is a gas – is made of individual neon atoms.
Other substances are made up of little groups of atoms.
The atoms join together in groups of two or more.
A group of atoms joined tightly together is called a molecule.
Some elements are made up of molecules.
For example, in oxygen the atoms are joined together in pairs.
Sulfur molecules are made up of eight sulfur atoms joined together.
They can be joined in slightly different ways.
Chemical symbols
There are more than 100 different elements (including those that have been made
in laboratories).
Chemists use a shorthand way of referring to them.
They have given each element a symbol.
• the symbol for oxygen is O (first letter of the English name )
• the symbol for helium is He (first letter of the English name of the element plus
another letter from its name).
• the symbol for sodium is Na, (from the old Latin name ‘natrium’).
• The first letter of the symbol is always upper case.
The second letter, if there is one, is always lower case.
A flask of chlorine gas, symbol Cl
Iodine crystals, symbol I
Mercury, symbol Hg
Sodium, symbol Na
Elements on Earth
The pie chart shows the approximate
proportions of the various elements
that are found in
the Earth’s crust.
The structure of the atom
Atom
so small that you cannot see them without using the most
powerful microscopes yet invented
comes from a Greek word that means ‘cannot be split’
All the atoms in a particular element are the same.
Different elements have different atoms.
For example the atoms in sodium are all the same as one
another. They are different from the atoms in potassium.
Scientists have discovered that atoms are made up of even smaller
particles.
Atoms are made up of three kinds of particles: protons, neutrons and
electrons.
These particles are arranged in a similar way in every atom.
The protons and neutrons are grouped closely together in the centre of the
atom.
They form the nucleus of the atom.
(Take care not to confuse the nucleus of a cell with the nucleus of an atom!)
The electrons move around the nucleus.
The three different particles in an atom have different properties.
• Protons and neutrons have much more mass than electrons.
In fact, electrons have almost no mass at all.
• Protons have a positive electrical charge. (a property of an object which
causes it to attract or repel other objects with charge)
• Neutrons have no electrical charge.
• Electrons have a negative electrical charge.
There is a lot of empty space between the parts of the atom. This space
really is completely empty - there is nothing in it at all.
1. Which particle in an atom has a positive electrical charge?
2. Which of the three particles that make up an atom has the smallest
mass?
3. Which particles make up the nucleus of an atom?
4. The size of the negative charge of an electron is exactly the same as the
size of the positive charge of a proton. What is the overall charge of the
helium atom shown in the diagram?
Answers
1.
2.
3.
4.
proton
electron
protons and neutrons
no charge
How did scientists come up with this model of the atom?
Scientists from different parts of the world have worked on a number of
different ideas that have led to the model of the atom which we use today.
In the late 1890s a British scientist called J.J. Thompson discovered the
electron. His model for the atom was that the different parts of the atom
were scattered throughout the structure.
Sir Joseph John Thompson (J.J
Thompson)
(British physicist)
(1856-1940)
How did scientists come up with this model of the atom?
One of Thompson's research students originally came from New Zealand.
His name was Ernest Rutherford.
Rutherford discovered the proton in 1909 and the nucleus in 1911.
Rutherford's most famous experiment was the gold foil experiment.
Rutherford
was
awarded the Nobel
Prize for Chemistry
in 1908.
Ernest Rutherford
(New Zealand physicist)
(1871–1937)
His portrait appears
on the New Zealand
$100 bill.
WIn this experiment Rutherford fired fast-moving particles - smaller
than an atom - at very thin gold foil.
WMost of the particles passed straight through the foil.
WOnly a few of these particles (about 1 in 8000) were deflected in
various directions. ('Deflected' means that their direction was changed.)
WThis led Rutherford to have the idea that gold atoms must be mostly
empty space, with their particles packed into a dense nucleus at the
centre.
WThis helped to move towards the model of the atom we use today.
WJames Chadwick worked with Rutherford and Thompson. In 1932, he
proved that neutrons exist.
James Chadwick
(British physicist)
(1891–1974)
WThe stories of these men show how scientists from
all over the world work together and develop their
ideas.
WEach scientist builds on the discoveries of others.
WThese scientists won Nobel prizes for their work.
WTheir experiments and ideas have helped us to
understand the structure of the atom.
There is still a lot that we do not know about atoms.
Scientists continue to work to improve our understanding of the structure
of the atom.
For example, scientists from all over the world are using the Large Hadron
Collider in Switzerland to further understand the structure of matter.
Tunnel in the Large Hadron Collider
x
x
x
x
1p
2p 2n
3p 4n
Hydrogen
x
Helium
2
1
x
Lithium
2, 1
nucleus
electron arrangement
electron shell (or) (or) electronic structure
energy levels (or) orbit
x x
x
4p 5n
x
Beryllium
2, 2
xx
x
xx
x
x
5p 6n
6p 6n
x
x
boron
2, 3
carbon
2, 4
xx
x
xx
x
x
x
x
7p 7n
x
nitrogen
2, 5
x
x
x
x
8p 8n
x
oxygen
2, 6
x
x
xx
x
xx
x
x
x
9p 10n
x
x
fluorine
2, 7
x
x
x
x
10p 10n
x
xx
neon
2, 8
xx
xx
x
x
xx
x
x
x
x
x
11p 12n
x
xx
sodium
2, 8,1
x
x
x
x
12p 12n
x
xx
magnesium
2, 8,2
x
x
The Periodic Table
Trends in Groups within the Periodic Table
Group 1
(Alkali metals)
Group 8
•the columns in the Periodic Table are called
(noble gases)
groups
Group 7
•the row in the Periodic Table are called
(halogens)
periods
Group 1: The alkali metals
the atomic number increases as you go down the group.
The mass number also increases as you go down the group.
These increasing numbers tell you that the size of the atom is increasing.
the melting points you can see that they go down as you go down the group.
The next element down in the group is the metal rubidium.
We can predict it would have a melting point lower than 63 °C.
The structure of the Group 1 metals
The properties of lithium, sodium and potassium are similar and so is their atomic structure.
Lithium has an atomic number of 3 and a mass number of 7.
This atom contains three protons, three electrons and four neutrons.
The electrons are arranged as 2,1. This means there are two electrons in the lowest electron
shell (which is full) and one electron in the second electron shell.
Sodium has an atomic number of 11 and a mass number of 23.
This atom contains 11 protons, 11 electrons and 12 neutrons.
The electronic structure is 2,8,1. This means there are two electrons in the lowest electron
shell (which is full) and eight electrons in the second electron
shell (which is full) and one electron in the third electron shell.
Potassium has an atomic number of 19 and a mass number of 39.
This atom contains 19 protons, 19 electrons and 20 neutrons.
The electronic structure is 2,8,8,1. This means there are two electrons in the lowest electron
shell (which is full), eight electrons in the second electron shell (which is full), eight
electrons in the third electron shell (which is full) and one electron in the fourth electron
shell.
Questions
1. Where in the Periodic Table do you find the metals?
2 What is the trend in the boiling points of Group 1 elements?
3. What prediction can you make about the boiling point of rubidium?
4. How many more electrons than lithium does sodium have?
5. What happens to the size of the atoms as you go down this group?
6. What similarity is there in the structure of these atoms? (Hint: look at the
electron shell arrangements.)
7. Suggest why this group of metals is called Group 1?
8. What are the trends in the structure and behaviour of these elements in
Group 1?
Group 7: The halogens
The group includes fluorine, chlorine and bromine.
These elements have a number of properties in common.
The first two elements (fluorine and chlorine) are gases at room temperature.
Bromine is a liquid.
The most reactive is fluorine, then chlorine.
Bromine is the least reactive of the three.
Questions
9. Are the halogens metals or non-metals?
10. What is the trend in melting points in Group7?
11.What is the trend in colour in Group 7?
12. What is the trend in boiling points in Group 7?
13. What would you predict about the boiling and melting points of iodine the
next element in Group 7?
14. Would you expect iodine to be more or less reactive than bromine?
15. The electronic structure for bromine is missing from the table.
How many electrons do you predict will be in the outer shell?
The structure of fluorine and chlorine
Fluorine has an atomic number of 9 and a mass number of 19.
This atom contains 9 protons, 9 electrons and 10 neutrons.
The electronic structure is 2,7. This means that the first electron shell has 2 electrons and is
full. The second electron shell has seven electrons.
Chlorine has an atomic number of 17 and a mass number of 35.
This atom contains 17 protons, 17 electrons and 18 neutrons.
The electronic structure is 2,8,7. This means that the first electron shell has two
electrons and is full. The second electron shell has eight electrons and is full. The third
electron shell has seven electrons.
Questions
16. What happens to the size of the atoms as you go down this group?
17. What similarity is there in the structure of these atoms?
18. Suggest why this group is called Group 17.
်
Group 8: The noble gases
Group 8 includes the elements helium, neon and argon.
They are all gases.
They are inert (unreactive) and do not form compounds.
They are called the noble gases.
Helium has an atomic number of 2 and a mass number of 4.
This atom contains 2 protons, 2 electrons and 2 neutrons.
The electrons are arranged with 2 in the first shell. The shell is full.
Neon has an atomic number of 10 and a mass number of 20.
This atom contains 10 protons, 10 electrons and 10 neutrons.
The electrons are arranged with two in the first shell and eight in the second shell. Both
shells are full.
Argon has an atomic number of 18 and a mass number of 40.
This atom contains 18 protons, 18 electrons and 22 neutrons.
The electrons are arranged with 2 in the first shell and 8 in the second and third shells.
All three shells are full.
Questions
19. What trend in melting points can be seen in Group 8?
20. What happens to the size of the atoms as you go down Group 8?
21. What similarity is there in the structure of these atoms?
22. Suggest why this group is called Group 8.
23. What predictions can you make about the melting and boiling points of krypton,
which is the next gas in this group?