Download 9.3 Atoms and Elements notes

Atomic Structure
An atom is the smallest possible particle of an element that can exist.
An element is made up of only one type of atom.
The Structure of the Atom
An atom consists of a tiny, massive central nucleus surrounded by the
electrons.
The electrons occupy shells around the nucleus which is made up of the
protons and neutrons.
Sub-atomic
particle
proton
neutron
Symbol
electron
Relative
Charge
+1
0
p
n
e-
-1
The nucleus




contains the protons and neutrons
is overall positively charged
is the heavy part of the atom
is very small compared to the rest of the atom
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Relative Mass
1
1
1/1836
(0.000545)
The electron shells (energy levels)
 electrons are arranged in shells (or ‘energy levels’) around the nucleus
 the electrons are fast moving
 each shell can only contain a fixed number of electrons. Once a shell is full,
any remaining electrons must go into the next shell.
Atoms of different elements contain different numbers of protons. If two atoms
have the same number of protons then they must be atoms of the same element.
Atoms have no overall charge, therefore:
The number of protons is the same as the number of electrons
Atomic Number and Mass Number
Each element in the Periodic Table has a chemical symbol and has two numbers
associated with it e.g.
The top number (the ‘mass number’ or ‘nucleon number’) and the bottom number
(the ‘atomic number’ or ‘proton number’) give us information about the structure of
an atom of that element. It can be seen that the elements are arranged in the
Periodic Table in order of atomic number.
The atomic number gives the number of protons in the nucleus of an atom
The mass number gives the number of protons and neutrons in the nucleus
Looking again at the information for chlorine given above:
This tells us:
Symbol for chlorine = Cl
Atomic number of chlorine = 17
Mass number of chlorine = 35
We can work out the number of protons, neutrons and electrons from this
information:
 Number of protons = atomic number = 17
 Number of neutrons
= mass number – number of protons = 35 – 17 = 18
 Number of electrons
= number of protons = 17
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Atomic Structure Worksheet 1
Use your Periodic Table to complete the following table. The last three
columns require you to write in the number of protons (p), electrons
(e-) and neutrons (n).
Element
Symbol
Mass Atomic
Number number
silicon
argon
oxygen
beryllium
sodium
boron
potassium
fluorine
neon
lead
platinum
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p
e-
n
Isotopes
The number of protons in an element is fixed e.g. carbon will always have 6 protons
in its nucleus. The number of neutrons, however, can vary slightly. There are three
kinds of carbon atom: carbon-12, carbon-13 and carbon-14. They have different
mass numbers as each has a slightly different number of neutrons in the nucleus:
Atoms of the same element with the same number of protons but different
numbers of neutrons are known as isotopes
Thus there are 3 isotopes of carbon. This has no effect at all on their chemical
properties (how they react), since this is determined by the number and
arrangement of electrons and all three of the isotopes have the same number of
electrons.
Carbon-14 can be known as a radioisotope because it is radioactive. This means its
nucleus is unstable and it can break down to form a more stable form, emitting
radiation as it does so. This property of carbon-14 is used in ‘carbon dating’ ancient
artefacts. The other two isotopes of carbon are not radioactive.
Radioisotopes of elements can be dangerous due to the radiation they emit, but
they can also be extremely useful in, among many other things, medicine (cancer
treatment) and checking oil and gas pipes for leaks in conjunction with a Geiger
counter..
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Complete the ‘p e- n’ numbers for the following sets of isotopes:
Isotope
Symbol
Mass
Number
Atomic
number
p
e-
n
uranium235
uranium238
chlorine-35
chlorine-37
Electron arrangements (Electron configurations)
Working out the number of electrons an atom has is easy – find the proton number
from the Periodic Table, and the number of electrons will be the same:
number of electrons in an atom = number of protons
Electrons are arranged in energy levels (also known as shells) around the nucleus.
The lowest energy levels are always filled first. These are closer to the nucleus
and hold the least numbers of electrons.
 The first energy level can only hold 2 electrons.
 The second energy level can only hold 8 electrons
 The third energy level can hold 8 electrons*
Potassium has an atomic number of 19. An atom of potassium has 19 protons and
therefore has 19 electrons. Arranging the electrons in levels, filling the inner
(lower energy) levels first gives 2 in the first level; 8 in the second level; 8 in the
third level; 1 in the fourth level. This can be shown on a diagram:
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Or it can be written as: 2,8,8,1.
So the electron arrangement of potassium is 2,8,8,1.
Exercise: Work out the electron arrangement for chlorine. Draw a diagram and
give the number form of the electron arrangement.
*
The third and fourth shells can expand to hold a maximum of 18 electrons, but not
when they are the outer shells. For example Rubidium has 37 electrons arranged
2,8,18,8,1.
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Electron Arrangement Worksheet
Use your Periodic Table to complete the following table. Draw the diagram AND give the number
version of the electron arrangement.
hydrogen
helium
lithium
beryllium
2
boron
carbon
nitrogen
oxygen
fluorine
neon
sodium
magnesium
2,8,1
aluminium
silicon
phosphorus
sulphur
argon
potassium
calcium
2,8,3
chlorine
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Link between electron arrangement and the Periodic Table
We have already seen that elements are arranged in the Periodic Table in order of
their atomic number. Clues about the electron arrangement of an element can also
be determined by considering its position in terms of Groups (columns)and Periods
(rows) in the Periodic Table. Complete the following to illustrate this (the first one
has been started for you):
Element
Electron
Arrangement
sodium
2,8,1
Number of
occupied
shells
Number of
electrons
in OUTER
shell
Period
Group
3
1
aluminium
chlorine
magnesium
carbon
oxygen
What is the relationship between the electron arrangement and the position of the
element in the Periodic Table?
The number of electron shells =
The number of outer electrons =
Using this relationship, the electron arrangement can be worked out very quickly.
Try it!
P
Ca
Ar
‘Full’ outer shells
Atoms with ‘full’ outer shells appear in Group 0. The Group 0 elements are known as
the noble gases because they are almost completely unreactive. This is because of
their electronic structure (with ‘full’ outer shells). An electronic structure such as
this is known as a noble gas structure.
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Elements, mixtures and compounds
A substance which contains just one type of atom is known as an
element.
There are well over 100 types of atom. For example, the element carbon contains
only carbon atoms. An atom is the smallest part of an element you can get elements consist of just one type of atom (but there are countless numbers of
atoms, not just one)! Each element has its own symbol. For example the symbol for
carbon is C and the symbol for iron is Fe.
The atoms of different elements can join together in chemical reactions, forming
compounds.
A COMPOUND is two or more different elements which are chemically
bound together.
The properties of compounds are usually very different from the properties of the
elements they contain. For example sodium is a very reactive shiny metal and
chlorine is a very poisonous green gas. Sodium chloride, which is made when these
two elements react, is commonly known as salt and is safe to eat.
A MIXTURE is when two more elements (or compounds) are mixed
together but not chemically joined.
Element
Element
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Compound
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Mixture
Mixture
Metals and non-metals
About one-quarter of the elements are non-metals. They are found on the righthand side of the periodic table.
Nonmetals
Non-metals have different properties to metals and are summarised in the table
below:
Properties of metals
Properties of non-metals
Shiny (when polished)
They are solids at room
temperature (except mercury)
They conduct electricity
Dull
They can be solids, liquids or gases
at room temperature
They don’t conduct electricity
(apart from carbon (graphite)
They don’t conduct heat
Brittle
They conduct heat
Strong (apart from group 1 metals
which can be cut with a knife)
Malleable
Ductile
Non-malleable
Not ductile
Alloys such as steel or brass are not elements and so are not found on the periodic table
They are mixtures of metals often with several other metals and non-metals too. The
properties of the component metals are modified by alloying, allowing the alloy to be used
in new applications.
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Topic 3:Atoms & Elements
Summary questions
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