ncea1 chemistry basics ca 2005 element structure and the periodic table chemical bonding
TRANSCRIPT
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NCEA1 Chemistry Basics
CA 2005
Element Structure and the
Periodic Table
Chemical Bonding
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The smallest whole particles are ATOMS
An atom consists of a central nucleus
surrounded by much smaller electrons
There are over 100 different types of atom. Each one is
called an ELEMENT.
nucleus
electron
The electrons are arranged in layers
called shells.
shells
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These elements are arranged into the PERIODIC TABLE
The horizontal rows are called the PERIODS
Period 4
Elements in the same period all have the
same number of electron
shells
So elements in Period 4 all have four shells of electrons
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These elements are arranged into the PERIODIC TABLE
The Vertical columns are called GROUPSGroup 18
Elements in the same group all have the same number of electrons in their outer shell so they all have similar properties.
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For NCEA 1 you only have to know how the rules apply for the first 20 elements (Hydrogen to Calcium)
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Inside the nucleus of the atom are subatomic particles called protons and neutrons.
Protons have a mass of 1 and a charge of +1
Neutrons have a mass of 1 but no charge.
Electrons have no mass but a charge of -1
Between the electron and the nucleus is a vacuum
(nothing!)
Vacuum
+
+
++
+
Proton
Neutron
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In an element there is always the same number of
protons and electrons.
Every different element has a unique number of these.
This element has 5 protons and 5 electrons. The 5th element in the periodic
table is Boron
B5
This is called the element’s Atomic Number
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Remember: In the periodic table all the elements in a horizontal period have the same number of electron
shells.
H1
Hydrogen
He2
Helium
The first electron shell can only hold 2 electrons. So Period 1 only
contains two elements.
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Period 2 elements all have two shells. The first one contains two electrons but the second
one can have up to eight.
Li3
Lithium is the smallest element of period 2 with just
one electron in shell 2
Ne10
Neon is the largest with a full second shell of 8 electrons.
Period 3 and the first elements of Period 4 follow the same rules with up to 8 electrons in their outer shell.
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Electron Arrangements.
The full electron arrangements in their shells for the first 20 elements are shown on the next slide.
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shell 1 shell 2 shell 3 shell 4
Hydrogen 1Helium 2
Boron 2 3
Lithium 2 1Berylium 2 2
Carbon 2 4
Oxygen 2 6Nitrogen 2 5
Fluorine 2 7Neon 2 8
Aluminium 2 8 3
Sodium 2 8 1Magnesium 2 8 2
Silicon 2 8 4
Sulphur 2 8 6
Phosphorus 2 8 5
Chlorine 2 8 7Argon 2 8 8Potassium 2 8 8 1Calcium 2 8 8 2
So the electron arrangement for Sulphur is normally written:
S: 2, 8, 6
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How and Why Elements Bond Together
H2
H2SO4
PO3-
Mg2+
(NH4)2CO3
H2O
CO2
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AgK Na
Al Fe
O2
Cl2
H2
N2
CO2
HCl
H2SO4
Al(HCO3)3
These are all atoms of different ELEMENTS
The symbol for an element may be a single capital letter (eg K) or a
capital followed by a lower case letter (eg Na).
These are all MOLECULES. They all have more than one atom in them joined together
chemically.
These MOLECULES contain atoms from just one
element
These molecules all contain atoms
from more than one element.
They are COMPOUNDS
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Elements join together chemically to form molecules because their outermost shell of electrons is incomplete.
The only elements that never form molecules are in Group 18
This is because they already have a full outer shell.
He2
Eg Helium
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To get a complete outer shell atoms can bond with others in two ways:
1. They can share some of their electrons.
This is called COVALENT bonding.
or
2. They can exchange electrons.
This is called IONIC bonding.
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Covalent Bonding.
An oxygen atom has 2 electrons in its first shell and 6 electrons in
its outer one.O
8oxygen
This leaves it with 2 gaps to fill in its outer shell.
By sharing 2 of their electrons each these two atoms of oxygen now have a full 8 in their outer shell each
This produces a covalent molecule of
oxygen gas: O2
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Non-metals often form covalent compounds with other non-metals, but the commonest ones you will
come across are the gases.
Oxygen gas = O2 Hydrogen gas = H2
Nitrogen gas = N2Chlorine gas = Cl2
Fluorine = F2
Bromine = Br2
You will need to remember when you are balancing equations that these gases exist as molecules of two
atoms each!
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Elements always have the same number of protons and electrons.
But many of them can gain or lose electrons from their outer shell to become charged particles called ions
Elements that lose electrons become positive ions.
This is what the metals do.
Elements that gain electrons become negative ions.
This is what the non-metals do.
Ionic Bonding
Positive ions attract negative ions to form ionic compounds
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For instance:
Magnesium is in Group 2. It has 2 electrons in its outer shell. It can lose these to become a 2+ positive ion
Mg Mg2+ + 2e-
Oxygen is in Group 16. It has 6 electrons in its outer shell. It can gain 2 more to become a 2- ion
O + 2e- O2-
Positive and negative ions attract each other to form ionic compounds
Mg2+ + O2- MgO
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You will always be given a table of common ions in the exam
Positive Ions
H+
Li +
Na +
K +
NH4 +
(ammonium)
Be2+
Mg 2+
Ca 2+
Cu 2+
Fe 2+
Zn 2+
Al3+
Cu3+
Fe 3+
Negative Ions
F-
Cl -
Br -
I -
NO3 -
HCO3 -
O2-
S 2-
SO4 2-
CO3 2-
PO43-
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Positive Ions
H+
Li +
Na +
K +
NH4 +
(ammonium)
Be2+
Mg 2+
Ca 2+
Cu 2+
Fe 2+
Zn 2+
Al3+
Cu3+
Fe 3+
Negative Ions
F-
Cl -
Br -
I -
NO3 -
HCO3 -
O2-
S 2-
SO4 2-
CO3 2-
PO43-
Group 1 elements form 1+ ions
Group 2 form 2+ ions
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Positive Ions
H+
Li +
Na +
K +
NH4 +
(ammonium)
Be2+
Mg 2+
Ca 2+
Cu 2+
Fe 2+
Zn 2+
Al3+
Cu3+
Fe 3+
Negative Ions
F-
Cl -
Br -
I -
NO3 -
HCO3 -
O2-
S 2-
SO4 2-
CO3 2-
PO43-
A few metals can form two different ions
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Positive Ions
H+
Li +
Na +
K +
NH4 +
(ammonium)
Be2+
Mg 2+
Ca 2+
Cu 2+
Zn 2+
Fe 2+
Al3+
Cu3+
Fe 3+
Negative Ions
F-
Cl -
Br -
I -
NO3 -
HCO3 -
O2-
S 2-
SO4 2-
CO3 2-
PO43-
Ammonium is the only polyatomic positive ion.
( polyatomic ions contain atoms from more than one element )
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Positive Ions
H+
Li +
Na +
K +
NH4 +
(ammonium)
Be2+
Mg 2+
Ca 2+
Cu 2+
Zn 2+
Fe 2+
Al3+
Cu3+
Fe 3+
Negative Ions
F-
Cl -
Br -
I -
NO3 -
HCO3 -
O2-
S 2-
SO4 2-
CO3 2-
PO43-
Group 17 elements all have one space in their outer shell. They gain one electron to become 1- ions
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Positive Ions
H+
Li +
Na +
K +
NH4 +
(ammonium)
Be2+
Mg 2+
Ca 2+
Cu 2+
Zn 2+
Fe 2+
Al3+
Cu3+
Fe 3+
Negative Ions
F-
Cl -
Br -
I -
NO3 -
HCO3 -
O2-
S 2-
SO4 2-
CO3 2-
PO43-
Group 16
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Negative Ions
F-
Cl -
Br -
I -
NO3 -
HCO3 -
O2-
S 2-
SO4 2-
CO3 2-
PO43-
hydrogencarbonate
nitrate
carbonate
sulphate
phosphate
The negative polyatomic ions
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Group 18 don’t form ions. They already have a full outer shell.
Group 14 don’t form ions either. They have 4 electrons in their outer shell - so it is neither easier to gain or lose them!
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It is important to balance the formula for compounds properly.
Fortunately the rules for doing this are quite simple.
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Potassium can lose an electron to become a 1+ ion : K+
Chlorine can gain an electron to become a 1- ion: Cl-
These charges are equal and opposite so they cancel each other out to form a molecule of potassium chloride : KCl
Magnesium can lose 2 electrons and forms a 2+ ion : Mg2+
Sulphur can gain 2 electrons to become a 2- ion : S2-
The charges cancel each other out to form a molecule of magnesium sulphide : MgS
So if the ions’ charges are equal and opposite the compound’s formula is easy to work out!
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Swap and DropYou use this rule when the charges are not the same
Mg2+
This means magnesium has to
give away 2 electrons
Cl-
But chlorine can only take 1 electron when it becomes a chloride
ion.
21
You “swap and drop” the charges from above one ion to below the other
This gives you the compound formula:
MgCl2
In other words, you need two chloride ions to
balance one magnesium one.
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Aluminium oxide
Al3+ O2-
The ions:
32
Swap and dropThe compound:
Al2O3
Iron III bromide
The ions: Swap and drop The compound:
Fe3+ Br-1 3
FeBr3
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Using Polyatomic IonsThese look difficult, but they work exactly the same way.
Sodium hydroxideThe ions:
Na+OH-
The compound:
NaOH
Magnesium hydroxideThe ions: The compound:
Mg2+OH-
21Mg(OH)2
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MgSO4
One molecule of magnesium sulphate
2MgSO4
One magnesium ion
One sulphate ion
Two molecules of magnesium sulphate
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(NH4)3PO4
3 ammonium ions 1 phosphate ion
1 molecule of ammonium phosphate
4Fe2(SO4)3
4 molecules of iron III sulphate
2 iron III ions 3 sulphate ions
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Increasing the Rate of Reactions.
Reactions occur when molecules collide.
Reaction Product
A
B
Reactants
There are various ways to increase the rate at which this occurs.
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1. Increase the concentration of the reactants.
A
A
AA
A
A
A
A
B B
BB
B
B
B
Reaction
Product
Product
Product
Product
More reactants = More collisions = More product!