CA 2005

NCEA1 Chemistry Basics

Element Structure and the Periodic Table

Chemical Bonding

nucleus electron shells The smallest whole particles are ATOMS An atom consists of a central nucleus surrounded by much smaller electrons The electrons are arranged in layers called shells.

There are over 100 different types of atom. Each one is called an ELEMENT .

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

These elements are arranged into the PERIODIC TABLE

The Vertical columns are called GROUPS Elements in the same group all have the same number of electrons in their outer shell so they all have similar properties.

Group 18

For NCEA 1 you only have to know how the rules apply for the first 20 elements (Hydrogen to Calcium)

Vacuum Neutron

+ + + + +

Proton 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!)

This is called the element’s

Atomic Number

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 5 th element in the periodic table is Boron B 5

Remember: In the periodic table all the elements in a horizontal period have the same number of electron shells.

Helium Hydrogen

He

2

H

1 The first electron shell can only hold 2 electrons. So Period 1 only contains two elements.

Period 2 elements all have two shells. The first one contains two electrons but the second one can have up to eight.

Lithium is the smallest element of period 2 with just one electron in shell 2 Neon is the largest with a full second shell of 8 electrons.

Ne 10 Li 3 Period 3 and the first elements of Period 4 follow the same rules with up to 8 electrons in their outer shell.

Electron Arrangements

.

The full electron arrangements in their shells for the first 20 elements are shown on the next slide.

Hydrogen Helium Lithium Berylium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulphur Chlorine Argon Potassium Calcium shell 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 shell 2 1 2 3 4 5 6 7 8 8 8 8 8 8 8 8 8 8 8 shell 3 7 8 8 8 1 2 3 4 5 6 shell 4 So the electron arrangement for Sulphur is normally written:

S:

2, 8, 6 1 2

H 2 H 2 O Mg 2+ PO 3 -

How and Why Elements Bond Together

(NH 4 ) 2 CO 3 H 2 SO 4 CO 2

These are all atoms of different ELEMENTS Ag Al K Na Fe 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.

O 2 Cl 2 H 2 N 2 These MOLECULES contain atoms from just one element CO 2 H 2 SO 4 HCl Al(HCO 3 ) 3 These molecules all contain atoms from more than one element. They are COMPOUNDS

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.

Eg Helium He 2

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.

O oxygen 8

Covalent Bonding.

An oxygen atom has 2 electrons in its first shell and 6 electrons in its outer one.

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: O 2

Non-metals often form covalent compounds with other non-metals, but the commonest ones you will come across are the gases.

Oxygen gas = O 2 Hydrogen gas = H 2 Nitrogen gas = N 2 Chlorine gas = Cl 2 Bromine = Br 2 Fluorine = F 2 You will need to remember when you are balancing equations that these gases exist as molecules of two atoms each!

Ionic Bonding

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.

Positive ions attract negative ions to form ionic compounds

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



Mg

2+

+ 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



O

2 Positive and negative ions attract each other to form ionic compounds

Mg

2+ +

O

2  MgO

You will always be given a table of common ions in the exam

Positive Ions H + Li + Na + K + NH 4 + (ammonium) Be 2+ Mg 2+ Ca 2+ Cu 2+ Fe 2+ Zn 2+ Al 3+ Cu 3+ Fe 3+ F Cl Br I NO 3 HCO 3 Negative Ions O 2 S 2 SO 4 2 CO 3 2 PO 4 3-

Group 1 elements form 1+ ions Group 2 form 2+ ions Positive Ions H + Li + Na + K + NH 4 + (ammonium) Be 2+ Mg 2+ Ca 2+ Cu 2+ Fe 2+ Zn 2+ Al 3+ Cu 3+ Fe 3+ F I Cl Br NO 3 HCO 3 Negative Ions O 2 S 2 SO 4 2 CO 3 2 PO 4 3-

A few metals can form two different ions Positive Ions H + Li + Na + K + NH 4 + (ammonium) Be 2+ Mg 2+ Ca 2+ Cu 2+ Fe 2+ Zn 2+ Al 3+ Cu 3+ Fe 3+ F I Cl Br NO 3 HCO 3 Negative Ions O 2 S 2 SO 4 2 CO 3 2 PO 4 3-

Ammonium is the only polyatomic positive ion.

( polyatomic ions contain atoms from more than one element ) Positive Ions H + Li + Na + K + NH 4 + (ammonium) Be 2+ Mg 2+ Ca 2+ Cu 2+ Zn 2+ Fe 2+ Al 3+ Cu 3+ Fe 3+ F Cl Br I NO 3 HCO 3 Negative Ions O 2 S 2 SO 4 2 CO 3 2 PO 4 3-

Group 17 elements all have one space in their outer shell. They gain one electron to become 1 ions Positive Ions H + Li + Na + K + NH 4 + (ammonium) Be 2+ Mg 2+ Ca 2+ Cu 2+ Zn 2+ Fe 2+ Al 3+ Cu 3+ Fe 3+ F Cl Br I NO 3 HCO 3 Negative Ions O 2 S 2 SO 4 2 CO 3 2 PO 4 3-

Group 16 Positive Ions H + Li + Na + K + NH 4 + (ammonium) Be 2+ Mg 2+ Ca 2+ Cu 2+ Zn 2+ Fe 2+ Al 3+ Cu 3+ Fe 3+ F Cl Br I NO 3 HCO 3 Negative Ions O 2 S 2 SO 4 2 CO 3 2 PO 4 3-

The negative polyatomic ions phosphate sulphate carbonate nitrate F Cl Br I NO 3 HCO 3 Negative Ions O 2 S 2 SO 4 2 CO 3 2 PO 4 3 hydrogencarbonate

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!

It is important to balance the formula for compounds properly.

Fortunately the rules for doing this are quite simple.

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 : Mg 2+ Sulphur can gain 2 electrons to become a 2- ion : S 2 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!

Swap and Drop

You use this rule when the charges are not the same Mg 2+ 1 Cl 2 You “

swap and drop

” the charges from above one ion to below the other This gives you the compound formula: This means magnesium has to give away 2 electrons But chlorine can only take 1 electron when it becomes a chloride ion.

Mg Cl 2 In other words, you need two chloride ions to balance one magnesium one.

The ions: Aluminium oxide

Swap and drop

The compound: Al 3+ 2 O 2 3 Al 2 O 3 The ions: Iron III bromide

Swap and drop

The compound: Fe 3+ 1 Br 3 Fe Br 3

Using Polyatomic Ions

These look difficult, but they work exactly the same way.

The ions: Na + Sodium hydroxide The compound: OH Na OH The ions: Mg 2+ 1 Magnesium hydroxide The compound: OH 2 Mg (OH) 2

One magnesium ion One sulphate ion

MgSO

4 One molecule of magnesium sulphate Two molecules of magnesium sulphate

2MgSO

4

3 ammonium ions 1 phosphate ion

(NH

4

)

3

PO

4 4 molecules of iron III sulphate 1 molecule of ammonium phosphate 2 iron III ions

4Fe

2

(SO

4

)

3 3 sulphate ions

Increasing the Rate of Reactions.

Reactions occur when molecules collide.

A Reactants B Reaction Product There are various ways to increase the rate at which this occurs.

1. Increase the concentration of the reactants.

A A A A A A A A Product Product Reaction Product B B B B B B B Product More reactants = More collisions = More product!