GCSE OCR Gateway Chemistry C2.2 d-i Bonding and the Periodic Table

New OCR Gateway specification from September 2016 Higher tier: grades 9 to 4: 
In this and subsequent posts I’m simply going to explain and illustrate each learning objective as they come up in the topics in the new GCSE specification. 

I’m giving you my notes from each lesson.

You can really get ahead of your class if you follow this blog and all the posts that will appear here about the new GCSEs over the coming months. 

This rejigging of the specification is just that: there is nothing really new here it has all been with us for the past half century at least. 

And remember all that’s written in italics is for the higher tier paper only.

C2 Elements, Mixtures and Compounds
C2.2 Bonding

C2.2d describe and compare the nature and arrangement of chemical bonds in:

i.        Ionic compounds


In ionic compounds bonds exist between electrically charged particles called ions.

Positive ions (cations) electrostatically attract negative ions (anions). 

The attraction is that of positive charge for negative charge, an electrostatic attraction.

The way the ions arrange themselves depends on their size but a common arrangement is for there to be six positive ions around each negative ion and six negative ions around each positive ion.

In the diagram below, of sodium chloride, sticks stand for ionic or electrostatic bonds.

Count the sticks to confirm the six—six arrangement of bonds and ions.

ii.            simple molecules 


Substances like chlorine gas, water , ammonia and methane exist as simple molecules or groups of atoms. 

Electrons shared in pairs join the atoms together in these molecules.

These are pairs of outer shell electrons.

These electrons hold both atoms positive nuclei together.

These bonds repel each other because they are formed from negatively charged electrons.

The repulsion results in simple molecules having simple shapes.

2 bonds give an angular molecule like water

3 bonds give a pyramidal molecule like ammonia

4 bonds give a tetrahedral molecule like methane.

All three are shown in this illustration below:

iii.          giant covalent structures 


Typical giant covalent structures are diamond, graphite, nano–tubes and grapheme sheets.

a) Diamond

Four bonds connect each carbon atom to four others in a tetrahedral arrangement.

These inter–atomic bonds are very strong.

The ball and stick model below shows this tetrahedral arrangement of atoms.

The arrangement is repeated continuously throughout a diamond crystal.

b) Graphite

Three bonds connect each carbon atom to three others in a sheet of atoms arranged in hexagons.

These inter–atomic bonds are very strong.

But the sheets bond weakly to each other so that the sheets can easily slide over each other.

As with diamond the arrangement is repeated through a crystal of graphite.

The giant structure is evident in this picture below:

c) Nano tubes

Nano-tubes are tubes of rolled up graphite sheets.

Each carbon atom is bonded to three others using three strong bonds.

The carbon atoms are arranged in hexagons.

d) Graphene

Graphene is a single sheet of graphite.

A one atom thick layer as in the diagram below. 

The carbon atoms are arranged in hexagons and each has three bonds connecting it to three others. 

iv.          polymers 


A typical polymer is polythene.

Polythene is a molecule formed from long string of — CH₂ —groups maybe about 1000 in a long line. 

Polythene is often represented by a long line.

There are very strong single bonds between the carbon atoms in the molecule.

There are much, much weaker bonds between the polymer chains. 

The chains can fold together to form areas of plastic that are more crystalline than others which are more amorphous.

A way of showing this difference is shown below. 

High density polythene (HDPE) is more crystalline than low density polythene (LDPE).

v.            metals

Atoms in metals are attached to each other using multi-directional bonds. 

Metal atoms have outer shells of electrons that combine with each other to form a sea of loose (or delocalized) electrons that can move from atom to atom. 

Best to show this idea using the kind of diagram below.

The atoms are arranged close packed like the reds in snooker.

You could be asked to draw the diagram above.

C2.2e explain chemical bonding in terms of electrostatic forces and the transfer or sharing of electrons

i) Ionic bonding

In ionic compounds, bonds exist between electrically charged particles called ions.

Ions form when the metal atom loses its outer shell electrons and they are transferred to the non–metal atom and complete its outer shell.

The result is that both positive and negative ions have noble gas electron arrangements with full outer shells.

Positive ions (cations) electrostatically attract negative ions (anions). 

The attraction is that of positive charge for negative charge, an electrostatic attraction.

The way the ions arrange themselves depends on their size but a common arrangement is for there to be six positive ions around each negative ion and six negative ions around each positive ion.

ii) Covalent bonding

Substances like chlorine gas, water , ammonia and methane exist as simple molecules or groups of atoms. 

Electrons shared in pairs join the atoms together in these molecules as in this example of the dot and cross diagram for water.

These are pairs of outer shell electrons.

These electrons hold both atoms positive nuclei together.

You can watch a good Youtube video here about ionic bonding (September 2016)

And on Youtube here is the quirkiest video about covalent bonding (September 2016)

C2.2f construct dot and cross diagrams for simple covalent and binary ionic substances

Let’s keep all this simple.

These are the examples of substances you’ll probably have to be able to construct.

Simple covalent molecules probably means Chlorine, Oxygen, Water, Ammonia, Methane, Carbon Dioxide

Here are the dot and cross diagrams for each one:

Chlorine

Oxygen

Water

Ammonia

Methane

Carbon Dioxide

Binary ionic substances means substances made from a metal and a non metal and probably refers to sodium chloride, magnesium oxide, sodium oxide and calcium chloride.

Sodium chloride

Magnesium oxide

Sodium oxide

Calcium chloride

C2.2g describe the limitations of particular representations and models to include dot and cross diagrams, ball and stick models and two- and three-dimensional representations.

No model can capture the fact that at all temperatures the atoms and ions in substances are vibrating non-stop even though the particles are bonded to others.

Dot and cross diagrams do not tell us anything about the shape of the molecule or the arrangement of the ions in an ionic crystal. 

Ball and stick models falsely assume that particles are solid and have no attraction for other particles beyond those to which they are attached. 

Ball and stick models forget that an atom or ion is bigger than its nucleus and that the electrons around the atom fill the space around the ball and stick: these models are called space–filling models.

Compare the ball and stick model with the slightly more accurate space–filled model of methane bottom right.

You can see in this illustration how difficult it is to show 3D in 2D where the methane displayed formula has two C—H bonds as thick lines, one bond as a wedge meant to be pointing out of the screen and one dotted line as a bond meant to be pointing behind the screen.

C2.2h explain how the reactions of elements are related to the arrangement of electrons in their atoms and hence to their atomic number

Elements with up to three electrons in their outer shells are metals and they react to lose those electrons to leave a full outer shell. 

These metals have atomic numbers that put them in Groups 1 to 3. 

Their atomic numbers (number of protons) remain the same on reaction and that means that losing up to three electrons their particles become positively charged ions or cations as we see in the illustration below the magnesium atom loses two electrons so it now has 10 electrons and 12 protons an overall electrical charge of 2+. 

Elements in Groups 5, 6 or 7 gain electrons on reaction with other elements so as to fill their outer shells.

Their atomic numbers (number of protons) also remain the same on reaction and that means that their particles become negatively charged ions (anions) on reaction as we can see in the diagram above where the oxygen atom gains 2 electrons to have 10 electrons and 8 protons, an overall charge of 2–.

C2.2i explain in terms of atomic number how Mendeleev’s arrangement was refined into the modern periodic table

Mendeleev’s arrangement was based on atomic mass and properties of the elements. 

Mendeleev correctly predicted on the basis of its properties the discovery of germanium which he called eka–silicon.

But Mendeleev’s Periodic Table had Beryllium in the wrong place between carbon and nitrogen where there was no space!

Atomic mass at the time was determined using the valency of the element  get the valency wrong and the atomic mass was out.

Mendeleev gave beryllium a valency of 2 instead of 3 and beryllium slipped into its right place between lithium and boron.

Iodine and tellurium could have been in the wrong places because tellurium’s atomic weight is greater than that of iodine.

But an examination of the properties of iodine placed it correctly in the group with chlorine etc..

It became clear much later that if instead the elements were ordered according to their atomic number (number of nuclear protons) then the anomalies were resolved. 

Today in a modern Periodic Table the elements are arranged in order of increasing atomic number. 

The history of the Periodic Table is well written at this level here (September 2016)