Covalent Bonding, Structures and Properties – GCSE Chemistry Revision (AQA Additional Science – Double and Triple)

How does it happen?

• Like with ionic bonding, this sort of bonding involves atoms filling their outer shells to gain stability
• However, in covalent bonding atoms share their electrons and don’t lose or gain them – so no ions are produced
• In order to keep sharing, the atoms have to be near each other - they are bonded!
• Hydrogen has only one electron. If it can get another it will have an outer shell of two, which is plenty when it’s your only shell
• Fluorine has an outer shell of 7 electrons. Obviously, it would like another.
• So hydrogen and fluorine team up to produce hydrogen fluoride, sharing electrons with each other as they go

Advanced Bonding

• Atoms can form more than one covalent bond at once – for instance, carbon can make four
• This is because it has four outer shell electrons, and needs to gain another four
• It can share an electron with a hydrogen atom to get one shared in return
• If it does this twice, it has six outer shell electrons, and two happy hydrogen molecules bonded to it
• It still needs two electrons, so it can do something special – it can find another carbon atom, and they can both share two electrons with each other, filling up their outer shells
• Since two electrons are being shared, this is called a double bond

Diagrams

• To draw diagrams of covalent bonding, make dot and cross diagrams like you do for ionic bonding, but draw them overlapping, so that the shared electrons are in the shells of both the atoms that need them
• There is another sort of diagram we can use to represent the complicated arrangement I described above – it involves letters and lines
• Here’s one for the Carbon and Hydrogen example:

H   H
|    |
C=C
|     |
H  H

• The lines represent the covalent bonds
• The double line represents the double bond
• Two carbon atoms have bonded to two hydrogen atoms each, then double bonded  to each other
• The chemical formula for that compound would be something like C2H4, because there are two carbon atoms and four hydrogen atoms, but I forget what the compound is called
• Double bonds are harder to break than single bonds

Simple Covalent Molecules

• Molecules like the one above are called simple covalent molecules
• They have low melting and boiling points, so they are often gases at room temperature
• The bonds between the atoms (shared electrons) are strong, but the bonds between the separate molecules are weak, so one of the molecules in the diagram above won’t stick to an identical one

Giant Covalent Structures

• Some covalent compounds form giant structures similar to the ionic lattices
• These strong bonds between atoms make them hard with high melting points
• Carbon atoms in a giant covalent structure make diamond – one of the hardest things on earth, because each atom makes four bonds
• Diamond, Graphite and Fullerenes are other giant covalent structures, but I won’t go into that here

Related articles

• Ionic Bonding – GCSE Chemistry Revision (AQA Additional Science – Double and Triple) (mattg99.wordpress.com)

Ionic Compounds – GCSE Chemistry Revision (AQA Additional Science – Double and Triple)

What do they do?

• After sodium and chlorine react to gain full outer shells, becoming ions in the process, they are doomed to stick together in a compound called sodium chloride (vinegar salt)
• But the chances are it wasn’t just one atom of sodium and one atom of chlorine in the reaction – there were probably thousands of atoms all doing the same thing and forming ions
• Because of their charges, all the ions stick together in a massive lump (a grain of vinegar salt)
• You’d think this would be a horrible mess, but it actually turns out to be a very neat arrangement we call a giant ionic lattice
• It’s made up of ions that would seem to go on and on forever in a regular arrangement

Why?

• Ionic compounds don’t make molecules – just those giant ionic structures I’ve already described
• This is because their strong electrostatic force of attraction works in all directions
• The ions pack together tightly, like words in a revision book

Drawing and Reading Ionic Bonding Diagrams – GCSE Chemistry Revision (AQA Additional Science – Double and Triple)

But…

• Yes, I know – how am I supposed to do a post about drawing diagrams, when I don’t even have any diagrams… well, I’ll do my best
• To show bonding, we use dot and cross diagrams
• To draw an atom, put the element’s chemical symbol in the middle (Na for sodium, for example)
• Now draw circles around it, to represent the electron shells
• Since this is bonding, you need to draw two atoms, so do the same for the second atom

The Electrons

• Now start to fill in the electrons – on one atom you should use dots to represent electrons, and on the other you should use crosses
• Remember, the first shell holds two electrons, then the maximum is eight (for a while, anyway – it’s enough for this course)
• Then, using your knowledge of ionic bonding, decide what electron moves where and draw an arrow to the knee draw an arrow to show the path it takes
• You should have two diagrams, showing the electronic structures of both atoms, and an arrow showing the electron that is transferred
• Write the electronic structure in brackets under the diagram, like this: (2,8,2)  – Magnesium, for example
• Sometimes, when people are feeling lazy, they deliberately ignore the inner electron shells and only draw the outer one… I have no idea whether you drop marks for this

Drawing the Ions

• So after the elements have reacted and bonded ionically, they are now ions
• You often have to draw or interpret these diagrams
• Basically they’re the same, but they show the ions – if the first diagram was a ‘before’ shot, this is the photo taken after
• Once again, you put the chemical symbol in the middle to represent the nucleus, then draw the electronic structure – but there are a few differences
• This time, you draw the electron that has moved in its new place it is in AFTER it has moved
• So you leave that electron out of the atom it came from and draw it in place in the atom that gained it
• Actually, these are ions now, since they’ve reacted, so to show this you put massive square brackets around the diagram, like this [Diagram]
• You also have to show the charge – so on the right of the square brackets you put + or -
• If it’s a double positive charge, you’d put 2+ (I’m pretty sure it’s 2+ not +2)
• When you write the electronic structure under the diagram that also has to be in square brackets and show the charge next to it
• Make sure its the up-to-date electronic structure as it would be after the reaction (so it has a nice full outer shell)
• Example: a magnesium ion’s electronic structure is [2,8] 2+

Ionic Bonding – GCSE Chemistry Revision (AQA Additional Science – Double and Triple)

Why bond?

• Elements react with other elements because they ‘want’ to end up with a full outer electron shell
• Electrons surround the nucleus of an atom in shells – remember Atomic Structure. The shells go (2,8,8) when full
• Getting the full shell will make them very stable
• The noble gases on the far right of the periodic table are like this – they don’t react with anything very much because they already have full shells
• Atoms can get a full shell by:
• Losing or gaining electrons in ionic bonding
• Sharing electrons in covalent bonding

Ionic:

• Ionic bonding happens between metals and non-metals
• Sodium and Chlorine can react together – they bond ionically to produce sodium chloride (table salt)
• Sodium’s electron shells go (2,8,1) – all its shells are full except the outer one, which only has a single electron in it… if it can get rid of that electron, its new outer shell will be full
• Chlorine’s shells go (2,8,7) – it just needs one more electron; then all its shells will be full
• The solution to this problem is for sodium to give its spare electron to chlorine – then everyone’s happy, so to speak… both atoms are now stable

The Consequences

• However, the balance of charges has been shifted!
• Normally, an atom has an equal amount of protons and electrons
• Electrons are negatively charged, and protons are positively charged, but they cancel each other out meaning the atom is neutral overall
• But this doesn’t happen after ionic bonding
• The sodium atom has sacrificed an electron to gain its outer shell – this means it has more protons than electrons, so the atom now has an overall positive charge
• Likewise, the chlorine atom has gained an electron, so it has more electrons than protons, and it has an overall negative charge

Ions

• The sodium and chlorine atoms have become ions
• An ion is an atom which has lost or gained an electron and thus has a charge
• The sodium is now a positive ion and the chlorine is now a negative ion
• And you know what happens to positives and negatives!
• Opposites attract – the ions get stuck together by their electrostatic charges!
• That’s why they call it ionic bonding

Magnesium and Oxygen

• Magnesium goes (2,8,2) – it needs to get rid of that 2 in its outer shell
• Oxygen goes (2,6) – it needs two more electrons to fill up
• So magnesium donates its two spare electrons to oxygen
• Because magnesium has lost TWO electrons, it now becomes an ion with a positive charge of 2 (because it has two more protons than electrons)
• Oxygen has gained two electrons, so it becomes an ion with a negative charge of two
• They bond together as usual
• Don’t forget that gaining electrons gets you a negative charge
• Losing electrons gets you a positive charge

Potassium and Oxygen

• Potassium’s electron shells go (2,8,8,1) – it really needs to get rid of that ONE pesky electron hogging its outer shell
• As I’ve said, oxygen goes (2,6) – it needs to gain TWO electrons
• So if the potassium gives the oxygen its spare electron it’s happy – it becomes a stable potassium ion with a full outer shell and a single positive charge
• BUT, the oxygen atom is not satisfied – it still needs one more electron to fill up its outer shell
• What are we gonna do?! Well, the solution is to bring in a second potassium atom
• Like its friend was, this new potassium atom is looking to lose one electron
• After gives an electron to oxygen, everything is fine – the two potassium ions both have single positive charges, and the oxygen has become an ion with a full outer shell and a double negative charge
• The three ions stick together – not so much because of their gratitude, but because of the strong electrostatic force that’s now bonding them
• So the moral of the story is, sometimes it takes two atoms to get the right amount of electrons