Bonding and structure, Year 10: covalent and metallic bonding

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HintEach hydrogen atom can make only one bond, and all of them are attached to the carbon.

The whyCarbon has four outer electrons and needs four more. It shares one pair with each of four hydrogen atoms, so the dot and cross diagram shows four shared pairs around the carbon.

Two shared pairs and two unshared pairs

HintOxygen has six outer electrons, and each hydrogen brings one to pair up with.

The whyOxygen shares one electron with each hydrogen, making two shared pairs. Its other four outer electrons stay as two pairs that are not in bonds. That gives oxygen eight outer electrons in all.

Each atom needs two more electrons to fill its outer shell

HintOxygen is in Group 6; compare that with a full set of eight.

The whyEach oxygen atom has six outer electrons. Sharing two pairs brings each atom up to eight. Hydrogen and chlorine atoms need only one more electron, so H2 and Cl2 have single bonds; nitrogen atoms need three, so N2 has a triple bond.

NH3 and HCl

HintLook for the formulae that contain no metal.

The whyAmmonia (NH3) and hydrogen chloride (HCl) contain only non-metals, so their atoms share electrons in small molecules. NaCl and MgO each contain a metal with a non-metal, so they are giant ionic lattices, not molecules.

Its first shell is full with just two electrons

HintHydrogen's single electron sits in the innermost energy level, which holds far fewer than eight.

The whyHydrogen has one electron, in a shell that holds only two. Sharing one pair gives it a share of two electrons, which is a full shell. That is why every hydrogen atom in H2, HCl, H2O, NH3 and CH4 makes exactly one bond.

A single covalent bond (one shared pair of electrons)

HintIt replaces the dot and the cross that would sit between the two symbols.

The whyA line is shorthand for two electrons shared between the atoms it joins. Drawing lines is quicker than dot and cross, but it no longer shows which atom each electron came from, or the outer electrons that are not in bonds.

A large number of repeating units

HintThe chain is far too long to draw in full, so the letter stands in for how often the bracketed piece occurs.

The whyA polymer molecule contains the same unit over and over, often thousands of times. Writing the unit once in brackets with n after it is a compact way of showing the whole chain.

The unit is bonded to the next units along the chain

HintA polymer molecule does not stop at the brackets.

The whyThe two lines are covalent bonds to the neighbouring repeating units on either side. Without them the drawing would show a small separate molecule, not one link of a long chain.

C2H6

HintCount every ball of each kind, not just the ones on one side.

The whyA molecular formula gives the actual number of each atom in one molecule: 2 carbon atoms and 2 × 3 = 6 hydrogen atoms. The molecule is ethane.

The three-dimensional shape of the molecule

HintOne is flat on the page; the other you could hold and turn in your hand.

The whyA dot and cross diagram shows which electrons are shared and where they came from, but it is flat. A ball and stick model shows the arrangement in space, but not the electrons, and it wrongly suggests that the atoms are held apart with gaps between them.

Delocalised electrons

HintThe word means "no longer tied to one place".

The whyIn a metal the outer electrons leave their own atoms and are shared by the whole structure. Graphite has them too. Because they can move, they can carry charge and energy from place to place.

outer

HintThese are the electrons furthest from the nucleus and the easiest to lose.

The whyOnly the electrons in the highest occupied energy level are free to move. The inner shells stay with their own atom.

In a giant structure with a regular pattern

HintThink of oranges stacked neatly on a market stall, row upon row.

The whyThe atoms are packed in a repeating pattern that carries on through the whole piece, so a metal has no separate molecules. The delocalised electrons move through this structure and hold it together.

Metallic bonding

HintThe loose dots are the clue: nothing has been handed to one partner or shared between just two.

The whyThe circles in a regular grid are the positive metal ions in the giant structure, and the scattered dots are the delocalised electrons. The sharing of those electrons by the whole structure is the metallic bond.

The electrons are shared by all the atoms, not just two

HintAsk how far one of them is able to wander.

The whyIn a covalent bond a pair of electrons stays between two particular atoms. In a metal the outer electrons are delocalised, so each one is shared by the whole structure and can move right through it.