Bonding and structure, Year 10: the three bonds and ionic bonding

Ionic

提示The metal atoms give electrons away, leaving charged particles that attract each other.

为什么Metal atoms lose outer electrons and non-metal atoms gain them, so oppositely charged ions form and attract. Two non-metals share electrons instead (covalent bonding), and metals on their own share a sea of electrons (metallic bonding).

A covalent bond

提示The name begins with "co-", as in co-operate: each of the two atoms has a part in it.

为什么Both nuclei attract the shared pair, and that attraction holds the two atoms together. Covalent bonds are found in most non-metal elements, such as oxygen, and in compounds of non-metals, such as water.

Metallic bonding

提示Both ingredients are the same class of element, so nothing is handed from one kind of atom to the other.

为什么Metallic bonding is found in metallic elements and in alloys. The atoms share delocalised electrons, which are free to move through the whole structure.

electrostatic

提示The word begins like "electron" and ends like "static".

为什么In every bond, opposite charges attract. In ionic bonding the charges are on positive and negative ions; in covalent bonding they are the positive nuclei and the shared negative electrons; in metallic bonding, the positive centres of the atoms and the delocalised electrons.

Sodium is a metal; hydrogen is a non-metal

提示Look at the element partnered with chlorine in each compound and ask which side of the periodic table it belongs to.

为什么A metal bonded to a non-metal transfers electrons, which gives ions. Two non-metals share electrons, which gives molecules. So the same element, chlorine, bonds in two different ways depending on its partner.

An ion

提示A three-letter word; table salt is built from two kinds of them.

为什么An atom is neutral because it has equal numbers of protons and electrons. Losing electrons leaves more protons than electrons, so the particle is positive; gaining electrons makes it negative.

It gains one electron to fill its outer shell

提示Count how many more it needs to match the noble gas next door, argon.

为什么A Group 7 atom has seven outer electrons, one short of a full shell. Taking one extra electron gives it one more negative charge than it has protons, so the ion is 1−. In the same way a Group 6 atom gains two and becomes 2−.

The electron transferred from the sodium atom

提示The two symbols are a way of labelling where each one started out.

为什么The seven crosses are chlorine's own outer electrons; the eighth is a dot because it came from sodium's outer shell. All electrons are really identical, and the two symbols only keep track of where each came from. The sodium ion is labelled with a + charge and the chloride ion with a − charge.

noble gas

提示Think of the family in Group 0, whose atoms hardly react at all.

为什么A sodium ion has the electronic structure 2,8, the same as neon; a chloride ion has 2,8,8, the same as argon. A full outer shell is a stable arrangement, which is why these ions form.

Two, because calcium loses two electrons and each chlorine gains one

提示Compare how many the metal must get rid of with how many a single non-metal atom has room for.

为什么A Group 2 atom loses its two outer electrons and becomes a 2+ ion. A Group 7 atom can take only one, becoming 1−, so two chlorine atoms are needed for every calcium. That is why the formula of calcium chloride is CaCl2.

A giant ionic lattice

提示The last word is also used for a criss-cross garden fence or the top of a pie.

为什么The pattern repeats in all three dimensions through the whole crystal, so there are no separate molecules. Sodium chloride is the one example whose structure you are expected to know.

In all directions

提示The pull of a charge is not aimed at one neighbour; picture it spreading out like light from a bulb.

为什么Each ion attracts every oppositely charged ion near it: above, below and on every side. This attraction throughout the lattice is what is meant by ionic bonding.

Packed closely together, with no sticks between them

提示The gaps in the model are only there so that you can see inside it.

为什么The sticks stand for the forces of attraction, which are not solid objects and act in every direction. A three-dimensional model with the ions touching shows the packing better, but then the ions in the middle are hidden.

That it is a giant structure of many ions

提示Think about the number of each kind in a single grain of salt, and the way they are stacked.

为什么A dot and cross diagram is good at showing how the electron is transferred and what charges result. It says nothing about the lattice: the huge number of ions, their regular arrangement, or the forces acting in all directions.

CaCl2

提示Write the metal first, then put a number after the ion there are more of.

为什么An empirical formula is the simplest whole-number ratio of the ions: one calcium to two chloride. The charges balance as well, because one 2+ charge is cancelled by two 1− charges.