1.6 Shapes of molecules and ions
Examples involving coordinate bonds Ammonium ion NH4+
- Ammonia reacts with hydrogen ions to form ammonium ion , NH4+. - The formation of the coordinate bond causes a change in shape. - Remember, ammonia (NH3) is pyramidal (three bonding pairs of electrons and one lone pair) but the ammonium ion is tetrahedral (four bonding pairs of electrons).
Examples involving coordinate bonds BF4- ion
- The BF4- ion is formed when BF3 reacts with F-. - The ion has four bonding pairs of electrons around the central boron atom so it takes up a tetrahedral shape (bond angle 109.5) to minimise repulsions.
Unusual examples Boron trifluoride BrF3
- Bromine (outer electronic configuration 4s2 4p5) has one unpaired electron, so to form the three covalent bonds required for BrF3, one electron in the bromine atom is promoted to a higher sub shell. - This gives bromine in this compound three unpaired electrons, which can form three bonding pairs of electrons leaving two lone pairs of electrons. - Five pairs of electrons would suggest a trigonal bipyramidal general shape with two of the pairs being lone pairs of electrons. The dots in the dot and cross diagram represent bromine electrons. - The shape is described as T shaped because the lone pairs of electrons take up positions 120º from each other. - The three fluorine atoms take up the three other positions in the trigonal bipyramid. - The repulsion from the lone pairs of electrons is greater than the repulsion from the bonding pairs so the 90º angle is reduced to 86º.
Examples involving coordinate bonds H3O+ ion
- The H3O+ ion is formed when H2O reacts with H+. - The H3O+ ion has three bonding pairs of electrons and one lone pair of electrons around the oxygen atom, so it takes up a pyramidal shape (bond angle 107º) to minimise repulsion.
Examples of bonding pairs of electrons and lone pairs of electrons Water H2O
- The basic arrangement of the electron pairs is tetrahedral around the oxygen atom but as there is no atom attached to the lone pair all you need is two bonds of the tetrahedron, which appears bent. - The extra repulsion from the lone pairs squeezes the bonds closer giving a *bent* (or V) shape and decreasing the bond angle to 104.5º
Examples with only bonding pairs of electrons Carbon dioxide
- The carbon in carbon dioxide has two sets of bonding pairs of electrons. - A double bonding pair of electrons repels in the same way as a single pair. - The two sets of bonding pairs of electrons repel each other equally so CO2 takes up a linear shape to minimise repulsions.
Examples with only bonding pairs of electrons Phosphorous pentafluoride PF5
- There are five bonding pairs of electrons around the central phosphorus atom. - These bonding pairs repel each other equally and the molecule takes up a *trigonal bipyramida* shape. - There are 2 bond angles in a trigonal by-pyramid; 90º and 120º If the points where the fluorine atoms are placed are connected, the shape formed is a triangle with a pyramid above and below. This is called a trigonal bipyramid.
Examples with only bonding pairs of electrons Methane CH4
- There are four bonding pairs of electrons around the carbon atom in methane. - These repel each other equally and the molecule takes up the *tetrahedral* shape to minimise repulsions. - The bond angle is 109.5º The tetrahedral refers to the solid shape formed when all the hydrogen atoms are connected - a triangular based pyramid with four sides is called a tetrahedron.
Examples with only bonding pairs of electrons Sulfur hexafluoride SF6
- There are six bonding pairs of electrons around the central atom. - These repel each other equally and the molecule takes up an *octahedral* shape to minimise the repulsion. The octahedral shape is also called square bipyramidal. The term octahedral comes from the fact that the solid shape formed from connecting all the fluorine atoms forms an eight-sided figure called an octahedron.
Examples of bonding pairs of electrons and lone pairs of electrons Ammonia NH3
- There are three bonding pairs of electrons and one lone pair of electrons around the central nitrogen atom in NH3. - The basic arrangement of the electron pairs is tetrahedral around the nitrogen but since there is no atom attached to the lone pair, all you see is the bottom of the tetrahedron, which looks like a pyramid. - The extra repulsion from the lone pair squeezes the bonding pairs of electrons closer together, decreasing the bond angle to 107º
Examples with only bonding pairs of electrons Boron trifluoride BF3
- There are three bonding pairs of electrons around the boron atom in BF3. - These repel each other equally and so the molecules takes up a *trigonal planar* shape with a bond angle of 120º
Examples with only bonding pairs of electrons Beryllium chloride BeCl2
- There are two bonding pairs of electrons around the beryllium atom. - These repel each other equally so the molecule takes up a *linear* shape to minimise the effect of repulsion. - The bond angle is the angle between the two covalent bonds, which in beryllium chloride is 180º.
The order of strength of repulsions
- This means that lone pairs of electrons repel lone pairs more than they repel bonding pairs of electrons. - The lowest kind of repulsion is between bonding pairs of electrons. - The molecule or ion will take up a shape that minimises these repulsions.
Questions frequently asked about shapes of molecules or ions
1. sketch of the shape 2. the name of the shape 3. the bond angle 4. the explanation of the shape
Nonpolar molecule
A molecule that does not have a permanent dipole is referred to as a non polar molecule. - Molecules in which the bona dipoles are arranged more symmetrically about the central atom(s) will have smaller permanent dipoles than molecules in which the bond dipoles are less symmetrically arranged.
Lone pair
A pair of electrons in the outer shell of an atom that is not involved in bonding Lone pairs are more compact to the central atom so they have a greater repulsive effect on the other pairs of electrons.
Bonding pair
A pair of electrons on the outer shell of an atom that is shared to form a covalent bond
Molecule Shape Bent
Bond angle - 104.5º Explanation - Two bonding pairs of electrons and two lone pairs of electrons; the lone pairs of electrons have a greater repulsion than the bonding pairs of electrons; the molecule takes up this shape to minimise repulsion.
Molecule shape; Pyramidal
Bond angle - 107º Explanation - Three bonding pairs of electrons and one lone pair of electrons; the lone pair of electrons has a greater repulsion than the bonding pairs of electrons and the molecule takes up this shape to minimise repulsion.
Molecule shape; Tetrahedral
Bond angle - 109.5º Explanation - Four bonding pairs of electrons repel each other equally and the molecule takes up this shape to minimise repulsions.
Molecule shape; Trigonal planar
Bond angle - 120º Explanation - Three bonding pairs of electrons repel each other equally and the molecule takes up this shape to minimise repulsions.
Molecule shape; Linear
Bond angle - 180º Explanation - two bonding pairs of electrons repel each other equally and the molecule takes up this shape to minimise repulsions.
Molecule shape T shaped
Bond angle - 86º Explanation - Three bonding pairs of electrons and two lone pairs of electrons; lone pairs of electrons have a greater repulsion than the bonding pairs of electrons; the molecule takes up this shape to minimise repulsions.
Molecule shape; Octahedral
Bond angle - 90º Explanation - Six bonding pairs of electrons repel each other equally and the molecules takes up this shape to minimise repulsions.
Molecule shape; Trigonal bipyramidal
Bond angle - 90º and 120º Explanation - Five bonding pairs of electrons repel each other equally and the molecule takes up this shape to minimise repulsions.
What is the shape of a molecule determined by ?
The shape of a molecule or ion is determined by the repulsion between the electron pairs surrounding a central atom - the total number of electron pairs around a central atom - the number of bonding pairs of electrons - the number of lone pairs of electrons
Valence Shell Electron Pair Repulsion (VESPR)
This theory asserts that a molecule or ion will always try to minimise the repulsion between outer shell electrons by arranging the outer shell electrons as far away from each other as possible.
