Shapes of molecules

what is a chemical bond

an electrostatic force of attraction between positively and negatively charged species

what are the three types of chemical bonding

1. covalent bonds
2. ionic bonds
3. metallic bonds

what is a covalent bond

where electrons are shared by two positive nuclei

what is an ionic bond

a bond formed when positive and negative ions are attracted to each other

what is a metallic bond

a bond formed when metal atoms are packed closely together and are bonded to each other by delocalised, free-moving valence electrons

description of a covalent molecule/bond

* negatively charged bonding electrons are attracted to the positively charged nuclei of both atoms
* this attraction holds the atoms together strongly
* force of attraction is as a result of sharing a pair of electrons

description of an ionic lattice

* strong electrostatic attraction between ions
* each ion is attracted to the neighbouring, oppositely charged ion
* resulting in a three-dimensional array of positively and negatively charged ions

description of a metallic lattice

* three dimensional arrangement of metal ions
* surrounded by delocalised valence electrons

when does a covalent bond form

when a pair of electrons are shared between the two atoms (usually between two non-metals)

description of a covalent bond forming

* as the atoms get closer, valence electrons from each atom are attracted to the protons from the neighbouring atoms
* this attraction pulls the atoms close together and the electron from each atom are shared

what are lewis structures used to demonstrate

only valence electrons

what is the centre atom for lewis diagram

whichever atom is closest to the left hand side

exceptions of having an octet of electrons

H, Be and B

where will regions of high electron density be arranged

as far as possible from the each other

why are regions of high electron density arranged so far away

to minimise the repulsion between the regions of high electron density

what does the shape of the molecule depend on

* the number of regions of electron density around the central atom
* the number of atoms bonded to the central atom

description of tetrahedral shape

4 sets of electrons arranged as far apart as possible

what is the bond angle for tetrahedral

109

description of trigonal planar shape

3 sets of electrons arranged as far apart as possible

what is the bond angle for trigonal planar

120

description of linear shape

2 sets of electrons arranged as far apart as possible

what is the bond angle for linear

180

what are the steps to determine the shape of the molecule

1. draw lewis structure for molecule
2. count the sets of electron on the central atom
3. count the number of bonded atoms (bonding sets of electrons)

overall molecule shape table