General Hints for Making the “Skeleton” of Molecules
1) Carbon is usually central.
2) Hydrogen is NEVER central.
3) Halogens are seldom central.
4) Oxygen is seldom central, but may link carbons.
5) An atom that appears only once is usually central.
6) If you can’t decide which atom should be central, pick the one with the lowest electronegativity.
7) Balance out the atoms around the central atom (make it symmetrical).
Steps for Placing the Valence Electrons
1) First, calculate the total number of valence electrons for the entire molecule (i.e. H2O has 8 total valence electrons)
2) Remember if the substance is an ion it has gained or lost electrons. Add or subtract electrons as needed. DO NOT forget your brackets with the charge of the ion on the outside…
3) Place pairs of electrons between every 2 atoms in the skeleton (Bonding Pairs).
4) Use the remaining electrons to complete the octets of all outer atoms (Non-Bonding Pairs). CAUTION: H and He are complete with only 2 valence electrons.
5) Any left over electrons are added in pairs to the central atom.
6) When all electrons have been placed, the outer atoms will all have octets (8 electrons around it…unless it is H or He then they will only have 2). The central atom may or may not have an octet.
a) If the central atom has an octet it is complete.
b) It is OK if the central atom has fewer than 8 electrons if it is BORON. If it isn’t, make multiple bonds to get an octet.
c) A central atom may have more than an octet if the central atom is in Periods 3-7. If it is in Period 2, it cannot have more than 8.
7) Draw the line structure. To do so, replace any bonding pair with a single line. Represent all lone pairs or free radicals by leaving the dots. Do not include non-bonding pairs. Double check to make sure you have included your brackets and charge if you drawing an ion.