Dot&Cross diagrams
Covalent Bonding
Steps to determine central atom in a
covalent molecule
• Lewis
electron
dot
structures
are
representations of the distribution of
electrons in molecules and ions. They are
useful in determining the three-dimensional
shape of a molecule or ion.
A Lewis structure can be drawn for a molecule
or ion by following three steps:
• Step 1: Count the total number of valence
electrons.
• (For a neutral molecule, sum the numbers of
valence electrons of each atom in the
molecule. For a negative ion, add to the sum
the magnitude of the charge. For a positive
ion, subtract from the sum the magnitude of
the charge.)
• Remember the valence electrons for each
atom is the same as the A group number in
the periodic table. Also remember that a
negative charge will add to the valence
electron count.
What is the total number of valence electrons
for each of the following molecules or ions?
PH3 , CF4 , NO3-
• Step 2: Decide on the arrangement of atoms.
• The central atom is usually the atom with the
lowest subscript in the molecular formula and
the atom that can form the most bonds. If all
of the atoms usually form the same number of
bonds, the least electronegative atom is
usually the central atom.
• P H 3 , C F 4 , N O 3-
https://www2.chem.wisc.edu/deptfiles/genchem/netorial/rottosen/tutorial/modules/
intermolecular_forces/01review/review2.htm
• Step 3: Arrange electrons around the atoms so
that each atom has an octet.
• EXCEPTIONS TO THE OCTET RULE:
• Hydrogen will only have two electrons.
• Group 3A (boron, aluminum, etc.) may only have
six electrons.
• Atoms in the third row and beyond may expand
their octet (have more than eight electrons) if
needed.
•
OBJECTIVES
• *describe the formation of a covalent bond by the
sharing of a pair of electrons in order to gain the
electronic configuration of an inert gas
• describe, using ‘dot-and-cross’ diagrams, the formation
of covalent bonds between non-metallic elements, e.g.
H ; Cl ; O ; HCl; N ; H O; CH ; C H ; CO
• deduce the arrangement of electrons in other covalent
molecules
• relate the physical properties (including electrical
properties) of covalent compounds to their structure
and bonding
2
2
2
2
2
4
2
4
2
1. Single covalent bond
• E.g.1 Chlorine (Cl2)
• E.g.2 Hydrogen chloride (HCl)
• E.g.3 water (H2O)
• E.g.4 methane (CH4)
2. Double covalent bond
• E.g.1 Oxygen (O2)
• E.g.2 Carbon dioxide (CO2)
• E.g.3 Ethene (C2H4)
3. Triple covalent bond
• E.g. Nitrogen (N2)