2.2.1 Covalent Bonds and molecules

• These bonds form when atoms ( not necessarily different ) share a pair of valence electrons

• A covalent bond is resulted from the electrostatic attraction between the positive nuclei of the atoms and the shared pair of electrons

Electronegativity ( $X$)

• Generally forms between atoms of relatively high electronegativity, so non-metals.

• When the difference in electronegativity is less than 1.8, the bond is predominantly covalent

  • A contrast to if greater than 1.8 results in a more Ionic bond

Note unlike the M-NM nature of Ionic compounds, Covalent compounds are NM-NM and thus can possiblity be both elements ($\Delta X = 0$ )& compounds

Lewis Formulas

• Atoms that are covalently bonded are called molecules

  • Can represent the Bond

• Dashes ( - ) ,pairs of dots,pairs of crosses, a combination, or all of the above--> refers to pairs of the valence electrons

• Whatever preference chosen, some universal rules to follow:

1. Only Valence Electrons shown

2. Each pair shared between atoms represent covalent bond

3. Bonding Electrons are positioned between the two atoms involved in the bond

4. Non-bonding electrons ( lone pairs ), are positioned away from the two atoms in the bonding electrons

Usually Lewis Formulas generally have noble gas configurations as they are the most stable ( Octet rule ) , but it is important to note not all atoms can behave like this. They can form with even less than an added up full electron shell. Chemical Species that with odd number of valence electrons are also exceptions.

• Additional Rules

  1. get # of valences for each atom

  2. Divide total # of valence by 2 to get how many pairs

  3. Draw atom symbols on page. Element w/ least atoms found in centre. ( Hydrogen always surround central atom(s))

  4. Bond central and the side atoms together by drawing single bonds. Each bond represents a single pair.

  5. Lone pairs go to the side atoms, keep going until noble gas configuration achieved

  6. Assign remaining pairs go to the central atom(s)

  7. Check if central has full octet. IF not:

     1. Get electrons from lone pairs to get more pairs in the central atom

     2. Check if molecule is an exception to octet rule.

IN SL you should be aware of Lewis Formulas that contain atoms with fewer than eight valence electrons. Like Boron Trifluoride,$BF_3$

Polyatomic Ions are a charged group of atoms bonded covalently.