Topic 4.3: Covalent structures Flashcards
Definition of lewis structure
A diagram of molecules in which the valence e- of the atom are represented by dots, and the sharing of e- to form a covalent bond is shown
How to write a lewis structure (6 steps)
a) Calculate the total number of valence e- in the molecule
b) Draw the skeletal structure of the molecule
c) Determine the central atoms (the lowest electronegative)
d) Draw single bonds to the central atom
e) Put all remaining valence e- on atoms as lone pairs
f) Turn lone pairs into double or triple bonds to give every atom an octet (except H)
g) Put the Lewis structure in a square bracket with the charge shown outside if it’s an ion
Definition of coordinate covalent bonds
Type of covalent bond in which both shared electrons in a molecule come from the same atom
Symbol of a coordinate covalent bond
An arrow on the head of the bond is used to indicate the origin of the electron pair
Definition of “octet rule”
Tendency of atoms to gain a valence shell with a total of 8 e-
Exceptions to octet rule
a) Be and B form stable molecules such as BeCl2 and BF3 (incomplete)
b) Elements in period 3 and below may expand their octet by using d-orbitals
Definition and properties of electron deficient molecules
Molecules with incomplete octets, which tend to accept an electron pair from a molecule with a lone pair. (NH4)
Valence Shell Electron Pair Repulsion (VSEPR) Theory
The total number of electron domains (# lone or bond pairs) determines the shape of a covalent molecule
a) Electron domains in the same valence shell carry the same charge, they repel each other and spread themselves as far as possible
Explanation of the order of electron repulsion among lone pair and bonding pair
LP – LP > LP – BP >
BP – BP
a) Orbitals that hold lone pairs are rounder and shorter and spread more easily
b) Orbitals that hold bonding pair are more elongated
Tendency of # lone pairs and bond angles
Since lone pairs cause more repulsion than bonding pairs, the angle is reduced as the # lone pairs increase
Difference between electron domain geometry and molecular geometry
a) The electron domain geometry is determined by the positions of all the electron domains,
b) Molecular geometry (arrangement of atoms in space) depends on the positions of the bonded atoms
Shape of molecules with 2 electron domains and 0 lone pairs
a) Molecular geometry
b) Angle
c) Example
a) Linear
b) 180°
c) CO2
Shape of molecules with 3 electron domains and 0 lone pairs
a) Molecular geometry
b) Angle
c) Example
a) Planar triangular
b) 120°
c) BF3
Shape of molecules with 3 electron domains and 1 lone pair
a) Molecular geometry
b) Angle
c) Example
a) Bent (V shaped)
b) < 120°
c) SO2
Shape of molecules with 4 electron domains and 0 lone pairs
a) Molecular geometry
b) Angle
c) Example
a) Tetrahedral
b) 109.5°
c) CH4
Shape of molecules with 4 electron domains and 1 lone pair
a) Molecular geometry
b) Angle
c) Example
a) Triangular pyramidal
b) < 109.5°
c) NH3
Shape of molecules with 4 electron domains and 2 lone pairs
a) Molecular geometry
b) Angle
c) Example
a) Bent (V shaped)
b) < < 109.5°
c) H2O
Shape of molecules with 5 electron domains and 0 lone pairs
a) Molecular geometry
b) Angle
c) Example
a) Triangular bipyrimidal
b) 90° / 120° / 180°
c) PF5
Shape of molecules with 5 electron domains and 1 lone pair
a) Molecular geometry
b) Angle
c) Example
a) See saw
b) 180° / 90° / < 120°
c) SF4
Shape of molecules with 5 electron domains and 2 lone pairs
a) Molecular geometry
b) Angle
c) Example
a) T-shaped
b) 90° / 180°
c) BrF3
Shape of molecules with 5 electron domains and 3 lone pairs
a) Molecular geometry
b) Angle
c) Example
a) Linear
b) 180°
c) I5 -
