Covalent bonding Flashcards
A covalent bond
A shared pair of electrons between non -metals.
why do covalent bonds form
- each atom wants to achieve a full outer shell of electrons.
- this is achieved by sharing electrons
Why is difficult for covalent bonds to come apart?
The attraction of the positive nuclei and the negative shared pair of electrons is GREATER than the repulsion between the positively charged nuclei of the bonded atoms.
Single and double covalent bonds and why they are described as LOCALISED
A single covalent bond ~ where atoms are bonded by a single shared pair of electrons, a single bond is formed.
Double covalent bond ~ two pairs of electrons are shared in total and a double bond forms.
LOCALISED ~ the attraction is acting solely between the shared pair of electrons and the nuclei of the two bonded atoms.
A dative covalent bond
OR
A coordinate bond
A COVALENT BOND in which the SHARED PAIR OF ELECTRONS only comes from ONE of the BONDED ATOMS.
- an arrow head is used to show the direction in which the electron pair has been donated.
- The acceptor atom must be electron deficient (there are available orbitals for the electrons to occupy.)
- A dative covalent bond is exactly the same as a normal covalent bond as all bond are the same length and have the same average bond enthalpy( tells us the strength of the bond)
The octet and duet rules
- The elements immediately before and after NEON in the periodic table , follow the octet rule , attaining the same electron configuration as neon via the gain , loss or sharing of electrons.
- This is because the second shell becomes full ~ an octet or closed shell is achieved.
- ARGON has the electron configuration 3s2 3p6. The empty 3d subshell is higher in energy than the 3s and 3p.
- The elements immediately before and after ARGON also tend to attain this configuration in compounds as the third shell becomes full ~ an octet or closed shell is achieved.
- HELIUM has the electron configuration 1s2.
- The atoms before and after helium ( H and Li ) follow a DUET RULE and tend to have the same electron configuration as helium.
Elements that are not able to attain an octet.
Period 2 ~ Be , B
Period 3 ~ Al
- None of these elements have enough unpaired electrons to reach a noble gas configuration.
- However they can pair up any unpaired electrons.
Elements that expand their octet
- occurs in elements in PERIOD 3 and BELOW because the d - subshell is available to accept electrons.
- P and As pair 3 or 5 electrons to form 3 or 5 covalent bonds.
- S , Se and Te pair 2 , 4 or 6 electrons to form 2 , 4 or 6 covalent bonds.
- Halogens excluding fluorine , pair 1 , 3 , 5 or 7 electrons to form 1 , 3 , 5 or 7 covalent bonds.
Valence - Shell Electron - Pair Repulsion Theory (VSEPR theory)
- Predicts the SHAPE of covalent molecules and ions.
- Based on the number of ELECTRON PAIRS surrounding the central atom and the
REPULSION between them. - Every electron pair moves as FAR WAY as possible from the others , generating the shape of the molecule.
SOLID LINES ~ the bond lies on the plane of the screen or page.
SOLID WEDGE ~ the bond is coming out of the plane of the page.
DOTTED WEDGE ~ the bond is projecting back behind the plane of the page.
SHAPES of molecule with NO lone pairs of electrons.
Central atom with TWO bonding pairs:
Shape ~ linear
Bond angle ~ 180 degrees.
Central atom with THREE bonding pairs:
Shape ~ trigonal planar
Bond angle ~ 120 degrees
Central atom with FOUR bonding pairs:
Shape ~ tetrahedral
Bond angle ~ 109.5 degrees
Central atom with FIVE bonding pairs:
Shape ~ trigonal bipyramidal
Bond angle:
- Between the 3 bonding pairs lying a right angle ~ 90 degrees
- Between the other two bonding pairs ~ 120 degrees.
Central atom with SIX bonding pairs:
Shape~ octahedral
Bond angle ~ 90 degrees
The effect of lone pairs & ammonia and water as examples.
The lone pair’s electron CHARGE CLOUD has a greater width.
This causes EXTRA REPULSION than a bonding pair.
This extra repulsion decreases other bond angles by 2.5 degrees.
Examples:
Ammonia ~
- 3 bonding pairs & 1 lone pair.
- The bond angle is 2.5 degrees less than
the bond angle in TETRAHEDRAL molecules.
- PYRAMIDAL shape
- Bond angle ~ 107 degrees
Water~
- 2 bonding pairs & two lone pairs
- The bond angle is 2.5 degrees less than the bond angle in pyramidal molecules
- NON -LINEAR shape
- 104.5 degrees
Shapes of molecules containing multiple covalent bonds
- Multiple bonds ( double or triple) are called BONDING REGIONS.
- They are treated as if they were single bonds when deciding the shape of a molecule.
Are lone pair of electrons or bonding pair of electrons closer to the nucleus ?
- LONE PAIRS of electrons are pulled CLOSER to the nucleus of the atom because they are not involved in bonding.
- BONDING PAIRS are FURTHER AWAY as the electrons are ATTRACTED to both nuclei and are located between atoms.
Shapes of IONS
A DATIVE covalent bond acts as a NORMAL covalent bond.
The bonding pairs will still repel each other as far as possible.
How many covalent bonds will and element form
Carbon -four
Nitrogen - three
Oxygen - two
Hydrogen - one
