Covalent Bonding Flashcards
Covalent bonding definition?
The strong electrostatic attraction between the nuclei of two atoms and the shared pair of electrons between them.
- single, double & triple covalent bonds
- formed between 2 atoms when: atomic orbital containing single unpaired electron from 1 atom overlaps w atomic orbital (also containing single unpaired electron) of another atom
Valence electrons?
electrons in outer shell of atom
- involved in forming bonds to adjacent atoms - atoms share these to complete their octet of electrons
A lone pair of electrons?
a pair of valence electrons that isn’t used in bonding
4 ways orbitals overlap?
- end on overlap of 2 s-orbitals (sigma bond)
- end on overlap of 2 p-orbitals (sigma bond)
- sideways overlap of 2 p-orbitals (pi bond) *cant form until sigma bond been formed so - only exist between atoms that r joined by double/triple bonds
- end on overlap of an s- & p-orbital (sigma bond) (only atoms of 2 diff elements… leads to formation of a type of covalent bond aka ‘polar’)
Sigma bond definition?
Formed by the ‘head-on’ overlap between 2 atomic orbitals in which the overlap lies on the line between the centre of the atoms
How can carbon form 4 bonds with only 2 unpaired electrons?
- electronic structure: 2s2, 2px1, 2y1, 2pz0 (so - 2 unpaired electrons)
- as only small energy gap between 2s & 2p orbitals - C provides small amount of energy to promote (aka give) an electron from 2s to empty p orbital so -> 4 unpaired electrons
- energy released/gained from forming 4 instead of 2 bonds is much greater than energy required when promoting
Dative covalent/coordinate bonding?
*A covalent bond in which both electrons in shared pair provided by 1 atom
- result of overlap of: empty orbital in 1 atom & orbital containing a lone pair in other atom
- arrow represents dative covalent bond - pointing away from ‘donor’ atom e.g. N to H in NH4+ (NH3 & H+)
e.g. H3NBF3, Al2Cl6 (one Cl in each AlCl3 donates their lone pair aka dative bonds to to other Al - forming 2 dative - giving Al full outer shell)
The Octet Rule?
- many atoms share electrons in such way that achieve an octet of electrons (noble gases)
- molecules which violate rule…
1. Molecules w too few electrons aka electron deficient compounds e.g. BeCl2 (Be only has 4 electrons in outer shell so - 2 empty orbitals), BF3 (B only has 6 electrons in outer shell)
2. Molecules w too many electrons aka w an expanded octet e.g. SF6 - only atoms from 3rd period onwards can expand their octets
Why can P form 5 bonds and N can’t (same groups)?
- electron configuration of P: [Ne] 3s2,3px1,3py1,3pz1
- in PCl3: 3 unpaired electrons be used to form bonds w 3 Cl atoms
- to make unpaired electrons: P promotes a 3s electron into 1 of its empty 3d orbitals (next available higher energy orbital)
Bond enthalpy?
(energy needed to disintegrate a chemical bond)
*linked to length of bond - the shorter the bond, the higher
- in covalent molecules: + nuclei attracted to area of electron density between them (aka where shared electrons r) but…
- also repulsion: 2 +vely charged nuclei & electrons repel e/o so - to maintain covalent bond - balance between forces
- dis between 2 nuclei = dis where attractive & repulsive forces balance e/o = bond length (result of balance)
- the greater the electron density (i.e. the more electrons in bond) between nuclei, the stronger the attractive force so - atoms pulled in further towards e/o so - shorter bond length, higher bond enthalpy.
e.g. C-C < C=C etc.
Simple molecular structure?
- diatomic, small polyatomic molecules
- tend to be gases at room temp
- while have strong INTRAmolecular forces, weak forces BETWEEN molecues
Properties of simple molecular structures?
- low mp/low bp/softness: to melt - only weak intermolecular forces need to be overcome - doesn’t require much energy so mp low
- non-conduction of electricity: no charge carriers (no ions/free electrons) to carry a charge
Giant covalent lattice: Diamond?
- each C atom forms 4 sigma bonds to 4 other C atoms
- in a giant 3-D tetrahedral arrangement, bong angles: 109.5°
- hard (due to very strong C-C), used as an abrasive (polishing/cleaning hard surfaces)
(structure similar to silicon & SiO2)
Giant covalent lattice: Graphite?
- each C atom bonded to 3 other C atoms by sigma bonds - forming interlocking hexagonal rings
- 4th electron on each atom is in p-orbital - C atoms close enough for p-orbitals to overlap - produce cloud of delocalised electrons above & below plane of rings
- strong bonds within layers
- weak (& long) bonds between layers
- delocalised electrons between layers allow graphite to conduct electricity
- layers can slip over e/o
- soft & used as lubricant
Giant covalent lattice: Graphene?
- single layer of graphite - 1 atom thick
- each C covalently bonded to 3 others
- 1 delocalised electron per C atom
