chem bonding– the covalent model Flashcards
define covalent bonding
the electrostatic FOA between the nucleus of each of the 2 bonded atoms and their shared pair of e-
what is another name for dative bond
coordinate bond
what are dative bonds
the covalent bond established by the donation of an e- pair from one atom to another
to form a dative bond,
1. the donor atom must contain ______________________
2. and the acceptor atom must have a _______________
- lone pair of e-
- vacant orbital
what are the exceptions to the Lewis structure?
BCl3 -> only 6 valence e-
BeCl2 -> only 4 valence e-
what type of orbital overlap do sigma bonds have? (letters)
s-s, p-p, s-p
pi bonds are the _______ overlap of ______ orbitals
side on, p
draw a sigma bond
-
draw a pi bond
-
is the pi or sigma bond stronger?
sigma
why is the sigma bond stronger
it has head on overlap of orbitals
does the covalent bond strength decrease or increase down the group?
decrease
why does the strength of covalent bonds down the group decrease?
- the atomic radius incr
- effectiveness of orbital overlap decr
- thus strength decr
the stronger the bond, the [shorter/longer] the bond length
shorter
across the period, electronegativity [incr/decr]
down the group, electronegativity [incr/decr]
increases
decreases
why can some central atoms expand their octet?
these elements can utilise their energetically accessible vacant 3d orbitals
what is the bond angle of a molecule that is trigonal planar, bent
less than 120º
what is the bond angle of a tetrahedral molecule
109.5º
the bond angle of a tetrahedral trigonal pyramidal molec is [greater/lesser] than a tetrahedral bent molec
greater
if an atom has 5 electron pairs what is its electron domain geometry?
trigonal bipyramidal
how many bond pairs on the central atom would lead to a see-saw shape?
4
draw the orbitals involved in the hybridisation of a carbon atom in methane
-
why does diamond have very high m.p.?
- each C atom is covalently bonded to 4 other C atoms in a tetrahedral shape
- has strong and extensive C-C covalent bonds throughout giant covalent lattice
- reqs large amts of energy to break
can diamond conduct electricity and why?
no, no mobile ions/e-
does Si or diamond have lower m.p.? (3)
- Si as it is lower than C in periodic table
- thus larger atomic radius
- Si-Si bonds poorer extent of orbital overlap than C-C bonds
- thus cov bonds weaker than diamond
- less energy needed to break bonds
why can graphite conduct electricity?
- has delocalised e-
- act as mobile charge carriers
- can conduct electricity parallel to its layers
why is graphite an effective lubricant?
- layers held by weak LDF
- thus layers can slide over e/o
why can graphene conduct electricity?
- has delocalised e-
- act as mobile charge carriers
is C60 a giant molecule?
no
why is C60 a poor conductor of electricity
- charge carriers confined within C60 molecule
- unable to move fr molec to molec to act as charge carriers
what is electronegativity
the tendency of an atom to attract a shared pair of e- in a covalent bond toward itself
why does electronegativity incr across the period
- incr nuclear charge
- shielding effect remains relatively constant
- incr ENC
- thus incr electronegativity
why does electronegativity decr down the group
- incr atomic radius
- incr shielding effect
- decr ENC
- thus decr electronegativity
IMFOA are responsible for the ___________ properties of simple molecules.
physical
IMFOA contributes to the _________ of substances, while the extent of orbital overlap contributes to the ___________ of the ______________ in the substance
m.p., strength, covalent bonds
the weakest IMFOA are…
LDF
what’s another name for dpdp forces
(permanent dipole) pdpd attractions
why does LDF increase down the group?
- e- cloud incr
- more easily polarisable
- thus stronger LDF
what is the criteria for H bonding?
- one molec must have H atom bonded to FON
- other molec must have lone pair of e- on FON
what affects the strength of the hydrogen bond? (2)
- electronegativity difference
- extensiveness of H bonding
how does the difference in electronegativity affect the strength of H bonding?
- greater the e- diff -> greater polarity
- thus stronger H bond
the _______ the number of H bonds formed __ molecule, the more ____________ and stronger the H bonds
greater, per, extensive
why would a covalent compound be less volatile?
- cmpd has stronger IMF
- thus lower volatility
why can ionic solutes dissolve in polar solvents?
- a lot of energy released from strong ion-dipole interactions
- thus sufficient to overcome energy between solvent molec
what is the formula for Rf value?
Rf = dist travelled by substance/dist travelled by solvent
how do you measure the distances in calculating Rf value in chromatography?
from the origin (start line)
recap: what is resonance?
delocalisation of π e- in a system
species that have resonance have identical_______ and ________
bond lengths and strengths
what is the formula for formal charge?
V - 1/2B - L
V=no. valence e- in unbonded atom
B= no. of e- in bonding domains
L= no. of e- in non-bonding domains
the most stable structure would be when all formal charges = __
0
what is lattice energy? (short defin.)
the amt of energy needed to break an ionic cmpd into the constituent gaseous ions
what affects the strength of the ionic bond?
lattice energy
what is the formula for lattice energy?
-
why do ionic compounds have a high melting point?
- strong electrostatic FOA btw oppositely charged ions
- large amts of energy is req to overcome
are ionic compounds good conductors of electricity?
- only in aqueous and molten state
- mobile ions can act as charge carriers
- in solid no
- ions are held in a fixed lattice
why would an ionic compound shatter if I drop it?
- brittle
- ions of the same charges will be forced next to eo -> repel eo
- causes lattice structure to break
why is there covalent character in all ionic compounds
- cation can polarise the e- cloud of the anion
- results in a degree of e- sharing
- thus covalent character exists in all ionic cmpds
what is metallic bonding?
the electrostatic FOA between metal cation and the sea of delocalised e-
what is the structure of a metal?
giant metallic lattice
electrostatic FOA exist in…
metals, ionic cmpds, covalent cmpds
draw a diagram of the bonding in Al
-
what is the formula for charge density?
-
why is the m.p. of Na lower than Mg
- Na has fewer no. of cations that delocalise into the sea of e-
- thus less metallic bonding
- lesser energy needed to break
why are metals ductile?
- when a large force is applied
- layers of metal cations slide over eo w/o disrupting e metallic bonds
why are metals good conductors of heat?
- delocalised e- and cations are closely packed together
- efficient transfer of heat
