S2.2: The Covalent Model Flashcards
(34 cards)
What is the basic theory of covalent bonds?
Covalent bonds formed when electrons from different atoms are shared so that each atom attains a noble gas configuration
What is a covalent bond?
Electrostatic attraction between shared pairs of electrons and the positively shared nuclei on either side of the electrons
Between 2 non-metals
Electrons shared, not transferred
-> not in fixed option -> electrons in constant motion (electron clouds)
Two atomic orbitals overlap -> molecular orbital formed
What are the steps for drawing a Lewis structure?
- count the total number ofvalence electrons
- draw theskeletal structure to show how many atoms are linked to each other
- use a pair of crosses or dot/cross to put an electron pair in each bond between the atoms
- add more electron pairs to complete the octets around the atoms ( except H which has 2 electrons)
- if there are not enough electrons to complete the octets, form double/triple bonds
- check the total number of electrons in the finished structure is equal to the total number ofvalence electrons
What are the ‘exceptions’ to the octet rule called?
More then 8 electrons -> ‘expanding the octet’ rule
Less than 8 electrons -> electron deficient
Examples of exceptions to the octet rule
Incomplète octets:
H -> achieve stable arrangement by gaining an electron
-> 1s2 (helium structure)
Li -> does the same thing as helium, but lose an electron
-> 1s2, 2s1 -> 1s2
Be -> has 2 valance -> forms stable with 4 valance
B and Al -> 3 valance -> forms stable with 6
Multiple covalent bonds
Nonmetals can share more than one pair of electrons -> different types of covalent bonds
Sharing electrons -> both atoms achieve octet -> stable
-> quadruple bond not possible -> repulsion from having 8 electrons in same region between 2 nuclei too great
Double covalent bond -> two shared pairs of electrons
Triple covalent bond -> three shared pairs of electrons
What is bond energy?
Energy required to break one mole of a particular covalent bond in the gaseous state
Unit: KJ/mol
Larger bond energy = stronger covalent bond
What is bond length?
Internuclear distance of two covalently bonded atoms
Greater force of attraction between electrons and nuclei -> atoms close -> decrease bond length, increases bond strength
Number of shared pairs of electrons increase -> bond length decrease, bond strength increase
-> increased attraction
What is a coordinate bond?
Normal covalent bond -> atoms share electrons
BUT
Some atoms have lone pair of electrons that are donated to form a bond with an electron-deficient atom
Formed when both of the electrons in shared pair originate from the same atom
Shown by arrows
Ex:
carbon monoxide, hydronium ion
ammonium ion -> nitrogen donate to hydrogen
What is electronegativity?
refers to the ability of an atom to draw an electron pair towards itself in a covalent bond
defined as a relative measure of the attraction that an atom has for a shared pair of electrons when it is covalently bonded to another atom (Pauling scale used at IB)
increases across a period and up a group
Explain bond polarity
Covalent bond between two atoms of same element -> bonding pair of electrons shared equally -> non-polar bond
Bond between atoms of different elements -> bonding pair will be closer to more electronegative atom -> polar bond
Bigger the difference in electronegativity -> high the polarity
What does bond polarity result in?
Neg charge center and Positive charge center do not coincide
Electron distribution is asymmetric
Less electronegative atom has partial charge of δ+(deltapositive)
More electronegative atom has partial charge of δ-(deltanegative)
-> difference > 1.8 -> bonding pair moves completely to more electronegative atom -> ionic
What is the dipole moment?
Dipole moment -> how polar a bond is
Direction of dipole moment shown by arrow (with vertical line through end) that points to the partially negatively charged end of the dipole
How do you determine molecular polarity?
Following things in consideration:
-> polarity of each bond in the molecule
-> how the bonds are arranged in the molecule
Some molecules have polar bonds, but overall non-polar
-> polar bonds arranged such that dipole moments cancel out
What is VESPR theory?
Covalent bonds -> electrons (in outer shell) behave as a negatively charged cloud and repel each other
Minimise repulsion -> all outer shell electrons spread out as far in space as possible
Molecular shape and angles -> predicted by VALENCE SHELL ELECTRON PAIR REPULSION THOERY (VESPR theory)
What are the basic rules of VESPR theory?
- All electron pairs and all lone pairs arrange themselves as far apart in space as is possible
- Lone pairs repel more strongly than bonding pairs
- Multiple bonds behave like single bonds
Used to predict shape and angle of covalent molecules/ions
What are domains (in terms of VESPR theory)?
The region of negative cloud charge
Can have 1, 2, 3 pairs of electrons
What is the order of repulsion for electron?
Non-bonding : non bonding > non-bonding : bonding > bonding : bonding
Explain/draw all the molecular geometry things
Look at picture flashcards
What is a covalent network structure?
Sometimes not possible to satisfy bonding capacity of substance in form of molecule -> covalent bonding between all adjacent atoms -> large lattice form
Basically one giant molecule
Strong covalent bonds:
High melting and boiling point
Poor conductors (exception is graphite because of delocalized electrons)
What are some common allotropes of carbon?
DIAMOND:
Carbon atoms covalently bonded tetrahedrally to four other carbon
Bond angle: 109.5°
Bond length: 0.154 nm
No place of weakness -> one of hardest natural substances
GRAPHITE:
Carbon atoms covalently bonded in a trigonal planar to 3 other carbon atoms -> hexagonal layers
Bond angle: 120°
Bond length: 0.142 nm
Layers held together by weak bonds (has spare delocalized electrons)
-> distance between layers: 0.335 nm
-> atoms on same layer -> strong covalent bonds
-> between layers: intramolecular forces
-> makes it a good lubricant, conductor
GRAPHENE:
Infinite lattice of covalently bonded atoms in 2D
Single layer of carbon in repeating hexagons
1 million times thinner than paper
What are the structure/properties of network covalent structure of silicon and silicon dioxide (silica)?
Si and SiO2 -> diamond like structure
Si: silicon bond covalently to four other silicon atoms tetrahedrally
SiO2: each silicon atom surrounded tetrahedrally by four oxygen -> each silicon bridged to enrichir via oxygen atom
Hard; high melting point, poor conductor
Insoluble in water and organic solvents
What are the properties of giant covalent structures/covalent network structures?
High melting and boiling point:
- many and strong covalent bonds -> lots of energy required to break
Can be hard or soft:
- graphite: soft because of weak carbon layers
- diamond and silicon (IV) oxide: hard -> difficult to break SD network of strong covalent bonds
(Mostly) insoluble in water
(Mostly) poor conductors:
- main exception is graphite -> delocalized electrons in layers
- graphene too
- buckminsterfullerene (allotrope of C) -> semiconductor
- diamond and silicon (IV) dioxide do not conduct -> all four outer electrons on carbon involved in covalent bond -> no free electrons
What are London dispersion forces?
A temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles
-> due to uneven spread of electrons
Electrons not static -> constant motion -> any given time distribution of electrons not symmetrical
Uneven distribution -> temporary dipole
Adjacent atom’s electrons repelled by neg part of dipole + attracted to pos -> temporary INDUCED dipole
Exist in all molecules -> sometimes weak
Responsible for attractive forces between non-polar molecules
Reason why all compounds can be liquified or solidified
