Lewis Structure & VESPR Flashcards
What is the octet rule?
Elements gain, lose or share electrons in order to obtain the noble gas configuration (in a chemical reaction) = full outer shell = stability
What is the goal of bonding according to the octet rule?
To achieve a full outer shell containing 8 electrons (an octet) for stability.
How does hydrogen achieve stability?
By sharing an additional electron to fill its 1s orbital with 2 electrons.
What type of bond is formed between a metal and a non-metal?
An ionic bond.
What happens in the formation of NaCl?
Na gives its electron to Cl, both achieve an octet, and become ions Na+ and Cl-.
Define ionic bond.
The force of attraction between oppositely charged ions.
What is the ionization energy of Na?
494 kJ/mol.
What is the electron affinity of Cl?
-349 kJ/mol.
What is the overall energy change when Na and Cl form NaCl?
-444 kJ/mol.
What is a covalent bond?
A pair of valence electrons shared between two atoms.
What types of atoms typically form covalent bonds?
Non-metal atoms.
What does electronegativity (χ) measure?
The ability of an atom to attract an electron when part of a compound.
What is the significance of a difference in electronegativity greater than 0.4 and less than 1.7?
It indicates a polar bond.
List the electronegativity values for H, C, N, and O.
- H = 2.2
- C = 2.6
- N = 3.0
- O = 3.4
What do Lewis structures represent?
The patterns of bonds and lone pairs in a molecule.
Only valence electrons are considered and this is represented by covalent bonding
How many bonding electrons are assigned to each bond in Lewis structures?
2 bonding electrons.
What is the formula to determine formal charge (FC)?
FC = V - L - (1/2)B.
What does each letter represent in the formal charge formula?
- V = Valence
- L = Lone pair
- B = Bonding
How do you determine the skeletal structure in drawing Lewis structures?
The element with the lowest ionization energy goes in the middle.
What is resonance in Lewis structures?
Structures with the same arrangement of atoms but a different arrangement of electrons.
What is an example of a molecule with an odd number of valence electrons?
The methyl group, CH3.
What are free radicals?
Molecules with unpaired electrons that are highly reactive.
Name one source of free radicals in biology.
Cigarette smoke.
What is the significance of the electronegativity order F > O > N > C?
It indicates that the negative charge will go on the most electronegative atom.
What happens in octet deficient molecules?
Certain elements do not achieve a complete octet, as seen in group 13 elements like B and Al.
What is the role of formal charge in determining the stability of Lewis structures?
The sum of all formal charges must equal the overall charge of the molecule.
What does experimental evidence show about bond lengths?
All bonds are the same length
This evidence suggests uniformity in bond characteristics despite variations in atomic structure.
What is the formula for calculating formal charge (FC) for Boron?
FC = 3 – 0 – (½)(8) = -1
This formula accounts for the number of valence electrons and bonding interactions.
What is the formal charge (FC) for Fluorine in the double bond with Boron?
FC = 7 - 4 – (1/2)(4) = +1
This calculation reflects the bonding situation and electron distribution around Fluorine.
What is the formal charge (FC) for Fluorine in a single bond?
FC = 7 – 6 – (1/2)(2) = 0
This indicates that Fluorine is in a stable bonding environment.
What is the breakdown of the octet rule regarding valence shell expansion?
Elements with n ≥ 3 have empty d orbitals allowing more than eight electrons around the central atom
This expansion is significant for elements in higher periods of the periodic table.
How many valence electrons are needed for five P-Cl bonds?
10 shared electrons
To satisfy the octet rule, each Cl atom requires 8 electrons, leading to a total of 40 electrons.
What is the steric number (sn)?
The number of atoms bonded to the central atom + the number of lone pairs of electrons
Each lone pair counts as one in the steric number calculation.
What is the VSEPR theory based on?
Valence electrons repel each other as far apart as possible, leading to geometrical arrangements that minimize repulsion
This theory helps predict molecular shapes based on electron pair interactions.
What is the order of repulsion in VSEPR theory?
Lone-pair/lone-pair > lone-pair/bonding pair > bonding-pair/bonding-pair
This hierarchy explains the spatial arrangement of electron pairs around the central atom.
What is the procedure for applying VSEPR theory?
- Determine Lewis structure
- Count BPs and LPs surrounding the central atom
- Establish electron pair geometry (parental geometry)
- Identify distortion factors
- Assign molecular geometry
This systematic approach allows for accurate predictions of molecular shapes.
What are the ideal bond angles for tetrahedral, bent, and trigonal planar geometries?
Tetrahedral: 109.5˚, Bent: 106.7˚, Trigonal Planar: 120˚
These angles reflect the spatial distribution of electron pairs around the central atom.
True or False: Lone pairs occupy less space than bonding pairs.
False
Lone pairs occupy more space and exert greater repulsion than bonding pairs.
What are the structures in a Lewis structure?
Two atoms share 1 electron pair = single bond
Two atoms share 2 electron pairs = double bond
Two atoms share 3 electron pairs = triple bond
Two unpaired electrons = lone pair
shared electron = represented as a line
unshared electron = represented as a dot
What are the steps in forming a Lewis structure?
- Draw skeletal structure - least electronegative element goes in the middle
- Count total number of valence electrons (electrons in outer shell) - if negative ion, add value to valence count, if positive, subtract
- Count total number of electrons needed to have a full valence shel (i.e. H needs 2)
- Subtract the number in step 2 (valence electrons) from number in step 3 (electrons for full valence shell) = number of bonding electrons
- Assign 2 bonding electrons to each bond
- If bonding electrons remain, make some double or triple bonds - double bonds form between C, N, O and S, triple bonds form between C, N and O
- If valence electrons remain, assign them as lone pairs
- Determine formal charge - FC = V - L - (1/2)B
This must add up to the overall charge on the molecule
What are the assumptions in VESPR theory?
- Atoms in a molecule are bound together by electron pair (bonding pairs = BP) and more than one pair may bind to 2 molecules
- Atoms in a molecule can possess electron pairs that are not involved in bonding (lone pairs = LP)
What are the rules in VESPR theory?
- BPs and LPs around an atom adopt positions in which their interactions with other electron pairs is minimised
- A LP occupies more space than a BP (repels more)
- Multiple bonds occupy more space than a single bond
List the options of parental geometry
Points of negative Charge and their shape:
2 = Linear
3 = Trigonal Planer
4 = Tetrahedral
5 = Trigonal Bipyramidal
6 = Octahedral
