2.2.2 Bonding And Structure Flashcards
Explain how an induced dipole dipole interaction forms
Movement of Electrons produces a Dipole in a molecule creating an instantaneous dipole which induces a dipole onto a neighbouring molecule this further induces more molecules which attract one another’s oppositely charged sides
Why do simple molecular compounds have low melting and boiling points?
Small amount of energy is needed to overcome the weak intermolecular forces between molecules
Why do simple molecular compounds not dissolve in water?
There is little interaction between the molecules in the lattice and the polar solvent molecules
Why do simple molecular compounds have poor electrical conductivity?
There are no free charged particles (delocalised electrons)
What is the structure of ionic compounds?
Giant ionic lattice
what are the properties of giant ionic lattices
- high melting points
- conduct electricity when molten or in an aqueous solution
- soluble in water
- hard and brittle
What are examples of giant covalent structures
Diamond graphite silicon dioxide SiO2
Why do ionic lattices have high melting points
Because a large amount of energies are required to overcome strong electrostatic bonds Between oppositely charged ions
What are the properties of Giant covalent lattices
> high melting and boiling points
do not conduct electricity except graphite
insoluble in Polar and nonpolar solvents
what type of solvents do Ionic lattices dissolve and why
polar solvents eg water Because the partially charged ions are attracted to the charged ions
What is the structure of simple covalent compounds?
Simple molecular
What are the properties of simple molecular lattices
> low melting points
do not conduct electricity
insoluble in water unless they are polar
soft
What are planar molecules
Can be arrange around central atom in 2D
Eg CO2 HCN H2O SCl2 BF3 SO3
Electronegativity
The ability of an atom to attract electrons in a covalent bond
What is a hydrogen bond
an attraction between a lone pair of electrons on an electronegative atom in one molecule and a hydrogen atom in a different molecule
explain why water molecules are polar
O is more electronegative than H so the electron pairs in the covalent bond are pulled towards it making it slightly negative So there is a difference in electronegativity and the molecule is not symmetrical, so dipoles do not balance out
how to explain the shape or bond angle
- state number of lone pair and bond pairs/regions
- say that electron pairs repel as far apart as possible
- state that lone pairs repel more than bond pairs
what makes a molecule polar
if there are polar bonds when there is a difference in electronegativity and the molecule is not symmetrical due to lone pairs so the dipoles don’t cancel out so there is an overall net charge
how can a molecule have polar bonds but not be a polar molecule
When dipoles cancel out in symmetrical molecules
what does electronegativity depend on
> the more protons, the more electronegative
the atomic radius (decreases left to right)
(the smaller atomic radius, more electronegative)
the less shells, the more electronegative
what are the most electronegative elements
F O N Cl
what is the definition of a dipole
The separation of opposite charges
why does water have relatively high surface tension
Because of the strong hydrogen bonds on the surface
why does ice have a higher melting point than expected
Hydrogen bonds are the strongest intermolecular forces so more energies needed to break them
Why is ice less dense than water
H2O molecules are held together by hydrogen bonds in open lattice structures.
Which one is the slightly negative atom of NBr and why
N because it is slightly more electronegative so electrons are pulled towards it
What is a covalent bond
the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
How can we predict the shape of a molecule from knowing the number of electron pairs surrounding the central atom
Pairs of Electrons surrounding a central atom repel so the shape is determined by the number of bond pairs and lone pairs Where lone pairs repel more
what are types of intermolecular forces In order of increasing strength
> Induced dipole-dipole (London forces)
Permanent dipole-dipole interactions
Hydrogen bonding
what is a permanent dipole
a polar covalent bond
What is a permanent dipole dipole interaction
The intermolecular bond between two polar molecules with permanent dipoles
if there are no hydrogen bonds why may boiling points still increase
The more electrons in each molecule, the larger the instantaneous dipoles, the more induced dipole-dipole interactions, the stronger the attractive forces between molecules, meaning more energy needed to overcome intermolecular forces which increases the boiling point.
What is an ionic bond
The strong electrostatic attraction between Oppositely charged ions ions
What is metallic bonding
Electrostatic attraction between the positive metal ions and the sea of delocalised electrons
Why do giant covalent structures have high melting and boiling points
Lot of energy is required for strong covalent bonds Between shared pair of Electrons and the nuclei to be broken
What structures can covalent bonding create
A small simple molecule
a giant covalent structure
a charged polyatomic ion
Why does giant ionic lattices conduct electricity when liquid but not when solid
When solid the ions are fixed in position when they are liquid they are free to carry charge
What is average bond enthalpy
A measure of average energy needed to break the bond
What is a dative covalent bond
A bond where both of the shared electrons are supplied by the same atom
what does the expansion of the octet mean
When a bonded atom has more than eight electrons in the outer shell
Describe the bonding in simple molecular structures
Atoms within the same molecule are held by strong covalent bonds and different molecules are held by weak intermolecular forces
How does graphite conduct electricity
Delocalised Electrons are present between the layers and are able to move freely carrying the charge Through the layers
What is the shape of a molecule depend on
Number of electron pairs in the outer shell and number of Electrons which are bonded and lone pairs
Bond angle - 2 bonded pairs 0 lone pairs
180 linear
Bond angle - 3 bonded pairs 0 lone pairs
120 Trigonal planar
Bond angle - 4 bonded pairs 0 lone pairs
109.5 Tetrahedral
Bond angle - 6 bonded pairs 0 lone pairs
90 Octahedral
Bond angle - 3 bonded pairs 1 lone pairs
107 Pyramidal
Bond angle - 2 bonded pairs 2 lone pairs
104.5 Nonlinear
how many degrees does each lone pair reduce the bond angle
2.5
How to draw a hydrogen bond
horizontal Dashed lines next to the H and the lone pair on the electronegative element
Why might an elements be unreactive
It is non polar
the bonds are very strong
It has no lone pairs to react
how is a polar bond formed
Bonding atoms have different electronegativities
What is a permanent dipole Dipole interaction
Force of attraction between two molecules with polar bonds
What are London forces
When a molecule with a permanent dipole (Polar bonds) or instantaneous dipole is close to a non-polar molecule it causes it to become slightly polar leading to A weak attraction
What are London forces
Caused by random movements of Electrons leading to instantaneous dipoles, inducing other dipoles nearby attracting one another
where can you find the strongest london forces
In bigger molecules as they have more electrons
Why is ice less dense than liquid water
In ice water molecules are arranged in a fixed lattice with hydrogen bonds
In water molecules are closer together and hydrogens bonds can break and reform so there is no fixed lattice
Why does water have a higher melting point than expected
hydrogen bonds are the strongest of the intermolecular forces so extra strength is required to overcome these forces
What is the difference between simple molecular structure and giant covalent structure?
Simple molecules are bonded by covalent bonds and joined to each other by weak intermolecular forces
Giant covalents lattices are many molecules joined by covalent bonds
HBr and HI have both permanent and induced dipole iodine has a lower electronegativity than Br so why does iodine have a higher boiling point?
Because the induced dipole interaction in iodine is higher
Four ionic compounds have different melting points why is that?
As ionic charge increases melting point increase, the greater the ionic charge, the greater attraction between ions, and the more energy required to break the ionic bonds
Which one has a higher ionic charge NaF Na2O MgO
MgO 2+ and -2
What makes stronger London forces between molecules
(The size of the molecule or atom has to be bigger) more electrons the bigger the molecule
What makes stronger hydrogen bonds
Higher difference in electronegativity
FH most strong hydrogen bonds
H2O mid strength hydrogen bonds
NH3 lower strength hydrogen bonds
Ice bond 109
Gaseous water bond 105
Why
Ice forms more hydrogen bonds
Water has two bond pairs, and two lone pairs
Lone pair repel more
Why is NCL3 a polar molecule?
This molecule has three bond pairs which repel equally but also one lone pair which causes asymmetry so dipoles don’t cancel out
