2: Structure & Bonding Flashcards
Define ionic bonding
Electrostatic attraction between two oppositely charged ions
What difference in electronegativity causes ionic bonding?
Typically greater than 1.5
How does the atomic radius change as you go down groups?
Atomic radius increases
How does the atomic/ionic radius of a group 1 metal compare to a group 2 metal?
Group 2 metals have a smaller atomic & ionic radius
Define covalent bonding
Electrostatic attraction between two nuclei and a shared pair of electrons between them
How are the electrons shared in normal covalent bonding?
Each atom provides one electron
Overlap of orbitals containing an electron from each
Define a dative covalent bond
Two atoms share a pair of electrons, with both electrons being donated by one atom
How does a dative covalent bond form?
An atom with a lone pair of electrons donates both electrons to another atom that needs 2 electrons to fill its outer shell
Name the shape and angles of a molecule with 2 electron pairs
Linear shape
180°
Name the shape and angles of a molecule with 3 electron pairs
Trigonal planar
120°
Name the shape and angles of a molecule with 4 electron pairs
Tetrahedral
109.5°
Name the shape and angles of a molecule with 5 electron pairs
Trigonal bi-pyramid
90° + 120°
Name the shape and angles of a molecule with 6 electron pairs
Octahedral
90°
Name the shape and angles of a molecule with 3 electron pairs + 1 lone pair
Trigonal pyramid
107°
Name the shape and angles of a molecule with 2 electron pairs and 2 lone pairs
Bent
104.5°
Why is ICl4- not tetrahedral?
6 electron groups
4 bonding pairs + 2 lone pairs
Square planar (90°)
Define polarity
In-balance of the sharing of electrons which causes a charge
Define electronegativity
A measure of the tendency of an atom in a molecule to attract the electrons in a covalent bond
Define a dipole
Sum of all the polarities on a molecule
What is the Pauling scale?
Measures electronegativity of elements from 4.0 to 0
Name some trends in electronegativity
Increases across a period
Decreases down a group
Non-metals tend to be more electronegative
What is the electronegativity in covalent bonding?
Very small/zero electronegativity
What is the electronegativity in ionic bonding?
Large difference in electronegativity
Describe ionic bonding using electronegativity
Atoms of significantly higher electronegativity (usually non-metals) form ionic bonds with atoms of significantly lower electronegativity (generally metals)
Why does the electronegativity increase across a period?
Nuclear charge increases
Atomic radius stays roughly the same
What is a polar covalent bond?
Electronegative difference not too small nor large
Covalent bond forms but electrons are attracted towards the more electronegative atom
What do polar covalent bonds form?
Slightly negative and slightly positive charge on the atoms
Bond is polarised so it has a slight ionic character
What causes a permanent dipole?
Shape of the molecules
Polarity of individual bond
What is a permanent dipole?
The overall molecule is polar
Meaning there must be a net dipole
What is a non-polar molecule made of?
All identical bonds
No lone electron pairs on central atoms
What is a polar molecule made of?
Different bonds or
Identical bonds and lone electron pairs on the central atom
What are the three types of dipole?
Permanent Dipole
Induced Dipole
Instantaneous Dipole
What is an induced dipole?
No permanent dipole (similar electronegativity)
If it is next to a molecule with a dipole, a dipole can be induced in it
What is an instantaneous dipole?
Electron cloud may not be evenly distributed leading to the formation of a temporary dipole
How can you determine if a liquid is polar?
Observing if a jet of liquid is deflected by a charged rod
What are London forces?
Instantaneous dipole - induced dipole
Caused by electrons constantly moving which can cause a charge
What affects the strength of instantaneous dipoles?
Number of electrons (the more electrons the stronger the dipole)
Proximity of molecules (closer they are the stronger)
Why do straight chain molecules have stronger id-id forces?
More points of contact as they pack better
What is a pd-pd interaction?
Two dipolar molecules are attracted to each other due to the different charges
What is hydrogen bonding?
The strongest form of pd-pd interactions
10x weaker than covalent, 10x stronger than normal pd-pd
What molecules can hydrogen bonding occur with?
X - H —Y
X & Y = N, O, F
How does hydrogen bonding explain the density of ice?
Density of ice is lower
As hydrogen bonds create a crystalline structure in ice, which means the molecules aren’t close together
Why can only some molecules form hydrogen bonds?
N, O, F
All very electronegative
Contain lone pairs
Why is ethanol soluble in water?
Forms hydrogen bonds with water
When will liquids mix (miscible)?
If it is energetically advantageous
What is the explanation if two liquids are mixed and the temperature went down?
Inter-molecular forces of the liquids before are stronger than the attractions which would be formed in the mix
This would be immiscible
What is the explanation if two liquids are mixed and the temperature stayed the same?
The bonding between the mix is the same as the individual liquids
What is the explanation if two liquids are mixed and the temperature went up?
Inter molecular forces in the individual liquids is weaker than that in the mixture
How are ionic solids soluble?
Ions surrounded by polar water molecules and energy is released therefore exothermic
(hydration enthalpy is negative)
What are cations surrounded by when dissolved in water?
Slightly negative oxygen atoms
What are anions surrounded by when dissolved in water?
Slightly positive hydrogen atoms
What is hydration enthalpy?
Enthalpy change when a solution of ions is made from 1 mole of gaseous ions
What is lattice enthalpy?
Enthalpy change when 1 mole of solid is formed from the separate ions, always exothermic
What is the enthalpy of a solution?
Enthalpy change when one mole of an ionic solid dissolves in excess water
If negative, the solid will dissolve
What is the equation of enthalpy for a solution?
Enthalpy of solution = Hydration enthalpy - lattice enthalpy
How does distance affect electrostatic attraction?
Gets weaker with distance
How does the small ionic radius of group 2 metals affect the electrostatic attraction?
As more closely packed they are able to have stronger ionic bonding than larger ions
How does the ionic radius change going down a group?
Increases
What are isoelectric ions?
Ions of different atoms with the same numbers of electrons
What is proof for ionic lattices having charged particles?
Migration of ions
Green copper chromate on paper with cathode and anode
Blue at cathode due to copper
Yellow at anode due to chromate
What is the bond length?
Distance between two nuclei in covalent bonding
How does bond length affect the bond enthalpy?
The shorter the covalent bond, the higher the bond enthalpy
What is electron density in relation to covalent bonding?
The number of electrons within the covalent bond
How does electron density of covalent bonds affect bond enthalpy?
Higher the electron density
Shorter the bond
Therefore higher the enthalpy
How much do lone pairs repel in comparison to bond pairs?
Lone pairs repel more than bonding pairs
Between which electron pairs are the angles largest?
Lp/lp is largest angles
Lp/bp is second largest angles
Bp/bp is smallest angels
How can lone pair repulsion affect bonding pairs?
Bonding pair angles reduced as they are pushed together by lone pair repulsion
What halogen bonded to hydrogen has the shortest bond?
H - F
most electronegative
Which of the following has the largest ionic radius?
S 2-, Cl -, K +, Ca 2+
S 2-
Same electrons but less protons so not as strong electrostatic pull on electrons
Why does H2O have a bond angle of 104.5?
Electrons repel as far as possible
Lone pair has greater repulsion than bond pair
Therefore less than 109
What is an ionic bond?
Electrostatic attraction between oppositely charged ions
What is an ionic giant lattice?
3-D arrangement of ions which is regular
Alternating between positive and negative
What properties do ionic compounds have?
Hard crystalline structures
High melting and boiling points
Generally high solubility in water
Good electrical conductivity when molten/dissolved
Why do ionic compounds not conduct electricity as a solid?
Ions cannot move out of the regular shape, therefore it cannot conduct electricity
What is a covalent bond?
Electrostatic attraction between two nuclei and a shared pair of electrons between them
What is a simple molecule?
Non-metals bonded together covalentlyUsually diatomic
What are the properties of simple covalent molecules?
Low melting point
Non-conducting of heat or electricity
More soluble in non-polar solvents (hexane not water)
Dissolved solutions don’t conduct electricity
What is a giant covalent structures?
Many atoms joined together by very strong covalent bonds
What is the structure of diamond?
Each C atom bonded to 4 others
Tetrahedral 3D arrangement
What is the structure of graphite?
Each C atom bonded to 3 others
Hexagons within a layer
4 outer electrons, 3 used in bonding
Free one delocalised and flow along layers
What keeps layers of graphite together?
London/van der Waals forces
What is graphene?
1 layer of graphite
Hexagon shape
Good electrical conductor
Relatively strong
What is the structure of metals?
Giant metallic 3D lattices
Metal cations surrounded by a sea of deloacalised electrons, attracted by electrostatic forces
What does the structure of metals cause?
High melting point - strong electrostatic attractions
High density - close packing
Conducts electricity - delocalised electrons
Malleable - bonding is non-directional so they can slide over each other
Conducts heat - electrons can pass kinetic energy to each other
Insoluble - strength of bonding
Why do ionic lattices form?
Each ion is electrostatically attracted in all directions to ions of the opposite charge
Why can ionic compounds not be shaped?
Same charged ions repel each other, so they break when its stretched or hammered Brittle
Why are ionic substances soluble in polar solvents?
Ions are pulled apart by polar molecules
What are the properties and explanations of giant covalent structures?
Very high melting points - need to break many strong bonds
Extremely hard - same as above
Good thermal conductors - Vibrations travel easily through the rigid structure
Insoluble - no ions, atoms more attracted to neighbours in the lattice than the solvent
Can’t conduct electricity - no charged ions or free electrons
Why can graphite conduct electricity?
As only bonds to 3 other carbon atoms, there is one delocalised electron per atom
Electrons can carry the charge
What bonding occurs in metallic bonding?
Electrostatic attraction between delocalised negative electrons and the positive metal ions
How does the number of electrons affect metallic bonding?
Stronger bonding as there is a larger electrostatic force
How do impurities affect the electrical conductivity of metals?
Reduce conductivity
Reduces number of electrons that are free to move and carry charge
As the electrons transfer to the impurity and forms anions
