Topic 2 Bonding and Structure Flashcards
What are ions?
Charged particles that is formed when an atom loses or gains electrons
What is the charge of the ion when electrons are gained?
Negative
What are molecular ions?
Covalently bonded atoms that lose or gain electrons
Which electrons are lost when an atom becomes a positive ion?
Electrons in the highest energy levels
Do metals usually gain or lose electrons?
Lose electrons
Which are the 4 elements that don’t tend to form ions and why?
The elements are beryllium, boron, carbon and silicon
Requires a lot of energy to transfer outer shell electrons
What are the 3 main types of chemical bonds?
● Ionic
● Covalent
● Metallic
Define ionic bonding
The electrostatic attraction between oppositely charged ions
What determines the strength of an ionic bond?
- Ionic radius and ionic charge
- Ionic bonding is stronger and the melting points higher when the ions are smaller and/ or have higher charges.
How to draw dot and cross diagram for ionic bonding
Brackets with charges outside with the outershell displayed on the inside
Explain the trend in ionic radius down a group
Ionic radii increases going down the group. This is because down the group the ions have more shells of electrons and thus the outermost electron experience less pull from positive
nucleus.
Explain the trend in ionic radius for this set of isoelectronic ions, N 3-, O 2-, F-, Ne, Na +, Mg 2+, Al 3+
There are increasing numbers of protons from N to F and then Na to Al but the same number of electrons. Therefore nuclear attraction between the outermost electrons and nucleus increases and ions get smaller
What are the physical properties of ionic compounds?
*high melting points
*non conductor of electricity when solid
*conductor of electricity when in solution or molten
*brittle
In a solution of CuCrO 4 with connected electrodes which electrode will the 2 ions migrate to?
Cu 2+ - migrates to negative electrode
CrO 4 2- - migrates to positive electrode
Define covalent bonding
Electrostatic attraction between a shared pair of electrons and the nuclei
Draw a dot and cross diagram for covalent molecules
Overlapping circles of outer shells
Define metallic bonding
Electrostatic attraction between the positive metal ions and the sea of delocalised electrons
Why does giant ionic lattices conduct electricity when liquid but not when solid?
In solid state the ions are in fixed positions and thus cannot move. When they are in liquid state the ions are mobile and thus can freely carry the charge
Giant ionic lattices have high or low melting and boiling point?
Explain your answer
They have high melting and boiling point because a large amount of energy is required to overcome the electrostatic bonds
In what type of solvents do ionic lattices dissolve?
Polar solvents
E.g water
Why are ionic compounds soluble in water?
Water has a polar bond. Hydrogen atoms have a δ+ charge and oxygen atoms have a δ- charge. These charges are able to attract charged ions
What is the effect of multiple covalent bonds on bond length and strength?
Double/triple bonds exert greater electron density therefore the attraction between nucleus and electron is greater resulting in a shorter and stronger bond.
What is a dative covalent bond?
A bond where both of the shared electrons are supplied by one atom
How are oxonium ions formed?
Formed when acid is added to water,
H 3 O +
What does expansion of the octet mean?
When a bonded atom has more than 8 electrons in the outer shell
What are the types of covalent structure?
● Simple molecular lattice
● Giant covalent lattice
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
Why do simple molecular structures have low melting and boiling point?
Small amount of energy is enough to overcome the intermolecular forces
Can simple molecular structures conduct electricity?
No, they are non conductors.
Why do simple molecular structures not conduct electricity?
The have no free charged particles to move around
Simple molecular structures dissolve in what type of solvent?
Non polar solvents
Give examples of giant covalent structures
● Diamond
● Graphite
● Silicon dioxide, SiO
List some properties of giant covalent structures
● High melting and boiling point
● Non conductors of electricity, except graphite
● Insoluble in polar and non polar solvents
How does graphite conduct electricity?
Delocalised electrons present between the layers are able to move freely carrying the charge
Why do giant covalent structures have high melting and boiling point?
Strong covalent bonds within the molecules need to be broken which requires a lot of energy
Draw and describe the structure of a diamond
3D tetrahedral structure of C atoms, with each C atom bonded to four others
What does the shape of a molecule depend on?
Number of electron pairs in the outer shell
Number of these electrons which are bonded and lone pairs
What is the shape, diagram and bond angle in a shape with 2 bonded pairs and 0 lone pairs?
Linear 180°
What is the shape, diagram and bond angle in a shape with 3 bonding pairs and 0 lone pairs?
Trigonal planar
120°
What is the shape, diagram and bond angle in a shape with 4
bonded pairs and 0 lone pairs?
Tetrahedral
109.5°
What is the shape, diagram and bond angle in a shape with 5
bonded pairs and 0 lone pairs?
Trigonal bipyramid
90° and 120°
What is the shape, diagram and bond angle in a shape with 6
bonded pairs and 0 lone pairs?
Octahedral
90°
What is the shape, diagram and bond angle in a shape with 3
bonded pairs and 1 lone pairs?
Pyramidal
107°
What is the shape, diagram and bond angle in a shape with 2
bonded pairs and 2 lone pairs?
Non linear / v shape
104.5°
What is the shape, diagram and bond angle for NH 4+
Tetrahedral
109.5°
By how many degrees does each lone pair reduce
the bond angle?
2.5°
Define electronegativity
The ability of an atom to attract the pair of electrons (the electron density) in a covalent bond
What scale is electronegativity measured on?
Pauling scale
In which direction of the periodic table does electronegativity
increase?
Top right, towards fluorine
What does it mean when the bond is non-polar?
The electrons in the bond are evenly distributed
How is a polar bond formed?
Bonding atoms have different electronegativities
Why is H 2
O polar, whereas CO 2 is non polar?
CO 2 is a symmetrical molecule, so there is no overall dipole
What are the 3 types of intermolecular forces?
● Hydrogen bonding
● Permanent dipoles
● London forces
Describe permanent dipole- induced dipole interactions
When a molecule with a permanent dipole is close to other non polar molecules it causes the non polar molecule to become slightly polar leading to attraction
Describe permanent dipole- permanent dipole interactions
Some molecules with polar bonds have permanent dipoles → forces of attraction between those dipoles and those of neighbouring molecules
Describe London forces
● London forces are caused by random movements of
electrons
● This leads to instantaneous dipoles
● Instantaneous dipole induces a dipole in nearby
molecules
● Induced dipoles attract one another
Are London forces greater in smaller or larger molecules?
Larger due to more electrons
Does boiling point increase or decrease down the noble gas
group? Why?
Boiling point increases because the number of electrons increases and hence the strength of London forces also increases
What conditions are needed for hydrogen bonding to occur?
O-H, N-H or F-H bond, lone pair of electrons on O, F, N
Because O, N and F are highly electronegative, H nucleus is
left exposed
Strong force of attraction between H nucleus and lone pair of
electrons on O, N, F
Describe the diagram of a hydrogen bond
dotted line between a atom of a positive dipole and a negative dipole
Why is ice less dense than liquid water?
● In ice, the water molecules are arranged in a
orderly pattern. It has an open lattice with
hydrogen bonds.
● In water, the lattice is collapsed and the
molecules are closer together.
Why does water have a melting/ boiling point higher than expected?
Hydrogen bonds are stronger than other intermolecular forces so extra strength is required to overcome the forces
What type of intermolecular forces do alkanes have? Why?
London force → induced dipole-dipole interaction, because the bonds are nonpolar
What happens to the boiling point as alkane chain length increases? Why?
The boiling point increases because there is more surface area and so more number of induced dipole- dipole interaction. Therefore more energy required to overcome the attraction
Does a branched molecule have lower or higher boiling point
compared to equivalent straight chain? Why?
The branched molecule has a lower boiling point because they have fewer surface area and hence less induced dipole -dipole interactions.
Are alkanes soluble in water? Explain your answer.
Insoluble because hydrogen bonds in water are stronger than alkanes’ London forces of attraction
What kind of intermolecular forces do alcohols have? Why?
Hydrogen bonding, due to the electronegativity difference in the OH bond
How do alcohols’ melting point and boiling point compare to other hydrocarbons’ of similar C chain lengths? Why?
Higher, because they have hydrogen bonding (strongest type of intermolecular force) → stronger than London forces
Are alcohols soluble in water? Why does solubility depend on chain length?
Soluble when short chain - OH hydrogen bonds to hydrogen bond in water
Insoluble when long chain - non-polarity of C-H bond takes precedence
Explain the trend of boiling temperatures of hydrogen halides HF to HI
There is a general increase of boiling point from HCl to HI which is caused by increasing London forces because of increasing number of electrons. There is a big drop in boiling point from HF to HCl because fluorine is very electronegative therefore the hydrogen bonding is much stronger.
