Topic 2 - Bonding and Structure Flashcards
What is an ion?
A positively or negatively charged atom (or group of atoms).
How are ions formed?
When electrons are transferred from one atom to another.
What are positive ions called?
Cations
What are negative ions called?
Anions
How do single atoms form ions?
They gain or lose 1, 2 or 3 electrons so that they have a full outer shell.
Is the charge on a metal ion positive or negative?
Positive
Is the charge on a non-metal ion positive or negative?
Negative
What is an ionic bond?
The strong electrostatic attraction between two oppositely charged ions.
What is the force responsible for ionic bonding?
Electrostatic attraction
What does the fomula of an ionic compound tell you?
The ratio of the ions in that compound.
How does the electrostatic attraction relate to the strength of an ionic bond?
The stronger the attraction, the strongere the bond.
What two factors affect the strength of an ionic bond?
- Ionic charges
* Ionic radius
How does ionic charge affect the strength of an ionic bond?
The greater the charge, the stronger the bond.
How does ionic radius affect the strength of an ionic bond and why?
- The smaller the radii, the stronger the bond.
* This is because smaller ions can pack more closely together and electrostatic attraction gets weaker with distance.
What is charge density of an ion?
The amount of charge per unit area or volume.
How does charge density affect the strength of an ionic bond?
Ions with high charge density form stronger bonds than those with low charge densities.
What is an ion with high charge density?
An ion with a large charge spread over a small area.
What is an ion with low charge density?
An ion with a small charge spread over a large area.
NaF and CaO.
Which has the higher melting point and why?
- CaO
- It is made up of Ca2+ and O2- ions, while NaF is made up of Na+ and F- ions.
- So the charges in the CaO are greater, resulting in stronger ionic bonds.
NaF and CsF.
Which has the higher melting point and why?
- NaF
- The ionic radius of Ca+ is greater than that of Na+.
- So Na+ and F- ions can pack more tightly than Ca+ and F- ions, resulting in stronger ionic bonds.
Describe how the size of an ion changes as you go down a group. Why?
It is increased, because extra electron shells are added.
What are isoelectronic ions?
Ions of different atoms with the same number of electrons.
Describe how the radius of isoelectronic ions changes as the atomic number increases.
It decreases, because there is greater attraction per electron from the protons, so they are pulled in closer.
What type of diagram is used to show ionic bonding?
Dot-and-cross diagrams
Remember to practise drawing out dot-and-cross diagrams for ionic bonding.
See pg 20 of revision guide
Describe the structure that ionic compounds form.
Giant ionic lattice
In an ionic compound, why does a giant ionic lattice form?
Each Ion is electrostatically attracted in all directions to ions of the opposite charge.
What are the physical properties of ionic compounds?
- High melting point
- Soluble in water, but not in non-polar solvents
- Non-conductors when solid, but do conduct when molten or dissolved
- Brittle
Describe how the physical properties of ionic compounds is evidence for the giant ionic lattice model.
- High melting point -> Tells you that the ions are held together by a strong attraction, like that between positive and negative ions
- Soluble in water, but not in non-polar solvents -> Particles must be charged
- Non-conductors when solid, but do conduct when molten or dissolved -> Ions are present, which are only free to move when molten or dissolved
- Brittle -> Layers can’t be pulled over each other, or the electrostatic repulsion would be too great
Aside from the physical properties of ionic compounds, what else is evidence for the theory of ionic bonding? Explain how this works.
Migration of ions in electrolysis:
• When you electrolyse a green solution of copper(II) chromate(VI) ions in some wet filter paper, the filter paper turns blue at the cathode and yellow at the anode.
• This is due to blue copper(II) ions in solution and yellow chromate(VI) ions in solution, which move to the electrodes.
• The solution is yellow to start off with because the ions mix.
What is the colour change observed in the migration of ions experiment observed? Why?
- Green colour (copper(II) chromate(VI) ions) splits into blue and yellow at the electrodes
- Blue is due to copper(II) ions
- Yellow is due to dichromate(VI) ions
What colour are dichromate(VI) ions?
Yellow
What types of bonds hold a molecule together?
Covalent
Describe how a covalent bond forms.
- Two atoms share electrons so they’ve both got full outer shells of electrons.
- A covalent bond is the strong electrostatic attraction between the two positive nuclei and the shared electrons in the bond
What diagrams can be used to represent covalent bonding?
Dot-and-cross diagrams
Remember to practise drawing out dot-and-cross diagrams for covalent bonding.
Pg 22 of revision guide, including examples
Describe the dot-and-cross diagram for the bonding in CO.
- C has two non-bonding electrons
- O had two non-bonding electrons
- In the covalent bond -> 2 electrons from the C + 4 electrons from the O (this includes a dative covalent bond)
What things balance at the given bond length for a bond?
- Repulsion between positively charged nuclei
* Attraction between the positive nuclei and the area of electron density
What determines the enthalpy of a bond?
Its length.
How does the length of a bond affect its enthalpy?
The shorter the bond, the higher the bond enthalpy.
Compare the length and bond enthalpy of C-C, C=C and C triplet bond?
- C-C is the longest and has the smallest enthalpy
* C triplet bond is the shortest and has the highest enthalpy
What is a dative covalent bond?
A covalent bond where bond of the electrons are donated by one atom.
Describe how an ammonium Ion involves a dative covalent bond.
- Ammonia has a lone pair of electrons on the N
* This lone pair is donated to form a dative covalent bond with a H⁺ ion to give an ammonium ion
When a dative covalent bond is being drawn, which way does the arrow point?
The direction in which the electrons are being donated.
Describe the structure of AlCl₃.
- Each Al forms single covalent bonds with 3 Cl’s
- However, in certain conditions, two AlCl₃ molecules can combine to form Al₂Cl₆.
- This happens when one Cl in each molecule donates a lone pair to the Al in the other molecule, forming two dative covalent bonds
- This gives aluminium a full outer shell
(See diagram pg 23 of revision guide)
Remember to practise drawing out the structure of Al₂Cl₆.
See diagram pg 23 of revision guide
Give some examples of molecules where dative covalent bonding is seen.
- CO
- NH₄⁺
- Al₂Cl₆
Order these in terms of the angle size:
• Bonding pair/bonding pair
• Lone pair/bonding pair
• Lone pair/Lone pair
- Smallest angle: Bonding pair/bonding pair
- Lone pair/bonding pair
- Largest angle: Lone pair/lone pair
Do lone pair electrons or bonding pair electrons repel more?
Lone pair electrons
What is the name for the way of predicting molecular shapes according to how much electron pairs repel?
Electron pair repulsion theory
Describe how much an extra lone pair decreases the bonding angles by in a molecule with 4 electron pairs around the central atom.
2.5°
Describe a tetrahedral molecule’s bond angle and how they change with extra lone pairs.
- 109.5°
- Decreases by 2.5° for every extra lone pair, so:
- 109.5° -> 107° -> 104.5°
Describe how you can predict the shape of a molecule around a central atom.
1) Find the central atom.
2) Work out the number of bonding pairs and lone pairs of electrons around it.
3) Use this information to predict the shape of the molecule.
For a molecule with 2 bonding pairs around the central atom, give the:
• Bond angles
• Name
- Bond angles = 180°
* Name = Linear