AS/A-Level Chemistry Edexcel- Bonding, Polarisation, Trends, Electronegativity Flashcards
Define Ionic Bonding
The electrostatic force of attraction between oppositely charged ions
Explain the reason for Giant Ionic Lattice structures:
Why is it structured this way?
[] They are arranged in this way to maximise the attraction of oppositely charged ions
[] Minimises repulsion of similarly charged ions
[] All forces act equally in all directions
[] Form ionic crystals
How do Ionic bonds form?
Electrons are transferred from one atom to another forming charged ions which are held together by electrostatic forces of attraction.
- Metals lose e- to form positive cations
- Non-metals gain e- to form negative anions
What is the charge of an Ionic compound?
0 because the positively charged ions cancel out negative
What are the physical properties of ionic compounds?
1- High melting point due to strong electrostatic forces of attraction
2- Brittle due to layered structure
3- Doesn’t conduct electricity when solid
4- Insoluble in non-polar solvents but soluble in water
Why are ionic compounds brittle?
Any force on the structure causes the layers to move meaning similarly charged ions will end up together resulting in the charges to repel and split up the crystal.
Why do ionic compounds have high Mp and Bp?
This is due to the strong electrostatic force of attraction needing large amounts of energy to overcome the forces and break it down
Why can they only conduct electricity when molten?
When they are solid the ions are held together strongly (compact) meaning the ions are no free to move whereas when molten or aqueous they are mobile and free to move meaning conduction can occur
Trend in periodic table: Atomic Radius
What trends can we see across and down the group?
Across a group (left to right)- Atomic radius/atomic size decreases
Down a group- Atomic radius/ size increases
Explain why Atomic radius increases down a group?
A new shell is added
Explain why Atomic size decreases across a period for metals:
Atomic size decreases across a period for metals because…
They lose an electron to form a cation
Explain why Atomic size increases down a group for metals:
Non-metals, they increase down a group because it forms anions meaning they gain an electron resulting in more shells and more no. of electrons than no. of protons leading to an increase in atomic size.
Explain how the ionic radius and charge of an ion can affect the bonding: (4 marks)
- Size of ions- smaller sizes mean they have a stronger force of attraction resulting in stronger ionic bonding. Larger size leads to less attraction resulting in weaker ionic bonding
- Higher charge- a stronger force of attraction hence stronger ionic bonding
Define Isoelectronic:
Same number of electrons and electronic configuration. e.g Aluminium 3+ ion and Oxygen 2- ion
Define Electronegativity:
The measure of the ability of an atom to attract the electrons when the atom is part of a compound
High electron density
More electrons in that area
Role of electron density maps
Show how electrons are distributed in a chemical substance. X rays passed through ionic crystals onto photographic film
- likelihood of finding electrons in that region
- contour lines of equal electron density, with greater electron densities being on contours closer to the nucleus.
Electrolysis- How can we use electrolysis as evidence of the existence of ions/movement of ions? (Copper(vI) Chromate)
the behaviour of ionic substances
1- Aqueous Copper(vI) chromate CuCrO4 is an olive green solution
2- It undergoes electrolysis to form a blue colour at the (negative electrode) cathode and a yellow colour at the anode (positive electrode)
Positive Anode Negative Is Cathode (PANIC)
What are the ions that turn colour in electrolysis?
Positive blue Copper ions are attracted to the cathode
Negative yellow Chromate ions are attracted to the anode
Electron Density structure of NaCl:
- Ions arranged in regular pattern
- Chloride ions are larger than sodium ions (remember not chlorine atom but ions are larger because it has Cl-)
- Ions are separate because the electron density falls 0 between ions
- Anions are a bit distorted
Describe the structure of Diatomic (atoms of the same element) molecules in electron density maps?
1- Ions are not separate because of the sharing of electrons
2-Equal size- identical electronegativity
3-Contour lines are similar because of the same electron density around ions
Describe the structure of covalent compounds in electron density maps?
1-Ions are not separate due to the sharing of electrons
2-Size of ions show little difference in electron density
3-Contour lines are not exactly the same
What evidence show that ionic compounds exist?
1- High melting points, strong electrostatic attractions between oppositely charged ions
2-Electrical conductivity can only occur in a liquid state or aqueous solution because ions need to move
3-Coloured ions can be observed migrating to electrodes during electrolysis
4-Electron density maps show low electron density between the oppositely charged ions
Why does the Ionic radius decrease as the number of protons increases?
As the positive charge of the nucleus increases, the electrons are attracted more strongly and therefore are pulled closer to the nucleus
What are the Fajan’s rules?
How can polarisation be increased?
- Cation= small size, high charge -> highly polarising and attracts electrons in the anion
- Anion= large size, high charge -> highly polarising and distorts easily
Define polarising power:
the ability for a cation to attract electrons from the anion towards itself
Charge density formula
charge/r^2
What causes covalent character in Ionic compounds?
- during the formation of an ionic bond 2 oppositely charged ions of unequal size attract each other
- smaller ion (cation) attract the outermost electrons of the other anion and repels the nuclear charge
- Results in distortion or polarisation of larger ion
- Electron cloud is no longer symmetrical and elongated towards the cation
Define polarizability:
The ability of the Anion to get polarised
What factors increase the covalent character?
Increases polarisation
1- Increase in anion size 2-Decrease in cation size 3-Increase in Nuclear charge 4-Transition metals 5-Highly charged density cations
Describe the relationship between melting point and covalent characters:
Bigger Anion, greater covalent character, low melting point (due to the sharing of electrons)
Ionic, higher melting point
Electrons in a covalent bond are not evenly shared. It all depends on the…..
….Electronegativity of the atoms in the bond
The greater the electronegativity…..
….the greater its tendency to attract the bonding pair of electrons. (creates poles at the end of the molecule, dipole)
Which element is the most electronegative?
Fluorine
What are the trends in electronegativity in the periodic table?
Increases across a period from group 1-17
Decreases down a group, Increases up a group
Explain the trend in electronegativity in the periodic table:
Why does it increase across a period and up a group?
Increases across a period (left to right) because the no. of protons increases, nuclear charge increases, greater attraction to the bond pair of electrons
Increases up a group because the atomic size/ radius decreases up a group, shielding effect is less, less repulsion and greater attraction
What do you mean by a bond is pure/non-polar?
diatomic molecules/ when 2 covalent bonds have the same electronegativity (no difference) and electron cloud is evenly distributed between 2 atoms.
-Bonding is shared equally, no charges, perfectly covalent
What do you mean by a polar covalent bond?
- Bonding is unequal
- Partial charges on atoms
- Small difference in electronegativity
Ionic bond:
- complete transfer of one or more valence electrons. Full charges on resulting ions.
- great difference in electronegativity
What are the 3 assumptions of electrostatic theory?
1- All bonds are ionic
2-All ions are perfect spheres
3-Charge is evenly distributed in these spheres/point charges
Electronegativity difference:
In pure covalent, there is 0 electronegativity difference
Polar covalent, electrons are unequally shared they have a slight electronegativity difference
Polar Ionic, ions are distorted, big electronegativity difference
Pure Ionic, very large electronegativity difference
Define polarisation:
covalent character in ionic substances is due to the polarisation of the anion by the cation
-causes a distortion of the electron density in the anion causing it to move closer to the cation resulting in a degree of covalent character.
How can we use the electrolysis of sodium chloride as evidence of the existence of ions?
1- Pass a direct electric current through molten sodium chloride
2-Sodium is formed at the negative electrode
3-Chlorine is formed at the positive electrode
[] Positive sodium ions migrate towards the negative electrode where they gain electrons and become sodium atoms
[]Negative chlorine ions migrate towards the positive electrode where they lose electrons and become chlorine molecules
Define covalent bond:
Strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
What do you mean by the term dipole?
The differing attraction for the pair of electrons allows there to be a small charge difference between the atoms.
What are the differences between polar and non-polar molecules?
Polar
- non-symmetrical
- difference in charge exists across the molecule
- overall dipole
Non-Polar
- symmetry of molecule means the effect of any permanent dipoles are cancelled out
- No difference in charge across the molecule
Exam-style question: In Magnesium Iodide some polarisation of the bond occurs. Explain using this example what the term polarisation means:
In Magnesium Iodide there is an unequal sharing of paired electrons meaning there is a difference in electronegativity resulting in polarisation occurring where the cation will attract electrons in the anion causing it to distort.
-unequal distribution of charge
Hydration:
process of water molecules being attracted to ions in solution and surrounding the ions; the oxygen ends of the water molecule will be attracted to the cations and the hydrogen ends will be attracted to the anions.
-Hydration of ions is an exothermic process
How are Sigma bonds formed? Where is the region of electron density located?
Formed by the head-on overlap of atomic orbitals
-electron density is concentrated in the nuclei of the bonding atoms
What are the types of sigma bonds?
1- 2 s orbitals overlap head-on (s-s)
2- S orbital overlaps with a p orbital (s-p)
3- 2 P orbitals overlap head-on (p-p)
How are Pi bonds formed? Where is the region of electron density located?
A pi bond is formed by the sideways overlap of atomic orbitals
-results in electron density above and below the plane of the nuclei of the bonding atoms
Describe the electron density of 2 atoms of the same element bonded together:
If 2 atoms of the same element bond together we see an identical electronegativity and an evenly distributed electron density. Electron density between the 2 nuclei is identical
Type of Covalent bond/ Sigma or Pi :
single covalent - 1 sigma
double covalent- 1 sigma 1 pi bond
triple covalent-1 sigma 2 pi bonds
Which bond is stronger? Sigma or Pi?
Sigma bond is a much stronger covalent bond than Pi bonds due to the head-on overlap of the orbitals which results in a stronger hold of the atoms.
The sideways overlap of Pi bonds are smaller compared to the head-on overlaps hence it has a weaker covalent bond.
