L7 Ionic bonding Flashcards
What is a chemical bond?
- Why do atoms form bonds?
= the attractive force that holds two or more atoms together in a molecule.
They form bonds to gain stability:
- e- distribute themselves in orbitals for the most stable/lowest energy configuration
- rearranging e- between atoms allow the combined molecule to achieve even lower energy
What is the Octet Rule?
= elements aim to become isoelectronic as Nobel gases - they adjust their valence shells by gaining/losing/sharing e- so they have full outer shells.
* Transition metals use an 18 e- rule instead
What is electronegativity?
- what does it depend on?
= the ability of an atom to attract e- in a chemical bond towards itself Depends on: - the no. of protons in the nucleus - the size of the atom - the amount of screening by inner e-
What elements are considered highly electronegative?
Any element with an electronegativity greater than 3 on the Paulings scale = top RHS of the periodic table.
* Fluorine is the most electronegative element - small atom, high nuclear charge
What is an ionic bond?
Electrostatic attraction between two oppositely charged ions, after one or more e- are fully transferred. Takes place between metals and non-metals and the formation of the bond involves a decrease in energy.
What is a cation?
Positive ion formed by a metal atom losing e-.
What is an anion?
Negative ion formed by a non-metal atom gaining e-.
Define isoelectronic?
An atom having the same electronic configuration (same no. of electrons/electronic structure) as another atom.
What are the properties of an ionic bond?
= electrostatic attraction
- Strong
- Non-directional
- Broken in polar solvents e.g. water -> dissolve
- Broken by physical changes e.g. melting
What does non-directional mean?
That the strength of the ionic bond (the interaction between ions) is dependant on distance but not on direction.
What are the features of ionic compounds?
- Formed between metals and non-metals
- Dissolve easily in water and other polar solvents - the bond is broken and the separated ions are solvated
- Conduct electricity whilst molten and in solution
- Form crystalline solids with high melting points
What is an ionic lattice?
= Repeating 3D pattern of anions and cations in the solid-state - the ions are attracted to all other ions with the opposite charge.
- Giant structure
- Hard crystals - shatter (the structure deforms when ions with the same charge are forced together and then repel)
What is lattice energy?
= The enthalpy of the formation when forming one mole of the ionic compound (solid) from its gaseous ions.
What factors favour ionic bonding?
- Easy cation formation - low ionisation of the metal
- Easy anion formation - high electronegativity, small size and low charge of the non-metal
- Large electronegativity difference between the atoms
- High lattice energy
How big are the ions?
- Cations (+) are smaller than the neutral, parent atoms: the remaining e- are held closer to the positive nucleus
- Anions (-) are larger than parent ions: larger e- clouds
What is polarisation?
= The distortion of the e- cloud of an atom, that results in non-spherical ions and covalent character in the bond.
- The larger the ion & the greater no. of e-, the higher its polarisability
- Anions tend to be more polarisable
What is polarising power?
= An atoms ability to distort the e- cloud of another atom.
- The smaller an ion & the higher its charge, the more polarising power it has
- Cations tend to have more polarising power
What is Fajan’s Rule?
= predicts covalent character in ionic bonds, that ionic compounds are likely to have some covalent character if:
- The cation is small and/or has a high charge = highly polarising
- The anion is large and/or has a high charge = highly polarisable
What are the features of purely ionic compounds v. ionic compound with covalent character?
Purely ionic: - Low positive charge - Large cation - Small anion Covalent character: - High positive charge - Small cation - Large anion
If a melting point is lower than expected, what is happening?
Covalent character/directionality weakens the strength of the ionic lattice.
What is a polyatomic ion?
= ions made up of more than one type of atom
- The bonding between the atoms in the ion is generally covalent
- The no. of e- means the group has an overall charge
How do you name ionic compounds?
1st word = Cation - usually the same name as the parent atom
(oxidation state of the metal)
2nd word = Anion:
- polyatomic = name of the ion
- one element = name of the element with suffix -ide
e.g. Copper (II) chloride
