topic 3 - electron affinity & types of bonding Flashcards
Electron Affinity
First electron affinity is the change in energy when 1 mole of electrons are added to 1 mole of gaseous atoms to form 1 mole of negative ions
X(g) + e- → X-(g)
Removing electrons requires energy, so gaining electrons releases energy
Therefore first electron affinities are negative – energy is released!
Group 7
The amount of energy released decreases down the group (except Fluorine – more later)
Electron affinity is the strength of the attraction between the nucleus and the incoming electron
A strong attraction means more energy is released
Electron affinity is affected by nuclear charge, distance from the nucleus to the outer orbital and shielding from inner electrons
Going down the group, the nuclear charge increases BUT shielding and distance also increase
Therefore the attraction gets weaker down the group so less energy is released
Fluorine
Fluorine is a very small atom!
There is very little shielding from the two electrons in the 1s orbital
There is a very short distance between the nucleus and incoming electron
BUT the extra electron is being added to an orbital already very dense with electrons
SO the repulsion from the existing electrons decreases the attraction to the incoming electron
This decreases the energy released
Group 6
The first electron affinity shows a similar pattern to group 7 – it decreases down the group apart from oxygen (for the same reasons as Fluorine)
Overall, group 6 elements have lower first electron affinity than group 7 as the shielding and distance are the same for both groups, but there is a greater nuclear charge in group 7
Therefore the nuclear charge will cause a greater attraction in group 7 so more energy is released by group 7
Second Electron Affinity
Singly charged negative ions can gain another electron to become 2- ions
Second electron affinity is the energy change when 1 mole of singly charged negative ions gains 1 mole of electrons to form 1 mole of doubly charged negative ions
X-(g) + e- → X2-(g)
Group 7 elements can also have second electron affinity (but it’s really hard to add another electron to a group 7 ion!)
Value of 2nd Electron Affinity
The two negative charges (the ion and the incoming electron) repel each other so energy is required to force another electron into the ion
This means the second electron affinity is positive (energy is needed)
E.g. O(g) + e- → O-(g) -142 kJ.mol-1
O-(g) + e- → O2-(g) +844 kJ.mol-1
Type of Bonding
The electronegativity of an element can predict the type of bonding it will undergo
Bonding is actually a spectrum, with very few bonds being purely ionic or purely covalent
Most are somewhere in between
Bonding with hydrogen
In hydrogen molecules (H-H), both atoms have the same electronegativity so they form a non-polar covalent bond
In Hydrogen Fluoride (H-F), Fluorine has high electronegativity so will attract the bonding pair of electrons. This forms a polar covalent bond with Fluorine having a δ- charge and Hydrogen having a δ+ charge
Difference in electronegativity
We cannot directly measure electronegativity but there is a scale of relative values produced by Chemist Linus Pauling
This allows us to predict how ionic or covalent a bond will be:
If the difference is low – non-polar covalent
As the differences increases – bond becomes more polar
If difference is very high – bond is ionic
Polarity in ionic bonds
Ionic bonds can also be polar
The extent of polarisation depends on:
Whether each ion is highly charged
If there is a small cation (positive)
If there is a large anion (negative)
A small, highly charged cation will attract electrons
A large, highly charged anion will be easily distorted
Therefore some negative charge is shared with the cation, giving the ionic bond some covalent properties
