The larger atoms have more completed shells, and the outer negative valance electrons are further from the neucleus ( positive protons).
So larger atoms further down the chart (decreasing electronegativity ) behave differently, than another lighter atoms in the same group.
Group 1 atoms form metalic bonds with each other, but the group 7 atoms form covalent bonds with each other F2, CL2, I2 etc.
Metalic bonds: The energy level of outer electrons overlap with neighboring metal atoms. The electrons are free to move from atom to atom, and gives rise to the property of conduction.
The negative electrons are strongly attracted to the positive protons in the neucleus, and the electrostatic attraction is strong.
Electrons can move, but in a shared way, so that an electron is not really lost, but another neighbor moves into place, so ions are not formed. Electrons are electrostatically attracted to the large group of protons ( other atoms). This produces a "cloud" of electrons that are closely held to the group of metalic atoms. They try to stick together as close as possible, and they can pack together in a crystaline pattern.
This also allows individual neucli to slide along from atom to atom. This allows metals to be reshaped, ( malleability and ductility).
Electrons and the positive proton neucleus, have strong attractive forces, so it can be difficult to remove any one atom from the group.
Smaller atoms have fewer completed electron shells, and the outer valence electrons in these feel a stronger pull from the positive protons.
Therefore, it is much more difficult to remove one of these outer electrons when compared to larger group 1 atoms. Larger group 1 atoms have many more completed electron shells, and the valence electrons are much further away from the positive protons. The attractive force is much less, than with lighter atoms. Therefore it is easier to remove one outer electron in Potassium, then to remove one outer electron from Lithium, even though they have the same outer electron configuration. This allows the larger group 1 atoms to move away from each other a bit more than lighter atoms, and allows them to appear more fluid like, (melt).
Covalent bonds of the Halogens:
The atoms of the halogens Flourine, Chlorine, etc, bond to each other to form covalent bonds.
Each has 7 valence electrons but 8 would be more stable, as the "p" orbital could be filled.
With a purely covalent bond ( electrons are evenly shared between the two atoms), there is no electric charge on the molecule F2, Cl2, Br2 etc.
Fl-Fl CL-CL Br-Br.
Because of this, molecules of these atoms are not attracted to each other. This lack of charge between molecules results, in most of these being in the gas state at room temperature.
But they can form liquids at very low temperatures, and Iodine is a solid at room temperature ( but will evaporate eventually if you let it).
The electrons are equally shared between the two atoms, but in any particular instant, the electron has a definite location, which may not be centered between the two atoms.
Statistically, the electron is centered between the two, but for an instant, it could be fully on one atom, or the other, or somewhere in-between.
This gives rise to a momentary charge imbalance.
( momentary di-pole). This momentary charge imbalance interacting with other such molecules, forms an attractive force known as London, or dispersive force. It is very weak, but is responsible for allowing purely covalent di-atomic molecules such as Hydrogen H2, Helium He2 to form a liquid at all.
So, why is there a trend of higher melting point as you progress down the chart?
Well, first what is temperature?
Temperature is the measure of molecular vibration. At high temperatures, molecules or atoms are vibrating with a great deal of energy. At low temperatures, the vibrational energy is less.
So at high temperatures, the atoms are bouncing around like ping pong balls, and appear as gases.
Lower temperatures produce liquids and solids.
Small atoms and large atoms (or molecules), have the same energy for the same temperature.
This means that small atoms are vibrating more because they are small, and larger heavy atoms vibrate less because they have more mass to move for the same given energy.
The small attractive London dispersive force, is weak, but Iodine is so large of an atom/molecule, that it doesn’t have enough energy to bounce around like a smaller molecule such as fluorine FL-FL can. So Iodine tends to be more solid, and Fluorine tends to be a gas. This directly describes the melting point trend of these halogens.
b) What is the expected trend in the melting points of the compounds LiF, NaCl, KBr, and CsI? Explain this trend using bonding principles."
These form ionic bonds.
Group 1 elements have 1 electron in their outer most shell. Group 7 elements Fluorine, Chlorine etc, have 7 electrons in their outer most shell.
When shells are filled, they have a lower energy state, and are more stable. Electron configurations of filled shells are more stable and have lower energy levels, and are preferred.
So Group 1 atoms prefer to loose one electron, in which case their electron configuration falls back to the previous filled shell, which has 8 electrons.
Those atoms in group 8 are the Nobel gases. Nobel gases have all outer shells (s and p orbital)filled. This preferred filling of the s and p orbital to achieve 8 electrons in the outer most shell is referred to as the octet rule.
Group 1 atoms prefer to loose one electron and fall back to the previous shell, but group 7 have 7, but need just one more to fill the shell.
As previously mentioned, filling the shell is a lower energy state, so when the shell is filled, considerable bonding energy is released, and the two atoms bond.
For example, if Potassium bonds with Chlorine, the Chlorine grabs the one electron from Potassium, giving Chlorine 8 electrons, and Potassium falls back to 8 from the previous shell.
Both now have 8 in their outer most shell, and are in the most stable ( lowest energy) configuration.
A great deal of energy was released when the bond was formed, so there was probably a considerable bang associated with their joining.
Lighter atoms in group 1 have fewer completed electron shells, and so the outer valance electron is relatively close to the protons. Therefore, the outer electrons are strongly attracted to the protons, and so there is some energy needed to remove this electron. Larger group 1 atoms such as Potassium, are much larger, and the outer electron is so far away from the protons, that the attractive force is much smaller. So it is much easier to remove an outer electron from potassium, and give it to a group 7 atom which desperately wants it to fill its shell.
Larger atoms of group 1 such as Potassium are more "electro-positive" then smaller group 1 atoms such as Lithium. This is a measure of how hard it is to remove an outer electron.
On the other hand, group 7 halogens such as Fluorine have 7 and want one more to have a total of 8, and a completed shell. Fluorine is the smallest, having the fewest lower shells, so the outer 7 electrons feel the protons much stronger, and will have more energy available to rip a stray electron from a group 1 atom. This is the most electro-negative element ( strongest affinity to rip an electron from another atom). Other group 7 atoms that are larger, and also want one more electron, but have so many lower shells, that the outer electrons don’t feel the protons as much.
So the lower left of the chart is the most electro positive, and the upper right of the chart is the most electro-negative.
By combining the lower left such as potassium, with the upper right Fluorine, will produce the most energy release ( "bang").
After the energy is released, the combination of
K-Fl is very stable. Both have filled shells. The energy released, is the same energy required to try to remove the 8th electron back from Fluorine and give it back to Potassium ( good luck with that). So combining the most electro positive, with the most electro negative will produce the strongest Ionic bonds. Combining Lithium with Iodine, will produce a weaker Ionic bond.
So what is an Ionic bond?
When the electron spends most of its time with one atom, such as the case of Fluorine and Potassium. Potassium doesn’t need the electron it already has 8 from the lower shell, and Fluorine needs it to keep it outer shell filled. So the electron is very strongly held by Fluorine, and statistically, it is nearly always on the Fluorine side.
This causes Potassium ( which lost the negative electron), to have a positive charge, and Fluorine, which has one extra is now negative.
( Normally, an element has one electron for every proton in the nucleus).
May The Force be with you . . .
Austin Semiconductor - Dallas TX