ionic compound with covalent charateristic

-pure ionic compounds are formed by complete transfer of electrons from metallic atoms to non-metallic atoms. the electron cloud of an ion is solely confined to itself and not shared with its neighbouring ions. Actually pure ionic compounds are hard to find as the elctron cloud of an ion is easily distorted by the neighbouring oppositely charged ions and hence bonding has some covalent character.

(1) In pure ionic compound, the electron cloud is held tightly by the nucleus in an ion. No sharing of electrons is allowed between ions.
(2) In reality, if two ions of opposite charges are placed together, the electron cloud of an ion will be distorted by the other, thus inducing some sharing of electrons (covalency).

but for #1, the electron cloud is held tightly by the nucleus in an ion <-- how can one lose electrons and transfer to the other ?

Besides,
2. Explain the following characteristic properties of metals in terms of their structure and bonding
I) high density
II) high melting point
3. there are several crystalline forms of metallic iron. Below 906C, iron exists in the (alpha) form which has a boday centred cubic structure. Above this temperature, iron changes to (gama) form, which as a face centred cubic structure. Describe, with explanation, the change in colume that would occur when the temperature of a piece of iron is increased from room temperature to 100 C
4. which compound, AgCl, AgBr or AgI has the highest covalent character in the ionic bond. explain.
5. compare and explain the polar covalent bond of HCl, HBr, HI.


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In part 1, it does not imply the full electron transfer. Just imagine a Li+ and a Cl- ions put nearby. The electron of cloud of Li+ is strongly held by the nucleus. So does the Cl nucleus to its electron cloud. However, the charge of Li+ will attract (though not strong enough to pull out electrons) the electron cloud of Cl-. This distortion of the theoretically spherical electron cloud (for the pure ionic model) accounts for the appearance of the covalent character of the so-called ionic bond.

2. Note that metal nuclei are held together by "electron sea", the free electrons that move around in metals. Therefore, metallic bonds are non-directional, which allows for the close packing of metal atoms. This results in the generally high density of metals.

Metallic bond is electrostatic in nature. Due to the efficient packing of atoms in metals, they usually have a high melting point (although this is not always the case; check the physical properties of group I and II elements).

3. Do you mean from RT to 1000C?
Alpha form of Fe is less packed (68%) compared to the gamma form of Fe (74%). Therefore, we would expect that the volume may reduce a bit during the phase transformation.

4. AgI should have the highest covalent character. It is due to the polarizability of the electron cloud of the halide ions whose order is:
F- < Cl- < Br- < I-

5. The bond length: HCl < HBr < HI
The dipole moment: HCl > HBr > HI
The anomaly arises from the increasing polarizability from Cl to I which results in the higher covalent character of the H-X bond down the group. The enhanced covalent character reduces the partial charges on H and X, thus decreasing the bond dipole moment.

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1. X possibly belongs to group 1. Since X is polarizing (large charge density), the resulting X+Br- bond possesses a certain degree of covalent character which weakens the electrostatic attraction a bit. Therefore, the theoretical lattice enthalpy, which assumes a pure ionic interaction between X and Br, is larger than the experimental value.

2. It depends on whether the ionic character or the covalent character dominates.

3. Mg has two valence electrons while Na has only one. Therefore, the metallic bond between Mg atoms in solid is stronger than that of Na

4. Similar reason as 3. The melting point of group 1 elements decreases down the group because the atomic size gets larger which reduces the attraction between the nucleus and valence electron of an atom.

5. Metals are shiny since they reflect all lights. In more details, the free electrons of metals absorb light of all wavelengths and re-emit them. It means that all light is reflected by metals. Therefore, they have a lustre.

6. It means that the more negative side of a molecule will point toward the positive side of another molecule.

It depends on the liquid, but usually not since the induced dipole-ion interaction may not be strong enough the cause an observable attraction.

7. C-H bond should be polar since there is a difference in the electronegativity of C and H, but the bond dipole may not be large.

The dipole moment decreases from HCl to HBr to HI; therefore, we expect that the attraction between HCl molecules should be stronger than that between HBr molecules, and so do the HI molecules. Note that the stronger the attraction between molecules, the higher the boiling point.

However, this is not the case; experimentally, it is observed that the boiling point increases from HCl to HBr to HI. Obviously, there are other factoring coming into play to change the trend of the boiling point. The factor is the van der Waals force, which increases with the size and mass of the molecule. Since HI is much larger and heavier than HCl, the van der Waals force between HI molecules is so stronge that it compensates for the weak dipole-dipole attraction, resulting in an overall stronger attraction and thus the higher boiling point of HI compared to HBr and HCl.(new concept)