Define the following terms as applied to solid state chemistry

Here are the solutions to the questions:

2. (b) Define the following terms as applied to solid state chemistry

• (i) Unit cell: The smallest repeating unit in a crystal lattice that, when repeated in three dimensions, generates the entire crystal structure. It represents the fundamental building block of a crystal.

• (i) Coordination number: The number of nearest neighbor atoms or ions that surround a central atom or ion in a crystal lattice.

• (ii) Give an example of a metal with a close packed structure.

  • Copper (Cu) (Other examples include Magnesium (Mg), Zinc (Zn), Silver (Ag), Gold (Au)).

2. (c) The table below shows the electronegativity of some elements

• (i) What do you understand by the term electronegativity? Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons towards itself in a chemical bond.

• (ii) Give the formula of an ionic compound formed by two elements on the table. An ionic compound typically forms between a metal and a non-metal with a large difference in electronegativity. Lithium (Li) is a metal (EN = 1.0) and Fluorine (F) is a non-metal (EN = 4.0). The difference is $4.0 - 1.0 = 3.0$, indicating a highly ionic bond.

  • Formula: $\boxed{\text{LiF}}$

• (iii) Suggest the formula of a molecule that has the least polar bond. The least polar bond will be formed between two elements with the smallest difference in electronegativity. Comparing the elements:

  • H (2.1) and B (2.0) have a difference of $2.1 - 2.0 = 0.1$.
  • H (2.1) and C (2.5) have a difference of $2.5 - 2.1 = 0.4$.
  • O (3.5) and F (4.0) have a difference of $4.0 - 3.5 = 0.5$. The smallest difference is between Hydrogen (H) and Boron (B).
  • Formula: $\boxed{\text{BH}_3}$ (Borane)

• (iv) The boiling points of three molecules are given in the table below

**Explanation of the trend (assuming this is the implied question):**  
The boiling points increase due to the strength of intermolecular forces:
*   $\text{F}_2$ is a nonpolar molecule, so it only experiences weak **London Dispersion Forces**.
*   $\text{CH}_3\text{F}$ is a polar molecule, experiencing **London Dispersion Forces** and stronger **dipole-dipole forces**.
*   $\text{HF}$ is a highly polar molecule that can form **hydrogen bonds** (due to H being directly bonded to the highly electronegative F atom), which are the strongest type of intermolecular force among these molecules, leading to the highest boiling point.