Explanation:​
SiO2​ has a giant covalent structure where each Si atom is singly bonded to four oxygen atoms (while maintaining a Si

ratio of 1:2).
As there are four electron domains around each silicon atom, the structure has a tetrahedral arrangement. SiO2​ is held together by strong covalent bonds requiring a large amount of energy to overcome, resulting in a high melting point.
The strong bonds, together with the fact that there are no charged particles present, lead to insolubility in polar solvents such as water.

Explanation​:
Simple molecular covalent structures cannot conduct electricity due to an absence of charged particles. $\mathrm{NaCl}$ is an ionic compound that has ions that are free to move when in a molten state.

Explanation:
The higher the charge of the ions, the stronger the electrostatic forces of attraction and, therefore, the greater the lattice enthalpy.
The smaller the ion, the greater the charge density and the stronger the electrostatic attraction between oppositely charged ions.
Statement I is incorrect since the smaller the lattice enthalpy, the weaker the electrostatic attraction and the lower the melting point is.

Explanation:
Non-metal elements bond to each other covalently. Ca3N2​ is made up of a metal and a nonmetal (with a large difference in electronegativity) and has ionic bonding.

Explanation​:
N2 has a triple bond, O2​ has a double bond, and H2only has a single bond.
The greater the number of shared pairs of electrons, the stronger the bond.

Explanation:​
In SO2​, the sulfur is bound to two oxygen atoms.
The central sulfur has one non-bonding pair of electrons giving it three electron domains in total.
CS2​, BeCl2​, and C2​H2​ have two electron domains on their central atoms and have linear shapes.

Explanation:
Non-metal elements bond to each other covalently. Ca3N2CaX3​NX2​ is made up of a metal and a nonmetal (with a large difference in electronegativity) and has ionic bonding.

Explanation​:
Magnesium has two electrons in its outermost shell and loses both to achieve a full outer shell (Mg^2+). Fluorine has seven electrons in its outermost shell; it needs one more electron to achieve a full outer shell (F^1−).

Two fluorine atoms are therefore required for each magnesium atom in order to form a neutral ionic compound (where the charges of the positive and negative ions cancel out).

Explanation:​
There are three bonding pairs and no lone pairs around the central B atom giving it a trigonal planar shape. As this is symmetrical, there is no net dipole even though the individual bonds are polar. There are only six electrons around the central B atom making it electron deficient (it has not achieved its octet).

Explanation​:
There is no polyatomic ion with the formula of CO4 ^2−​. The correct formula for the carbonate ion is CO3^2−​. The formulas and charges of the other ions are correct.

Explanation​:
There is no polyatomic ion with the formula of CO42−COX42−​. The correct formula for the carbonate ion is CO32−COX32−​. The formulas and charges of the other ions are correct.

Explanation:
Ionic bonds form between oppositely charged ions; NH4^+ and SO4^2− in ammonium sulfate and Na^+ and CO3^2− in sodium carbonate.

Due to a smaller difference in electronegativity and both being nonmetals, $\mathrm{l}$ bonds are polar covalent.

Explanation​:
Metals and non-metals have the greatest differences in electronegativity and so ionic bonds are formed.

Explanation:​
Metals characteristically have high melting points due to strong metallic bonds.

Explanation​:
Choice A refers to ionic bonding;
Choice B could refer to ionic bonding or specifically coordinate (dative) bonding;
Choice C refers to hydrogen bonding.

Explanation​:
There is a non-bonding (lone) pair of electrons on the nitrogen atom and three on each chlorine. There are three N−Cl bonding pairs of electrons. Only the outermost electrons are shown for each element

Explanation:​
Atom 1 has four electron domains and so a tetrahedral shape (109.5$°$).
Atom 2 has three electron domains and so a trigonal planar shape (120$°$).
Atom 3 has four electron domains of which one is a lone pair giving it a trigonal pyramidal shape (107$°$).

Explanation:​
The strength of metallic bonding and therefore melting point, decreases down a group due to increasing atomic radius (more energy levels) and greater separation of the nuclei. Cs is in period 6, while all other 3 elements are in period 5. Hence, Cs has the lowest melting point. Since atomic radius decreases across a period due to increased effective nuclear charge, the strength of metallic bonding, and therefore melting point, increases from Rb to Zr.

Explanation:​
Alloys are homogeneous mixtures of metals with other metals or nonmetals.

Explanation​:
Strontium is in group 2 and so forms Sr2+ ions. The formula of the sulfide ion is S2− as sulfur is in group 16. For the positive and negative charges to cancel out, one of each ion is required.

Explanation:
Fluorine is the most electronegative element on the periodic table. Therefore, the greatest difference in electronegativity between $\mathrm{C}$ and $\mathrm{F}$ results in the most polar bond.

Explanation​:
Carboxylic acids have hydrogen bonding due to O−H bonds.

Explanation:​

Lithium is in group 1 and so forms Li+ ions. The formula of the nitride ion is N3−as nitrogen is in group 15. For the positive and negative charges to cancel out, three Li+ ions are needed for each N3− ion.