S2.2 The covalent model

The question asks for the correct description of covalent bond formation between two non-metal atoms.

A covalent bond forms when two non-metal atoms share a pair of electrons. The stability arises from the electrostatic attraction between the shared electron pair and the positively charged nuclei of both bonded atoms.

Complete electron transfer describes ionic bonding. A sea of delocalised electrons describes metallic bonding. Sharing of protons does not occur in any standard chemical bond.

The only option consistent with covalent bonding is: "A shared pair of electrons creates electrostatic attraction between the electrons and both nuclei."

We need to identify the correct description of covalent bond formation specifically between non-metal atoms.

"One atom completely transfers electrons to the other, creating oppositely charged ions" describes ionic bonding, not covalent bonding. Eliminated.

"Positive metal ions held together by a sea of delocalised electrons" describes metallic bonding. Non-metals are involved here, so this is eliminated.

"Protons are shared between two atomic nuclei" is scientifically incorrect — nuclei do not share protons in any type of chemical bond. Eliminated.

The remaining option — "A shared pair of electrons creates electrostatic attraction between the electrons and both nuclei" — correctly describes covalent bond formation.

$Si{O}_2$ is a giant covalent (network) structure — a continuous three-dimensional lattice of covalently bonded atoms, similar to diamond. Statement I is therefore correct.

In a giant covalent structure, all electrons are localised in covalent bonds. There are no free-moving charged particles, so $Si{O}_2$ is a poor conductor of electricity. Statement II is correct.

Silicon has four valence electrons and forms four bonds to oxygen atoms. Using VSEPR: 4 bonding pairs, 0 lone pairs → tetrahedral arrangement ($109.5°$). Statement III is correct.

All three statements are correct, so the answer is I, II, and III.

We need to determine which combination of three statements about $Si{O}_2$ is entirely correct.

The question states $Si{O}_2$ forms an extended three-dimensional lattice of covalent bonds — this is the definition of a giant covalent structure. Statement I is correct; options that exclude I ("II and III only") are eliminated.

Since all electrons are in localised covalent bonds with no free charge carriers, $Si{O}_2$ cannot conduct electricity. Statement II is correct; "I and III only" is eliminated.

Silicon forms 4 bonds to oxygen with no lone pairs, giving a tetrahedral geometry ($109.5°$). Statement III is correct; "I and II only" is eliminated.

All three statements are verified as correct, so the answer is I, II, and III.