Sorry, that is incorrect.

The only correct answer is: $CO_{2}$

• $H_{2}S$ is not correct because it has a similar shape to water.
• $SO_{2}$ is not correct because it has a trigonal planar shape, resulting from the lone pair and two groups of electrons in the two double bonds.
• $CH_{2} = CH_{2}$ is not correct because it is planar but not linear.

Sorry, that is incorrect.

The only correct answer is: hexagonal rings within a layer

• layers with each atom bonded to four others is not correct because, in the layers of graphite and graphene, the carbon atoms are bonded to three other carbon atoms and not four.
• hexagonal and pentagonal rings within a layer is not correct because graphite and graphene do not have pentagonal rings within their layers
• a three-dimensional structure is not correct because graphene is a two-dimensional structure consisting of a single layer

Sorry, that is incorrect.

The only correct answer is: $N^{3-}$

• $O^{2-}$ is not correct because oxygen has more protons to exert an attractive force to reduce the ionic radius
• $Na^{+}$ is not correct because sodium has more protons to exert an attractive force to reduce the ionic radius
• $Al^{3+}$ is not correct because aluminum has more protons to exert an attractive force to reduce the ionic radius

Sorry, that is incorrect.

The only correct answer is: 8 protons, 10 neutrons, and 8 electrons

• 9 protons, 9 neutrons, and 9 electrons is incorrect because oxygen atoms do not have this number of protons.
• 0 protons, 8 neutrons, and 10 electrons is incorrect because oxygen atoms do not have this number of protons.
• 18 protons, 18 neutrons, and 18 electrons is incorrect because oxygen atoms do not have this number of protons.

Sorry, that is incorrect.

The only correct answer is: 12 

• 5 is incorrect because this is the number of electrons in the 3p orbitals of the chlorine atom
• 6 is incorrect because this is only the number of electrons in the 3p orbitals of the chloride ion
• 18 is incorrect because this is the total number of electrons in the chloride ion, not just those in the p orbitals.

Sorry, that is incorrect.

The only correct answer is: $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p6$

• $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{2}$ is incorrect because two electrons have been removed instead of added to the sulfur atom.
• $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{4}$ is incorrect because this is the electronic configuration of the sulfur atom.
• $1s^{2} 2s^{2} 2p^{6} 3p^{6}$ is incorrect because this is the incorrect electronic configuration of the sulfur atom.

Sorry, that is incorrect.

The only correct answer is: 16 $\sigma$ bonds and 1 $\pi$ bonds

• 5 $\sigma$ bonds and 2 $\pi$ bonds is incorrect because there are 10 $\sigma$ C-H bonds, 6 C-C $\sigma$ bonds and 1 C-C $\pi$ bond
• 6 $\sigma$ bonds and 1 $\pi$ bonds is incorrect because there are 10 $\sigma$ C-H bonds, 6 C-C $\sigma$ bonds and 1 C-C $\pi$ bond
• 15 $\sigma$ bonds and 2 $\pi$ bonds is incorrect because there are 10 $\sigma$ C-H bonds, 6 C-C $\sigma$ bonds and 1 C-C $\pi$ bond

Sorry, that is incorrect.

The only correct answer is: no change

• orange to green is not correct because tertiary alcohol is not oxidized
• green to orange is not correct because this is an incorrect colour change for acidified dichromate
• purple to colourless is not correct because this is an incorrect colour change for these reagents.

Sorry, that is incorrect.

The only correct answer is: carbon particulates

• carbon particulates is incorrect because oxides of nitrogen are not black solids
• oxides of sulfur is incorrect because oxides of sulfur are not black
• unburnt hydrocarbons is incorrect because unburnt hydrocarbons are not black solids

Sorry, that is incorrect.

The only correct answer is: free radical substitution.

• electrophilic addition is not correct because electrophilic addition would be the reaction mechanism between alkenes, such as ethene, and chlorine but the substance in the question is the alkane, ethane
• electrophilic substitution is not correct because electrophilic substitution involves the reactions of aromatic systems but this question refers to the reaction of the alkane, ethane
• free radical addition is not correct because the mechanism of reaction between an alkane such as ethane and chlorine involves substitution and not addition