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Depending on the active compound's specific method of action, sunscreens are broadly classified into two groups: organic and inorganic. Organic sunscreens typically contain aromatic compounds that absorb ultraviolet (UV) radiation. The absorbed energy from the UV radiation causes these compounds to enter electronically excited states which then relax back to the ground state and harmlessly re-emit the absorbed radiation as heat. Alternatively, inorganic sunscreens contain metallic nanoparticles that serve to reflect, scatter, and absorb incident radiation. The mechanism for UV protection of each group is shown in Figure 1.
Figure 1 Method of action for organic and inorganic sunscreensThe effectiveness of sunscreens is traditionally evaluated based on their ability to prevent reddening of the skin, as indicated by their sun protection factor (SPF) . However, studies have shown that additional deleterious effects, such as the suppression of T cell-mediated immune responses, can occur at lower UV doses than erythema, indicating the need for new metrics to judge sunscreen efficacy. For this reason, researchers conducted a study to compare the protective ability of six sunscreens against photo-isomerization of the photoreceptor trans-urocanic acid (UCA) to its cis form, a process believed to be linked to UV-induced immunosuppression. Table 1 lists the sunscreens tested and their relevant characteristics.Table 1 Results from Tested Sunscreens
In vivo experiments were conducted on two groups of human volunteers. Participants in the acute exposure group received a single UV dose of 100 mJ/cm² whereas participants in the chronic exposure group received the same dosage once daily over four consecutive days. Each sunscreen was applied to a different spot on the volunteers' back prior to irradiation. A portion of skin without sunscreen was also irradiated to serve as a control. After irradiation, the sunscreens were removed and the skin was wiped with filter papers. Filters were washed with potassium hydroxide, and the aqueous solution was analyzed for UCA using high-pressure liquid chromatography. Figure 2 shows the mean percentages of cis-UCA formed upon irradiation for each exposure type and sunscreen tested.
Figure 2 Percentage of UCA in cis configuration upon UV exposure with various sunscreens
Adapted from Van der molen RG, Out-luiting C, Driller H, Claas FH, Koerten HK, Mommaas AM. Broad-spectrum sunscreens offer protection against urocanic acid photoisomerization by artificial ultraviolet radiation in human skin. J Invest Dermatol. 2000;115(3) :421-6.
-Which of the following is the correct Lewis dot structure for the common inorganic sunscreen compound titanium dioxide (TiO₂) given that titanium has four valence electrons?

Question

Passage
Depending on the active compound's specific method of action, sunscreens are broadly classified into two groups: organic and inorganic. Organic sunscreens typically contain aromatic compounds that absorb ultraviolet (UV) radiation. The absorbed energy from the UV radiation causes these compounds to enter electronically excited states which then relax back to the ground state and harmlessly re-emit the absorbed radiation as heat. Alternatively, inorganic sunscreens contain metallic nanoparticles that serve to reflect, scatter, and absorb incident radiation. The mechanism for UV protection of each group is shown in Figure 1.

Figure 1 Method of action for organic and inorganic sunscreensThe effectiveness of sunscreens is traditionally evaluated based on their ability to prevent reddening of the skin, as indicated by their sun protection factor (SPF) . However, studies have shown that additional deleterious effects, such as the suppression of T cell-mediated immune responses, can occur at lower UV doses than erythema, indicating the need for new metrics to judge sunscreen efficacy. For this reason, researchers conducted a study to compare the protective ability of six sunscreens against photo-isomerization of the photoreceptor trans-urocanic acid (UCA) to its cis form, a process believed to be linked to UV-induced immunosuppression. Table 1 lists the sunscreens tested and their relevant characteristics.Table 1 Results from Tested Sunscreens

In vivo experiments were conducted on two groups of human volunteers. Participants in the acute exposure group received a single UV dose of 100 mJ/cm² whereas participants in the chronic exposure group received the same dosage once daily over four consecutive days. Each sunscreen was applied to a different spot on the volunteers' back prior to irradiation. A portion of skin without sunscreen was also irradiated to serve as a control. After irradiation, the sunscreens were removed and the skin was wiped with filter papers. Filters were washed with potassium hydroxide, and the aqueous solution was analyzed for UCA using high-pressure liquid chromatography. Figure 2 shows the mean percentages of cis-UCA formed upon irradiation for each exposure type and sunscreen tested.

Figure 2 Percentage of UCA in cis configuration upon UV exposure with various sunscreens
Adapted from Van der molen RG, Out-luiting C, Driller H, Claas FH, Koerten HK, Mommaas AM. Broad-spectrum sunscreens offer protection against urocanic acid photoisomerization by artificial ultraviolet radiation in human skin. J Invest Dermatol. 2000;115(3) :421-6.
-Which of the following is the correct Lewis dot structure for the common inorganic sunscreen compound titanium dioxide (TiO₂) given that titanium has four valence electrons?

Quizplus · Accepted Answer

11ecba2d_1196_2628_a3bf_4fa91cf70850_MD0008_00 Lewis dot structures are used to represent the covalent bonding between the atoms of a molecule. Single bonds, which consist of two electrons, are represented as a line, and unshared electrons are represented as dots. Lewis dot structures are constructed using the following steps: Determine the number of valence electrons, which is the number of electrons in the atom's outer shell, for every atom in the molecule. Titanium has 4 and each oxygen has 6, for a total of 16 in titanium dioxide (TiO₂). Draw the structure of the compound with the least electronegative atom (titanium) positioned in the center, and connect the atoms with single bonds. Add dots (representing electrons) to atoms surrounding the central atom so they have complete octets (be aware of atoms with exceptions to the octet rule. Remember that each bond represents two electrons, and the total number of electrons per atom should be eight. Place any remaining electrons on the central atom. In the case of TiO₂, all 16 electrons are already accounted for. Oxygen is a divalent compound, meaning it should have two bonds. Therefore, two electrons from the lone pairs on each oxygen are used to form an additional bond with titanium on each side, making two double bonds and completing titanium's octet. Ensure the resulting Lewis structure is the most accurate representation by calculating the formal charge, which should be equal to the final charge of the compound (or zero for neutral molecules such as TiO₂). (Choice A) Titanium should have eight electrons for a full octet, but only four are shown. (Choice B) The oxygen atoms each have 10 electrons, which is not allowed according to the octet rule. (Choice C) There are 18 valence electrons shown, but TiO₂ should only have 16. In addition, TiO₂ is a neutral molecule, and therefore the formal charge should be zero. Educational objective: Lewis dot structures represent the distribution of bonds and valence electrons between atoms in a molecule, with the least electronegative atom in the center. A correct Lewis structure will show all valence electrons and complete octets for each atom, and yield the correct formal charge.

Passage Depending on the Active Compound's Specific Method of Action, Sunscreens