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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.
-Zinc, titanium, iron, and aluminum are all ingredients found in inorganic sunscreens. Which of the following choices correctly lists the atoms in order of increasing atomic radius?

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.
-Zinc, titanium, iron, and aluminum are all ingredients found in inorganic sunscreens. Which of the following choices correctly lists the atoms in order of increasing atomic radius?

Quizplus · Accepted Answer

11ecba2d_1198_bd41_a3bf_7fb35d30f915_MD0008_00 Atomic radius is defined as the distance from the nucleus of the atom to the outermost electron. The periodic table has the following trends for the atomic radius of atoms: Within a group (column), the atomic radius increases from top to bottom because each successive atom in the group has an additional electron shell. The outer electrons are shielded from the pull of the protons by the inner electrons. The electrons experience a decreased attractive force toward the nucleus (effective nuclear charge, Z_eff), so they orbit farther from the nucleus. Within each period (row), the atomic radius gradually increases from right to left. With each step to the right, an electron and a proton are added. The charge in the nucleus increases with each additional proton, causing a gradual drawing in of the electron cloud. Because the number of electron shells is constant within a period, each new electron is added to the same shell and cannot effectively shield other electrons from the nucleus. Because the addition of shells has a strong effect on shielding, changes in atomic radius down a group are more pronounced than for those across a period. Aluminum has the smallest atomic radius because it is located in the highest row and farthest to the right in the periodic table. Zinc, iron, and titanium are all in the same period, so the trend from right to left is Zn < Fe < Ti. (Choice B) Aluminum has the smallest atomic radius rather than the largest. (Choice C) Both trends have been reversed. Atomic radius decreases to the right across a period and increases down a group. (Choice D) Although aluminum has the smallest atomic radius, the trend for the remaining atoms is reversed. Educational objective: Atomic radii are governed by the distribution of electrons surrounding the atom's nucleus. They quickly increase from top to bottom in a group and gradually increase from right to left across a period on the periodic table.

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