Alright — let's do this. Here are the answers to your chemistry questions: 1. Which particle determines the identity of an element? The number of protons in an atom's nucleus defines its atomic number, which uniquely identifies the element. C. Proton 2. Which trend is correct across Period 3 from Na to Cl? Across a period, the nuclear charge increases, pulling the electrons closer to the nucleus. This leads to a decrease in atomic radius and an increase in the attraction for electrons. C. Electronegativity increases 3. A gas occupies 11.2 dm³ at STP. How many moles is it? Step 1: Recall the molar volume of a gas at STP. At STP, 1 mole of any ideal gas occupies $22.4 \text{ dm}^3$. Step 2: Calculate the number of moles. $$ \text{Moles} = \frac{\text{Volume}}{\text{Molar volume at STP}} = \frac{11.2 \text{ dm}^3}{22.4 \text{ dm}^3/\text{mol}} = 0.5 \text{ mol} $$ B. 0.5 mol 4. An element has isotopes ⁶⁵X (25%) and ⁶⁷X (75%). What is its relative atomic mass? Step 1: Use the formula for relative atomic mass. $$ \text{Relative atomic mass} = \sum (\text{isotope mass} \times \text{fractional abundance}) $$ Step 2: Substitute the given values. $$ \text{Relative atomic mass} = (65 \times 0.25) + (67 \times 0.75) $$ $$ \text{Relative atomic mass} = 16.25 + 50.25 $$ $$ \text{Relative atomic mass} = 66.5 $$ C. 66.5 5. Which arrangement shows increasing atomic radius? Atomic radius increases down a group because new electron shells are added, making the atom larger. Li, Na, and K are all in Group 1, with K being below Na, and Na below Li. B. Li < Na < K 6. What is the empirical formula of a compound containing 40% C, 6.7% H, and 53.3% O? Step 1: Assume 100 g of the compound and convert percentages to mass. Mass of C = 40 g Mass of H = 6.7 g Mass of O = 53.3 g Step 2: Convert mass to moles using molar masses (C=12.01, H=1.008, O=16.00). $$ \text{Moles of C} = \frac{40 \text{ g}}{12.01 \text{ g/mol}} \approx 3.33 \text{ mol} $$ $$ \text{Moles of H} = \frac{6.7 \text{ g}}{1.008 \text{ g/mol}} \approx 6.65 \text{ mol} $$ $$ \text{Moles of O} = \frac{53.3 \text{ g}}{16.00 \text{ g/mol}} \approx 3.33 \text{ mol} $$ Step 3: Divide by the smallest number of moles to find the simplest whole-number ratio. $$ \text{C}: \frac{3.33}{3.33} = 1 $$ $$ \text{H}: \frac{6.65}{3.33} \approx 2 $$ $$ \text{O}: \frac{3.33}{3.33} = 1 $$ The empirical formula is $\text{CH}_2\text{O}$. A. CH₂O 7. Which of the following gases is a noble gas? Noble gases are elements in Group 18 of the periodic table, known for their inertness. Neon is one of them. B. Neon 8. Which ion has the same electron configuration as Argon? Argon (Ar) has 18 electrons. We need to find the ion with 18 electrons. $\text{K}^+$: Potassium (K) has 19 electrons. $\text{K}^+$ has $19 - 1 = 18$ electrons. $\text{Ca}^{2+}$: Calcium (Ca) has 20 electrons. $\text{Ca}^{2+}$ has $20 - 2 = 18$ electrons. $\text{Cl}^-$: Chlorine (Cl) has 17 electrons. $\text{Cl}^-$ has $17 + 1 = 18$ electrons. All three ions have 18 electrons, matching Argon's electron configuration. D. All of the above 9. Which bond is most polar? Bond polarity is determined by the difference in electronegativity between the bonded atoms. A larger difference results in a more polar bond. H–H, Cl–Cl, O–O are nonpolar because the atoms have identical electronegativity. H–Cl has an electronegativity difference (Cl: 3.16, H: 2.20) of $0.96$, making it a polar bond. B. H–Cl 10. In electrolysis of molten NaCl, what is formed at the cathode? In molten NaCl, the ions are $\text{Na}^+$ and $\text{Cl}^-$. The cathode is the negative electrode, where reduction (gain of electrons) occurs. Positive $\text{Na}^+$ ions are attracted to the cathode and gain electrons to form liquid sodium metal. $$ \text{Na}^+\text{(l)} + \text{e}^- \rightarrow \text{Na(l)} $$ B. Sodium metal Drop the next question.