has **5** unpaired electrons.\n\n* **Option C:** $\\text{Ni}^{2+}$\n Nickel ($\\text{Ni}$, $Z = 28$) is $\\text{[Ar]} \\, 4\\text{s}^2 3\\text{d}^8$. \n Removing two electrons gives $\\text{Ni}^{2+}$: $3\\text{d}^8$. Since $n > 5$, it has $10 - 8 =$ **2** unpaired electrons.\n\n* **Option D:** $\\text{Cu}^{2+}$\n Copper ($\\text{Cu}$, $Z = 29$) is $\\text{[Ar]} \\, 4\\text{s}^1 3\\text{d}^{10}$. \n Removing two electrons (one from $4\\text{s}$, one from $3\\text{d}$) gives $\\text{Cu}^{2+}$: $3\\text{d}^9$. Since $n > 5$, it has $10 - 9 =$ **1** unpaired electron.","highlights":[{"text":"ignored","index":1,"location":"eliminate"},{"text":"ignored","index":2,"location":"eliminate"},{"text":"ignored","index":3,"location":"eliminate"}]},{"type":"text","order":4,"title":"Confirm the correct option","content":"Now let's verify **Option A:** $\\text{Cr}^{2+}$\n\nChromium ($\\text{Cr}$, $Z = 24$) has an anomalous ground-state configuration of:\n\n$$\\text{[Ar]} \\, 4\\text{s}^1 3\\text{d}^5$$\n\nRemoving two electrons (one from $4\\text{s}$ and one from $3\\text{d}$) leaves $\\text{Cr}^{2+}$ with the configuration:\n\n$$\\text{[Ar]} \\, 3\\text{d}^4$$\n\nSince $n = 4 \\le 5$, $\\text{Cr}^{2+}$ has **4** unpaired electrons, which perfectly matches the 4 unpaired electrons of $\\text{Fe}^{2+}$.","highlights":[{"text":"correct","index":0,"location":"option"}]}]}],"generatedAt":"2026-06-11T10:08:24.448Z","modelVersion":"gemini-3.5-flash","explanationVersion":"v2"},"firstAttemptFullMarks":null},{"id":"f5dd450d-ad1d-46c7-a357-50610846d2d0","revisionId":"dd462e3c-e6b0-47ff-b825-85e18de4dd21","questionNumber":null,"questionType":"LA","specification":"A $^{137}\\text{Ba}^+$ ion is accelerated in a time-of-flight (TOF) mass spectrometer, gaining a kinetic energy of $2.12 \\times 10^{-16}\\text{ J}$. The ion then enters a field-free drift region (flight tube), taking $3.65 \\times 10^{-5}\\text{ s}$ to traverse its entire length and reach the detector.\n\n$$\\text{KE} = \\frac{1}{2}mv^2 \\quad \\text{where } m = \\text{mass (kg) and } v = \\text{speed (m s}^{-1}\\text{)}$$\n\nThe Avogadro constant, $L = 6.022 \\times 10^{23}\\text{ mol}^{-1}$","difficulty":6,"difficultyBand":"medium","totalMarks":5,"sourceTitle":null,"calculatorUse":null,"parts":[{"id":"7e1173d2-acb3-4e33-9ca3-0a50fed99998","label":null,"content":"Calculate the length of the flight tube in metres. Give your answer to the appropriate number of significant figures.","markscheme":"- Calculates the mass of a single $^{137}\\text{Ba}^+$ ion in kg:\n $$m = \\frac{137 \\times 10^{-3} \\text{ kg mol}^{-1}}{6.022 \\times 10^{23} \\text{ mol}^{-1}} = 2.275 \\times 10^{-25} \\text{ kg}$$ M1\n- Rearranges the kinetic energy equation to solve for velocity ($v$):\n $$v = \\sqrt{\\frac{2\\text{KE}}{m}}$$ M1\n- Substitutes the kinetic energy and calculated mass to find the velocity:\n $$v = \\sqrt{\\frac{2 \\times 2.12 \\times 10^{-16}}{2.275 \\times 10^{-25}}} = 4.317 \\times 10^4 \\text{ m s}^{-1}$$ M1\n- Calculates the length of the flight tube ($d$) using $d = v \\times t$:\n $$d = 4.317 \\times 10^4 \\times 3.65 \\times 10^{-5} = 1.5757 \\text{ m}$$ M1\n- Rounds the final answer to 3 significant figures (appropriate to match the given data):\n $$1.58 \\text{ m}$$ A1","marks":5,"order":0,"markingRubric":null,"explanation":{"methods":[{"id":"sequential_calculation","label":"Sequential Step-by-Step Calculation","steps":[{"type":"text","order":0,"title":"Calculate ion mass","content":"First, we calculate the mass ($m$) of a single $^{137}\\text{Ba}^+$ ion in kilograms.\n\nThe mass of one mole of $^{137}\\text{Ba}^+$ ions is approximately $137\\text{ g}$, which is $137 \\times 10^{-3}\\text{ kg mol}^{-1}$. To find the mass of a single ion, we divide this molar mass by Avogadro's constant ($L = 6.022 \\times 10^{23}\\text{ mol}^{-1}$):\n\n$$m = \\frac{137 \\times 10^{-3}\\text{ kg mol}^{-1}}{6.022 \\times 10^{23}\\text{ mol}^{-1}} = 2.2750 \\times 10^{-25}\\text{ kg}$$","highlights":[{"text":"The Avogadro

Atomic structure

Question 21

Atomic structure Questions

Atomic structure

Question 21

Atomic structure Questions