Getting Through the Nuclear Charge Problems
Most teachers hand out this sheet around mid-September, right after they've spent two weeks on subatomic particles and before the periodic table trends hit. The key is straightforward if you know what traps are built into it. The first section asks students to fill in protons, neutrons, and electrons for various isotopes given in standard notation. That part is mechanical. The second section is where people start losing points because the questions don't lay everything out neatly. I remember grading a batch where nearly half the class put the mass number wrong on question 7. The isotope was written as carbon-14, not in A/Z notation. Students kept using 6 as the mass number because that's the atomic number they had memorized. They were subtracting 6 from 6 instead of 6 from 14. I stopped trying to explain it in class and just made them write out the full notation — 14 over 6 C — before answering anything. It took twenty minutes but cut the error rate down to maybe ten percent on the next problem set.
Basic Atomic Structure Worksheet Key 2
Below is the working key. If you're a student checking your work, don't just look at the answers. The valuable part is the reasoning, especially on the electron configuration questions in section three. Section 1: Isotope Identification 1. Protons: 6, Neutrons: 8, Electrons: 6 — Carbon-14
2. Protons: 11, Neutrons: 12, Electrons: 11 — Sodium-23 3. Protons: 17, Neutrons: 18, Electrons: 17 — Chlorine-35 4. Protons: 20, Neutrons: 22, Electrons: 20 — Calcium-42
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5. Protons: 79, Neutrons: 118, Electrons: 79 — Gold-197 Section 2: Mass Number and Atomic Number 6. Atomic number 9, mass number 19 — Fluorine-19, neutrons = 10
7. Atomic number 6, mass number 14 — Carbon-14, neutrons = 8 8. Atomic number 26, mass number 56 — Iron-56, neutrons = 30 9. An atom with 47 protons and 61 neutrons is silver-108. The mass number is 108, not 61. This is the one most people miss because the neutron count is larger than the proton count and it looks wrong intuitively.
10. Potassium-40 has 19 protons and 21 neutrons. The neutron count being higher than the proton count is normal for heavier elements but threw a lot of students off on this one. Section 3: Electron Configuration 11. Sodium (Z=11): 1s² 2s² 2p 3s¹
12. Oxygen (Z=8): 1s² 2s² 2p 13. Aluminum (Z=13): 1s² 2s² 2p 3s² 3p¹ 14. Argon (Z=18): 1s² 2s² 2p 3s² 3p
15. Iron (Z=26): 1s² 2s² 2p 3s² 3p 4s² 3d. Note the 4s fills before 3d. This ordering matters and teachers deduct points when students write 3d 4s² even though the total electron count is correct. Section 4: Ion Formation 16. Mg² loses 2 electrons. Configuration becomes 1s² 2s² 2p, same as neon.
17. O² gains 2 electrons. Configuration becomes 1s² 2s² 2p, also isoelectronic with neon. 18. Al³ loses 3 electrons. 1s² 2s² 2p. Again, neon configuration. 19. Cl gains 1 electron. 1s² 2s² 2p 3s² 3p, isoelectronic with argon.
20. K loses 1 electron from the 4s orbital. 1s² 2s² 2p 3s² 3p. One thing that comes up repeatedly and isn't obvious from a glance at the key: questions 19 and 20 trip people up because of which electron gets removed or added. For cations, you always remove from the highest principal quantum number first, which means the 4s electron leaves before any 3d electrons. Students who memorized the Aufbau order as a simple filling sequence without understanding the removal order will get the ion configurations wrong even if they got the neutral atom right. The worksheet also glosses over one edge case that shows up in harder versions. Some isotopes listed use non-standard notation where the element symbol is given without the superscript mass number, and students have to infer the isotope from context or a data table. I've seen answer keys incorrectly list the most abundant isotope when the question actually specified a rarer one. Always double-check the question text against the data provided rather than defaulting to the periodic table average mass.
There isn't a single reliable download link I can vouch for. These worksheets circulate through teacher networks and sites like Lesson Planet or Teachers Pay Teachers under slightly different titles each year. The content stays roughly the same because the curriculum hasn't changed. If you need the original, search for the document through your school's learning management system or ask a colleague who taught it last year. The key I posted above matches the standard version that's been in circulation since around 2018. The main limitation of this worksheet is that it treats all isotopes as if they exist in stable form. Carbon-14, potassium-40, and a few others are radioactive and don't occur in measurable quantities in a normal lab setting. The questions don't address this, and it's fine for an introductory exercise, but if a student asks about half-lives or decay modes, the worksheet won't help. You'd need a separate nuclear chemistry worksheet for that. Another practical note: the answer key I've outlined here covers the standard 20-question version. Some schools use an extended version with 25 to 30 questions that adds Bohr model diagrams and valence electron counting. If your copy goes beyond question 20, the principles are the same but the diagrams require checking that the energy levels are labeled starting from n=1 at the nucleus going outward. I've seen students draw the shells in reverse order and lose points even though the electron counts were correct.
If you're self-studying or a teacher looking for practice material, the biggest waste of time is rushing through section 1 and section 2 without writing out the full A/Z notation for every isotope. That habit causes errors in section 4 when ions are involved. Taking thirty seconds per problem to write out the complete notation before moving on saves more time than it costs.
