Read Quantum Numbers Before Counting Electrons by Spin
Quantum-number questions reward writing out the actual orbital diagram for the shell in question rather than trying to recall a shortcut rule.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Separate the nucleus from the electron shell
Nucleus identifies the element · outer shell sets valency
Protons and neutrons are in the nucleus; electrons occupy shells. Atomic number identifies the element, while valency concerns outer-shell electrons.
Counting electrons of a given spin across a shell
To count electrons with a specific spin across an entire shell, fill every subshell's orbitals following Hund's rule first, then count — don't assume every orbital automatically has one electron of each spin.
Isoelectronic species and mass number
Working backward from an isoelectronic clue (same electron count as a named ion or atom) plus a separate neutron-count clue pins down both atomic number and mass number — solve for atomic number from the isoelectronic condition first, then add the given neutron count.
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- ✓Fill orbitals following Hund's rule before counting electrons by spin — don't assume automatic pairing.
- ✓Isoelectronic species share electron count, not proton count — use that to find atomic number first.
- ✓Mass number = atomic number + neutron count, applied only after atomic number is pinned down.
Don't shortcut a quantum-number counting question — write out the actual subshell filling for the specific shell asked about, since assumed patterns break down for partially-filled subshells.
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