Diagnose an atom or ion electron configuration

A correct total can hide the wrong arrangement

Two proposals for neutral nitrogen both contain seven electrons. Each puts two in 1s and two in 2s. In the first, the three remaining electrons occupy three separate 2p orbitals. In the second, two share one 2p orbital and one occupies another, leaving the third empty. Counting seven electrons cannot distinguish them, but a ground-state claim must.

Try nitrogen with the early-pairing proposal. Read the electron-total check before the orbital checks. Then switch only the proposal to the ground-state reference. Capture both and name the check that changed. The activity supplies an original diagnostic drawing; it is not an image of an atom or a calculation of its exact wavefunction.

Keep identity and charge in the same ledger

Let Z be proton count, N be electron count, and q be signed charge in elementary-charge units. Then q = Z − N, or N = Z − q. A positive ion has fewer electrons than protons; a negative ion has more. Chemical electron removal does not change Z.

For Na+, Z = 11 and q = +1, so N = 11 − 1 = 10. For Mg2+, Z = 12 and q = +2, so N = 10 again. Both have the closed-shell arrangement 1s² 2s² 2p⁶. They are isoelectronic, but their different nuclei mean different species and different interactions. An equal electron count does not turn magnesium into neon.

The “one extra electron” proposal changes that ledger deliberately. A proposed eleven-electron sodium arrangement has implied charge zero, even if someone writes Na+ beside it. Detect the inconsistency from counts rather than accepting the label. A capture records the selected proposal and your explanation; it does not certify that your reasoning is correct.

Separate three questions about the arrangement

An orbital is a quantum state with a spatial distribution, not a circular path. An s subshell has one orbital; p has three; d has five. Each orbital accommodates at most two electrons with opposite spin states. This Pauli restriction is different from a rule about which orbital is occupied first. The up/down arrows label spin states, not particles spinning like tops.

Under the ground-state filling description used here, equal-energy orbitals are occupied singly with parallel spins before pairing. This is Hund’s rule. Nitrogen’s three 2p electrons therefore appear as three single arrows. Oxygen has four 2p electrons: one opposite-spin pair and two single arrows. A same-spin pair in one 1s box violates the occupancy restriction even when the total is correct; early 2p pairing is a different failure of the ground-state proposal.

Finally, check which subshells are populated. Moving an electron from a closed 2p subshell into 3s can preserve the total and the local occupancy rules while producing a higher-energy arrangement. Such a proposal is not equivalent to an impossible same-spin pair. “Not the reference ground state” does not always mean “no electronic state could have these populations.” The boxes do not supply excitation energies.

Connect the pattern to shells and the Aufbau description

The Aufbau description fills available lower-energy states before higher-energy states when constructing the selected ground-state configurations. Shells are principal energy-level groupings; subshells distinguish states within those groupings. For the representative atoms considered here, inner electrons form a core and outer-shell electrons supply the valence pattern. A periodic table organizes recurring valence patterns, not merely a list of increasing masses. The positive nucleus contains protons and neutrons; negatively charged electrons occupy states around it, and the charge ledger depends on protons minus electrons rather than neutron count.

Changed construction: Build a neutral atom with Z=13 using the supplied nonexceptional ordering 1s, 2s, 2p, 3s, 3p. Then remove three valence electrons without changing the nucleus. Name the core and compare the resulting ion with Mg²⁺.

Check the construction and identity

The neutral arrangement is 1s² 2s² 2p⁶ 3s² 3p¹: ten core electrons and three outer-shell valence electrons. Removing those three gives Al³⁺ with ten electrons, isoelectronic with Mg²⁺ but containing thirteen rather than twelve protons. Equal electronic occupancy does not imply equal radius or identical nuclear attraction. The stated ordering is not a universal rule for reversing ionization in transition metals.

OpenStax configurations and periodicity, checked 31 October 2026.

An ion is not made by reversing a mnemonic

For neutral iron the reference configuration is [Ar] 3d⁶ 4s². The [Ar] abbreviation accounts for eighteen electrons. Forming Fe2+ removes the two 4s electrons, leaving [Ar] 3d⁶: 18 + 6 = 24 electrons and 26 − 24 = +2 charge. Removing another electron gives Fe3+, [Ar] 3d⁵, with 23 electrons.

The tempting proposal [Ar] 3d⁴ 4s² also totals 24, but it is not the reference ground state of Fe2+. Subshell energies and occupations depend on the species; a simple order used to construct neutral configurations is not a universal list to reverse during ionization. Compare the two iron ions and use both the charge ledger and reference populations in your explanation.

These selected configurations agree with the NIST neutral-atom reference and NIST iron ionization-stage reference, inspected 28 October 2026. The selected cases avoid exceptional neutral Aufbau configurations. Assigning individual electron quantum numbers and calculating term symbols are outside this activity.

Transfer the checks

A new proposal for Mg2+ is 1s² 2s² 2p⁵ 3s¹, with locally allowed spin boxes. Before opening feedback, calculate its total and implied charge. Decide whether a passing count proves the ground state, and state the repair.

Check the changed case

The total is 2 + 2 + 5 + 1 = 10, giving charge 12 − 10 = +2. The ground-state reference has 2p⁶ and no 3s electron. Moving the proposed 3s electron into the unfilled 2p position repairs the populations. The count alone never established the ground state.

Return to high-school electron notation or continue to ionization evidence. Your next explanation should connect a configuration to an actual energy comparison without pretending that the boxes calculated that energy.