ARCH-C-01: Subatomic Isoelectronic Ions, Isotopic A_r Weighted Means & Comma Electron Configs (Z ≤ 20)

Full 15-Q Timed Drill

Step 1 of 6 • The Core Idea

ARCH-C-01: Why the 125s Classroom Method Fails Under the 90s Clock

Standard Classroom Method (Too Slow)~125s (Blows 90s Clock)

Count protons from the atomic number

Z = 7

, subtract

Z

from the mass number

A = 15

to get

15 - 7 = 8

neutrons, and add

3

electrons for the

3-

charge to get

7 + 3 = 10

electrons, filling shells as

2,8

(~95s).

The Fast ShortcutTarget: 30s (Saves 95s)

Line 1 (Charge & Shell Prune):

\text{N}^{3-}

has

7 + 3 = 10\text{ e}^- \rightarrow 2,8

(isoelectronic with

\text{Ne}

).
Line 2 (Neutron Check):

n = 15 - 7 = 8 \rightarrow

7 protons | 8 neutrons | 2,8 (25s).

Where Students Lose Time: In 25 past UAT-UK questions (11.11% of Chemistry), candidates lose 95s by inverting the negative charge of an electron when adjusting ion electron counts. Instead, apply C1 with Maths 1 skill M6.3.

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