Transition Elements

Writing electronic configurations

Sublevel by sublevel, and the short form with [Ne] or [Ar]

1. Electronic configurations and oxidation states

Writing electronic configurations

2. A long line of electrons

Calcium has 20 electrons, so writing them all takes a long line. Is there a shortcut?
gathered₂₀Ca:\ 1s²\ 2s²\ 2p⁶\ 3s²\ 3p⁶ \\ 4s²gathered

Pick the right answer

Predict: which part of this line is exactly the configuration of argon, ₁₈Ar?
  • A. 1s²\ 2s²\ 2p⁶
  • B. 1s²\ 2s²\ 2p⁶\ 3s²\ 3p⁶
  • C. 4s²
الشرح
1s²\ 2s²\ 2p⁶\ 3s²\ 3p⁶ holds 2 + 2 + 6 + 2 + 6 = 18 electrons, exactly the configuration of argon. So calcium is argon's configuration plus 4s².

3. The rules for writing

Fill the sublevels in the filling order you studied, starting with 1s. Fill one sublevel before you start the next. Chapter 1 shows two exceptions, Cr and Cu.
  1. 1s (2) → 2s (2)
  2. 2s (2) → 2p (6)
  3. 2p (6) → 3s (2)
  4. 3s (2) → 3p (6)
  5. 3p (6) → 4s (2)
  6. 4s (2) → 3d (10)
Filling order, with the maximum number of electrons in each sublevel: s holds 2, p holds 6, d holds 10

4. Worked example: S (Z = 16)

Sulphur has 16 electrons. Let's place them, step by step.
Step 1
1s²\ 2s²\ 2p⁶ uses 10 electrons. Why? Here, a sublevel is filled to its maximum before the next one starts.
Step 2
6 electrons are left. 3s comes next and takes its 2. Then 3p takes the remaining 4.
₁₆S:\ 1s²\ 2s²\ 2p⁶\ 3s²\ 3p⁴
Check: 2 + 2 + 6 + 2 + 4 = 16
Step 3: the short form

noble-gas core

The configuration of the noble gas just before the element, written as its symbol in square brackets. Example: [Ne] = 1s²\ 2s²\ 2p⁶.
₁₆S:\ [Ne]\ 3s²\ 3p⁴
Neon (Z = 10) is the noble gas just before sulphur

5. Your turn: K (Z = 19)

Potassium has 19 electrons. The first part is done for you.

Fill in the blanks

Full: 1s²\ 2s²\ 2p⁶\ 3s²\ 3p⁶ 4s_____ (type the exponent). In the short form, the noble-gas core is [_____].
الشرح
₁₉K: 1s²\ 2s²\ 2p⁶\ 3s²\ 3p⁶\ 4s¹ = [Ar]\ 4s¹. Check: 18 + 1 = 19.
Need a hint?
  1. Add up the exponents already written.
  2. Subtract that from 19 to see what is left for 4s.
  3. For the core: which noble gas has that same number of electrons?

6. On your own

Pick the right answer

Which is the short-form configuration of phosphorus, ₁₅P?
  • A. [Ne]\ 3s²\ 3p³
  • B. [Ar]\ 3s²\ 3p³
  • C. [Ne]\ 3p⁵
  • D. [Ne]\ 3s²\ 3p⁵
الشرح
Phosphorus has 15 electrons, and the noble gas just before it is neon (10). So the short form is [Ne]\ 3s²\ 3p³: 10 + 2 + 3 = 15, with 3s filled before 3p.

Fill in the blanks

Short form of chlorine, ₁₇Cl: [_____] 3s² 3p_____ (type the core symbol, then the exponent).
الشرح
₁₇Cl: [Ne]\ 3s²\ 3p⁵. Check: 10 + 2 + 5 = 17.

7. Summary

  • Fill the sublevels in the filling order: 1s\ 2s\ 2p\ 3s\ 3p\ 4s\ 3d … s holds 2, p holds 6, d holds 10.
  • Check: the exponents add up to Z.
  • Short form: [the noble gas just before the element] + the electrons after it, e.g. ₂₀Ca: [Ar]\ 4s².
ElementFull configurationShort form
₁₆S1s²\ 2s²\ 2p⁶\ 3s²\ 3p⁴[Ne]\ 3s²\ 3p⁴
₁₉K1s²\ 2s²\ 2p⁶\ 3s²\ 3p⁶\ 4s¹[Ar]\ 4s¹
₂₀Ca1s²\ 2s²\ 2p⁶\ 3s²\ 3p⁶\ 4s²[Ar]\ 4s²
The examples of this lesson: full configuration and short form
Can you explain how to check in a few seconds that [Ne]\ 3s²\ 3p⁴ is sulphur?

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