Electron Configuration — Practice · Grade 11 Science

Chapter: Atoms, Electrons & Periodicity

What this practice covers

By the end of Electron Configuration you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: energy level (n), sublevel, orbital, electron configuration, Aufbau principle, Pauli exclusion principle, Hund's rule, valence electrons.

Problem 1

What is the maximum number of electrons a single orbital can hold?
  1. 8
  2. 1
  3. 2
  4. 6
Answer and explanation

2

By the Pauli exclusion principle, an orbital holds at most two electrons, and they must have opposite spins.

Problem 2

Which sublevel can hold a maximum of 10 electrons?
  1. f
  2. s
  3. p
  4. d
Answer and explanation

d

The d sublevel has 5 orbitals × 2 electrons = 10 electrons.

Problem 3

Using the rule 2n², what is the maximum number of electrons the n = 3 energy level can hold?
Answer and explanation

18

2n² = 2 × 3² = 2 × 9 = 18 electrons.

Problem 4

Which is the correct ground-state electron configuration of phosphorus (Z = 15)?
  1. 1 s² 2 s² 2p⁶ 3 s² 3p³
  2. 1 s² 2 s² 2p⁶ 3 s² 3p⁵
  3. 1 s² 2 s² 2p⁶ 3 s² 3d³
  4. 1 s² 2 s² 2p⁸ 3 s² 3p³
Answer and explanation

1 s² 2 s² 2p⁶ 3 s² 3p³

Filling in Aufbau order and stopping at 15 electrons gives 1 s² 2 s² 2p⁶ 3 s² 3p³ (2+2+6+2+3 = 15).

Problem 5

Explain why the 4 s sublevel is filled before 3d even though 3d has a lower principal quantum number.
Answer and explanation

Electrons fill in order of increasing energy, not simply increasing n. The 4 s sublevel happens to be slightly lower in energy than 3d, so by the Aufbau principle electrons occupy 4 s first.

Problem 6

Bromine (Z = 35) has the configuration [Ar] 4 s² 3d¹⁰ 4p⁵. How many valence electrons does it have?
Answer and explanation

7

Valence electrons are those in the highest energy level (n = 4): 4 s² + 4p⁵ = 7. The inner 3d¹⁰ electrons are core, not valence.

Problem 7

Give the ground-state configuration of neutral neon, Z=10. Show the reasoning, not just the conclusion.
Hints

Check electron total and distinguish an orbital from a shell.

Answer and explanation

1s² 2s² 2p⁶; exponents total 10.

Problem 8

What changes when magnesium forms Mg²⁺ in the simple configuration model?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

It loses the two outer 3s electrons, giving [Ne]; the nucleus remains magnesium.

Problem 9

Outer-electron patterns connect configuration with chemical behaviorOuter-electron patterns connect configuration with chemical behaviorMain-group speciesOuter configurationCommon ionNa3s¹Na⁺Mg3s²Mg²⁺Cl3s²3p⁵Cl⁻Valence patterns help predict chemistry, not every oxidation state.Stable compounds depend on the full energy and structure.
Original selected main-group configurations and common ions; not a table of all possible oxidation states.
Interpret the supplied visual. Why is an octet drawing alone insufficient to show that isolated sodium spontaneously loses an electron?
Answer and explanation

Removing an electron from an isolated atom requires energy. Other reaction steps and interactions must be included in the full favorable or unfavorable account.

Valence electrons are the electrons most involved in a chemical bonding model. Main-group outer configurations help explain recurring groups and common ion charges: sodium often loses its 3s electron and chlorine often gains one electron into its 3p subshell. The octet rule is a useful main-group pattern, not a cause or universal command that atoms want eight electrons. Whether a reaction occurs depends on the complete energy balance, interaction partners and conditions, including lattice or solution stabilization.

Removing an electron from an isolated atom requires energy. Other reaction steps and interactions must be included in the full favorable or unfavorable account.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 10

A neutral atom has Z=16. Write its ground-state configuration and electron total for its common 2− ion. Explain why gaining electrons does not change the element.
Answer and explanation

Neutral sulfur: 1s²2s²2p⁶3s²3p⁴, totaling 16 electrons. S²⁻ has 18 electrons, with 3p⁶. Its 16 protons retain sulfur identity.

Neutral sulfur: 1s²2s²2p⁶3s²3p⁴, totaling 16 electrons. S²⁻ has 18 electrons, with 3p⁶. Its 16 protons retain sulfur identity.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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