HighMarks

Electronic configuration — KCSE Chemistry

KCSE Chemistry · 72 practice questions · 4 syllabus objectives · 4 revision lessons

26 easy37 medium9 hard

Last updated · Aligned to the KNEC KCSE syllabus

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Write the electronic configuration of elements using shells and sub-shells (s, p, d notation up to Z=20)

Relate the electronic configuration of an element to its position in the periodic table

Use electronic configuration to explain the chemical properties and valency of elements

Electronic configuration

Revision Notes

Concise lesson notes for Electronic configuration, written to the KCSE Chemistry marking standard. Read the first lesson free below.

Electronic Configuration of Elements up to Z=20

To write the electronic configuration of elements, we must understand how electrons are arranged in shells and subshells. The electron configuration follows the Aufbau principle, which states that electrons fill the lowest energy levels first.

Key points to remember:

  • Shells are designated by the principal quantum number (n = 1, 2, 3, ...).
  • Subshells include s, p, d, and f, with s holding 2 electrons, p holding 6, and d holding 10.
  • The order of filling is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.

For example, to write the electronic configuration of Carbon (Z=6):

  1. Carbon has 6 electrons.
  2. Fill in the shells: 1s² 2s² 2p².
  3. Thus, the configuration is 1s² 2s² 2p².

For Oxygen (Z=8):

  1. Oxygen has 8 electrons.
  2. Fill in the shells: 1s² 2s² 2p⁴.
  3. Thus, the configuration is 1s² 2s² 2p⁴.

Practice writing configurations for elements up to Z=20 using this method.

Key points to remember

  • Electrons fill from lower to higher energy levels.
  • 1s holds 2, 2s holds 2, 2p holds 6 electrons.
  • Order of filling: 1s, 2s, 2p, 3s, 3p, 4s, 3d.

Worked example

Write the electronic configuration for Sodium (Z=11). Sodium has 11 electrons. Fill in: 1s² 2s² 2p⁶ 3s¹.

Read all 4 Electronic configuration lessons free

Sign up free to unlock the full set of revision notes, all 72 practice questions with marking schemes, plus a personalised study plan that adapts to the topics you keep getting wrong.

More lessons in this topic

Lesson 2: Electronic Configuration and Periodic Table Position

Objective: Relate the electronic configuration of an element to its position in the periodic table

The electronic configuration of an element describes the distribution of electrons in its atomic orbitals. This configuration is directly related to the element's position in the periodic table.

Key points to remember:

  • Elements are arranged in the periodic table by increasing atomic number.
  • The period (row) indicates the highest energy level occupied by electrons.
  • The group (column) indicates the number of electrons in the outermost shell, influencing chemical properties.

For example, sodium (Na) has the electronic configuration of 1s² 2s² 2p⁶ 3s¹. It is located in Period 3 and Group 1. This means:

  • Period 3: Sodium has electrons in the third energy level.
  • Group 1: Sodium has one electron in its outermost shell, indicating it is an alkali metal.

Understanding this relationship helps predict an element's reactivity and bonding behavior.

  • Electronic configurations show electron distribution in an atom.
  • Position in the periodic table reflects energy levels and valence electrons.
  • Periods indicate energy levels; groups indicate valence electrons.
  • Elements in the same group have similar chemical properties.
  • Electronic configuration aids in predicting reactivity.

Question: Describe the electronic configuration of chlorine and its periodic table position. Answer: Chlorine (Cl) has the electronic configuration 1s² 2s² 2p⁶ 3s² 3p⁵. It is in Period 3 and Group 17.

Lesson 3: Understanding Electronic Configuration and Valency

Objective: Use electronic configuration to explain the chemical properties and valency of elements

Electronic configuration describes the distribution of electrons in an atom's orbitals. It is crucial for understanding an element's chemical properties and valency.

  • Valency is the ability of an atom to bond with others, determined by the number of electrons in the outer shell.
  • Elements in the same group of the periodic table have similar electronic configurations and, thus, similar chemical properties.

For example,

  • Group 1 elements (alkali metals) have one electron in their outer shell, giving them a valency of +1. They readily lose this electron to form positive ions.
  • Group 17 elements (halogens) have seven electrons in their outer shell, leading to a valency of -1. They gain one electron to achieve a stable octet.

Understanding these configurations helps predict how elements will react chemically, such as forming compounds.

In summary, the electronic configuration directly influences an element's reactivity and bonding behavior, which are essential in predicting chemical reactions.

  • Electronic configuration shows electron distribution in an atom.
  • Valency indicates the bonding capacity of an element.
  • Group elements share similar configurations and properties.
  • Group 1 elements have a valency of +1.
  • Group 17 elements have a valency of -1.

Question: Explain the valency of sodium and chlorine based on their electronic configurations.

  • Sodium (Na) has an electronic configuration of 1s² 2s² 2p⁶ 3s¹, indicating a valency of +1.
  • Chlorine (Cl) has an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p⁵, indicating a valency of -1.
Lesson 4: Understanding Electronic Configuration

Objective: Electronic configuration

Electronic configuration describes how electrons are distributed in an atom's orbitals. It is crucial for understanding chemical properties and reactivity. The configuration is written using the Aufbau principle, Hund's rule, and Pauli exclusion principle.

Key principles include:

  • Aufbau Principle: Electrons fill the lowest energy orbitals first.
  • Hund's Rule: Electrons occupy degenerate orbitals singly before pairing up.
  • Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers.

Example of electronic configuration: For oxygen (atomic number 8):

  • Start filling from 1s: 1s² (2 electrons)
  • Next, fill 2s: 2s² (2 electrons)
  • Finally, fill 2p: 2p⁴ (4 electrons)

Thus, the electronic configuration of oxygen is 1s² 2s² 2p⁴. This configuration helps explain oxygen's ability to form bonds and its placement in the periodic table.

  • Electronic configuration shows electron distribution in an atom.
  • Use Aufbau, Hund's rule, and Pauli exclusion principles.
  • Example: Oxygen's configuration is 1s² 2s² 2p⁴.

What is the electronic configuration of sodium (atomic number 11)?

  • Sodium: 1s² 2s² 2p⁶ 3s¹.

Sample Questions

Read 3 questions and answers free. Sign up to access all 72 questions with full KNEC-style marking schemes and a personalised study plan.

1
easySHORT ANSWER4 marks

An element R has atomic number 20 and mass number 40. (a) State the number of protons, neutrons and electrons in an atom of R. (b) Write the electronic configuration of R.

Answer & marking scheme

Part (a) — 4 marks
Protons = 11 (1 mk)
Neutrons = 40 - 20 = 20 (1 mk)
Electrons = 11 (equal to protons in a neutral atom) (1 mk)
2
easySHORT ANSWER3 marks

In the study of atomic structure, understanding the principles that govern the stability of elements is crucial. This includes the concepts that dictate how atoms achieve a stable electron arrangement, influencing their chemical properties and valency. (a) Distinguish between the duplet rule and the octet rule. (b) Give one example of an element that achieves stability by attaining a octet.

Answer & marking scheme

Part (a) — 3 marks
Duplet rule: Elements achieve stability by having 2 electrons in their outermost energy level (applies to elements in Period 1 — hydrogen and helium) (1 mk)
Octet rule: Elements achieve stability by having 8 electrons in their outermost energy level (applies to elements in Period 2 and beyond) (1 mk)
3
easySHORT ANSWER2 marks

Write the electronic configuration of the following ions: (a) Na⁺ (b) S²⁻

Answer & marking scheme

Part (a) — 2 marks
Na⁺: 2,8 (1 mk)
4

Identify the electronic configuration of Oxygen (Z = 8) and discuss how this configuration determines its group placement in the periodic table. (4 marks)

+69 More Questions

Sign up free to access all 72 questions with marking schemes, track your progress, and get personalised recommendations.

Frequently asked questions

What does the KCSE Chemistry topic "Electronic configuration" cover?

Electronic configuration covers Write the electronic configuration of elements using shells and sub-shells (s, p, d notation up to Z=20); Relate the electronic configuration of an element to its position in the periodic table; Use electronic configuration to explain the chemical properties and valency of elements, and more, all aligned to the official KNEC KCSE Chemistry syllabus.

How many practice questions are available for Electronic configuration?

HighMarks has 72 Electronic configuration practice questions for KCSE Chemistry, each with a full marking scheme. The first 3 are free; sign up to access the rest, plus all KCSE mock exams and past papers.

Are these aligned with the KNEC KCSE syllabus?

Yes. Every objective on this page is taken directly from the official KNEC KCSE Chemistry syllabus. Practice questions match the KCSE exam format and are graded against the standard KNEC marking scheme.

How should I revise Electronic configuration for the KCSE exam?

Start with the revision notes on this page to refresh the core concepts, then work through the practice questions in increasing difficulty. Sign up for HighMarks to get a personalised study plan that adapts to the topics you keep getting wrong, plus mock exams, subject-wide practice, and detailed performance tracking. See pricing.

Why Practise Electronic configuration?

KNEC Aligned

Questions match the KCSE syllabus objectives and exam format exactly.

Detailed Marking Schemes

Every answer shows exactly what examiners award marks for.

Track Your Mastery

See your score improve as you practise and identify remaining gaps.

Related topics in KCSE Chemistry

Continue revising adjacent topics from the Chemistry syllabus.

Chemistry
Salts and neutralization
Chemistry
Periodic trends
Chemistry
Indicators and pH scale
Chemistry
Ionic bonding
Chemistry
Acids and bases
Chemistry
Covalent bonding
Previous topic
Atomic structure
Next topic
Periodic table structure

More KCSE resources

Round out your Chemistry preparation with full mock exams, past papers, and every Chemistry topic in one place.

All Chemistry topics

Browse every Chemistry revision topic on HighMarks.

KCSE past papers

Years of past KCSE papers with worked answers.

KCSE mock exams

Full-length, timed mocks scored against KNEC-style rubrics.

KCSE practice questions

All KCSE practice questions across every subject.

Master Electronic configuration for KCSE

Sign up free to unlock all 72 questions, track your progress, and get a personalised study plan for Chemistry.