What is the difference between an orbit and an orbital?

An orbit is a fixed circular path around the nucleus, as described by the Bohr model. An orbital is a three-dimensional region around the nucleus where there is a high probability (usually >90%) of finding an electron. Orbitals have specific shapes (s, p, d, f) and can hold a maximum of two electrons with opposite spins.

Why does chromium have an electron configuration of [Ar] 4s1 3d5 instead of [Ar] 4s2 3d4?

Chromium has an anomalous configuration because a half-filled d subshell (3d5) provides extra stability due to exchange energy and reduced electron-electron repulsion. The energy cost of promoting one electron from the 4s to the 3d is outweighed by the stability gained from the half-filled d subshell.

How do I determine the number of unpaired electrons in an atom?

Write the electron configuration and then draw the orbital diagram for the outermost subshells. Apply Hund's rule: each orbital in a subshell gets one electron before any pairing. Count the number of orbitals with a single electron. For example, oxygen (1s2 2s2 2p4) has two unpaired electrons in the 2p subshell.

What is the significance of the principal quantum number n?

The principal quantum number n indicates the main energy level (shell) of an electron. It determines the size and energy of the orbital; higher n means larger orbitals and higher energy. n also relates to the distance from the nucleus: electrons with higher n are generally farther away and less tightly bound.

Why does ionisation energy increase across a period?

Across a period, the nuclear charge (number of protons) increases, while electrons are added to the same principal energy level. This increases the effective nuclear charge experienced by the outer electrons, pulling them closer to the nucleus and making them harder to remove. Shielding by inner electrons remains roughly constant, so the attraction increases.

How do I write electron configurations for ions?

For cations (positive ions), remove electrons from the outermost shell (highest n value) first. For example, Fe ([Ar] 3d6 4s2) becomes Fe2+ by removing the two 4s electrons: [Ar] 3d6. For anions (negative ions), add electrons to the outermost shell following the Aufbau principle. For example, O ([He] 2s2 2p4) becomes O2- by adding two electrons to the 2p subshell: [He] 2s2 2p6.

IBO Level 3 Alternative Academic Qualification HL in Chemistry (Extended Certificate)

INTERNATIONAL-BACCALAUREATE-ORGANISATION

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The Core Content of the IB HL Chemistry course establishes fundamental chemical principles and their interconnections, spanning stoichiometry, atomic structure, bonding, energetics, kinetics, equilibrium, acids and bases, redox, organic chemistry, and measurement. Students develop the ability to conceptualise, analyse, and apply these concepts through theoretical problem-solving, practical investigations, and data evaluation, preparing them for the Internal Assessment and terminal examinations.

Learning Outcomes

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Key Skills

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Assessment criteria

IBO Level 3 Alternative Academic Qualification HL in Chemistry (Extended Certificate) - Core Content

Topic Overview

This topic covers the fundamental principles of atomic structure, including the arrangement of electrons in atoms, the concept of orbitals, and the quantum mechanical model. You will explore how electron configurations determine an element's position in the periodic table and its chemical properties. Understanding atomic structure is essential for explaining trends in ionisation energy, electronegativity, and bonding behaviour across the periodic table.

The quantum mechanical model replaces the outdated Bohr model, introducing orbitals as regions of high electron probability. Key concepts include the four quantum numbers (n, l, m_l, m_s), the Aufbau principle, Hund's rule, and the Pauli exclusion principle. You will learn to write electron configurations for atoms and ions, including exceptions for transition metals and lanthanides/actinides.

Mastering atomic structure is crucial for success in HL Chemistry because it underpins topics like periodicity, chemical bonding, and spectroscopy. It also connects to real-world applications such as understanding atomic spectra used in analytical chemistry and the design of new materials. A solid grasp of electron configurations will help you predict reactivity and bonding patterns with confidence.

Key Concepts

Core ideas you must understand for this topic

→Quantum numbers: n (principal), l (azimuthal, 0 to n-1), m_l (magnetic, -l to +l), m_s (spin, +1/2 or -1/2). These define an electron's unique state in an atom.
→Electron configuration: Follow the Aufbau principle (fill lowest energy orbitals first), Hund's rule (each orbital in a subshell gets one electron before pairing), and Pauli exclusion principle (no two electrons share the same set of four quantum numbers).
→Orbital shapes and energies: s orbitals are spherical, p orbitals are dumbbell-shaped, d orbitals have more complex shapes. Energy ordering: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p (note the 4s fills before 3d).
→Exceptions to the Aufbau principle: Chromium (Cr) and copper (Cu) have configurations [Ar] 4s1 3d5 and [Ar] 4s1 3d10 respectively, due to half-filled and fully filled d subshell stability.
→Ionisation energy trends: First ionisation energy increases across a period (due to increasing nuclear charge) and decreases down a group (due to increased atomic radius and shielding).

What You Need to Demonstrate

Key skills and knowledge for this topic

Award credit for consistent and correct use of significant figures and units in all calculations, reflecting appropriate precision and International System (SI) conventions.
Credit should be given for balanced chemical equations that include correct state symbols, as this demonstrates a thorough understanding of reaction conditions and products.
In explanations, examiners look for precise scientific terminology and logically structured reasoning that links cause and effect, such as using collision theory to explain reaction rate changes.

Assessment Criteria

Key criteria assessors look for in your portfolio

Award credit for consistent and correct use of significant figures and units in all calculations, reflecting appropriate precision and International System (SI) conventions.
Credit should be given for balanced chemical equations that include correct state symbols, as this demonstrates a thorough understanding of reaction conditions and products.
In explanations, examiners look for precise scientific terminology and logically structured reasoning that links cause and effect, such as using collision theory to explain reaction rate changes.

Assessment Guidance

Guidance for achieving higher grades

💡In Paper 2 data-response questions, always annotate given data with relevant formulas before attempting calculations, and show all working to maximise partial credit even if the final answer is incorrect.
💡For the Internal Assessment, explicitly address safety, ethical, and environmental considerations in your methodology, as these are distinct marking criteria in the exploration section.
💡When writing electron configurations, always use the noble gas core notation (e.g., [Ne] 3s1 for sodium) to save time and reduce errors. For ions, remember to add or remove electrons from the outermost shell (highest n value) first.
💡For ionisation energy questions, always consider the effect of electron shielding and nuclear charge. Be prepared to explain why successive ionisation energies increase and why there are jumps when removing an electron from a new shell.
💡Practice drawing orbital diagrams (boxes with arrows) for elements up to atomic number 36. This helps visualise Hund's rule and the Pauli exclusion principle, and is a common exam question.

Common Mistakes

Common errors to avoid in your coursework

Students frequently confuse the terms 'atom' and 'molecule', especially when describing diatomic elements or ionic compounds, leading to inaccurate representations in equations and diagrams.
A common misconception is that adding a catalyst increases the yield of a reversible reaction, when it only speeds up attainment of equilibrium without shifting its position.
Errors in sign conventions for thermodynamic quantities—such as misassigning positive or negative values for enthalpy changes in exothermic/endothermic processes—lead to incorrect numerical results and faulty predictions.
Misconception: Electrons orbit the nucleus like planets around the sun. Correction: Electrons do not have fixed orbits; they exist in orbitals, which are probability clouds where an electron is likely to be found.
Misconception: The 3d orbital fills before the 4s orbital. Correction: Actually, the 4s orbital has lower energy than 3d and fills first, but when writing configurations, we list 3d before 4s (e.g., [Ar] 3d10 4s1 for copper).
Misconception: All elements follow the Aufbau principle strictly. Correction: Transition metals like Cr and Cu have anomalous configurations due to extra stability from half-filled or fully filled d subshells.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Basic understanding of the atom (protons, neutrons, electrons) and the periodic table (groups, periods).
•Familiarity with energy levels and the Bohr model (though this will be refined).
•Simple algebra for understanding quantum numbers and energy level calculations.

Key Terminology

Essential terms to know

Core knowledge
Practical application

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IBO Level 3 Alternative Academic Qualification HL in Chemistry (Extended Certificate)

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Key Marking Points

IBO Level 3 Alternative Academic Qualification HL in Chemistry (Extended Certificate)

Assessment criteria

Topic Overview

Key Concepts

What You Need to Demonstrate

Assessment Criteria

Assessment Guidance

Common Mistakes

Frequently Asked Questions

What is the difference between an orbit and an orbital?

Why does chromium have an electron configuration of [Ar] 4s1 3d5 instead of [Ar] 4s2 3d4?

How do I determine the number of unpaired electrons in an atom?

What is the significance of the principal quantum number n?

Why does ionisation energy increase across a period?

How do I write electron configurations for ions?

Before You Start

Key Terminology

Ready to learn?

Related Topics in INTERNATIONAL-BACCALAUREATE-ORGANISATION vocational Chemistry

IBO Level 3 Alternative Academic Qualification SL in Chemistry (Certificate)