9701 · 1.3
Electrons, energy levels and atomic orbitals flashcards
Revision flashcards for Cambridge 9701 Electrons, energy levels and atomic orbitals (syllabus 1.3). Flip, recall, then mark a real past-paper question.
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What is a principal quantum shell?
A main energy level in an atom, denoted by the integer 'n' (n=1, 2, 3...). Electrons in shells further from the nucleus have higher energy.
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What is an atomic orbital?
A region of space around the nucleus that can hold up to two electrons with opposite spins. It represents a 3D volume where there is a high probability (typically >90%) of finding the electron.
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How many orbitals are in s, p, and d sub-shells?
s sub-shell: 1 orbital p sub-shell: 3 orbitals (px, py, pz) d sub-shell: 5 orbitals
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What is the maximum number of electrons that can occupy the s, p, and d sub-shells?
s sub-shell: 2 electrons p sub-shell: 6 electrons d sub-shell: 10 electrons
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What is the shape of an s orbital?
Spherical. The size of the sphere increases with the principal quantum number (e.g., a 2s orbital is larger than a 1s orbital).
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What is the shape of a p orbital?
Dumbbell-shaped. There are three p orbitals (px, py, pz) arranged at 90° to each other along the x, y, and z axes.
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State the Aufbau Principle.
Electrons fill atomic orbitals starting from the lowest energy level before occupying higher energy levels.
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State Hund's Rule of Maximum Multiplicity.
When filling a sub-shell containing degenerate (equal energy) orbitals, electrons will occupy each orbital singly before any orbital is doubly occupied.
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State the Pauli Exclusion Principle.
No two electrons in the same atom can have the same four quantum numbers. In practice, this means that if two electrons occupy the same orbital, they must have opposite spins.
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Why is the electron configuration of Chromium (Cr, Z=24) anomalous?
It is [Ar] 4s¹3d⁵, not [Ar] 4s²3d⁴. An electron is promoted from 4s to 3d to achieve a more stable half-filled d sub-shell.
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Why is the electron configuration of Copper (Cu, Z=29) anomalous?
It is [Ar] 4s¹3d¹⁰, not [Ar] 4s²3d⁹. An electron is promoted from 4s to 3d to achieve a very stable, fully-filled d sub-shell.
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How do you determine the electron configuration of a positive ion (cation)?
First, write the configuration for the neutral atom. Then, remove the required number of electrons from the highest principal quantum shell first. For transition metals, this means removing 4s electrons before 3d electrons.