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State the maximum number of electrons that can occupy a p sub-shell, and explain your answer in terms of orbitals.
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6 electrons; because a p sub-shell contains 3 orbitals and each orbital holds a maximum of 2 electrons
Write the full electronic configuration of a chromium atom (Z = 24) and of a Cr³⁺ ion.
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Cr: 1s²2s²2p⁶3s²3p⁶3d⁵4s¹; Cr³⁺: 1s²2s²2p⁶3s²3p⁶3d³; (4s electron removed first, then two 3d electrons)
Define the term orbital and state how many electrons one orbital can hold.
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A region of space around the nucleus where there is a high probability of finding an electron; one orbital holds a maximum of 2 electrons
Explain why the 4s sub-shell is filled before the 3d sub-shell when writing the electronic configuration of potassium.
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The 4s sub-shell is at a lower energy than 3d in the neutral atom; electrons fill the lowest available energy sub-shell first to give the most stable ground state
Discuss how the electronic configuration of an atom is determined by both the energy of the sub-shells and inter-electron repulsion. Use the configurations of nitrogen (Z = 7) and oxygen (Z = 8) to illustrate your answer.
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Electrons fill the lowest-energy sub-shells first to give the lowest-energy/ground-state configuration; this is why 1s fills before 2s and 2s fills before 2p; within a sub-shell containing several orbitals (such as 2p), electrons occupy separate orbitals singly before pairing/Hund's rule; this minimises inter-electron repulsion because two electrons in one orbital repel more strongly than two in separate orbitals; nitrogen has the configuration 1s²2s²2p³ with three singly occupied 2p orbitals (parallel spins); oxygen has the configuration 1s²2s²2p⁴, where one 2p orbital must contain a pair of electrons while the other two remain singly occupied