Question 1

How do you write the electron configuration for sodium using noble gas notation?

Accepted Answer

Sodium has 11 electrons with a core electron configuration identical to neon. The condensed configuration is written as [Ne]3s1, where [Ne] represents the neon core electrons and 3s1 represents the single valence electron in the 3s orbital. This notation emphasizes that sodium's valence shell resembles other alkali metals.

Question 2

Why does potassium's nineteenth electron enter the 4s orbital instead of the 3d orbital?

Accepted Answer

The 4s subshell has substantial penetrating ability and lower energy than the 3d subshell, which lacks radial nodes and experiences greater shielding from the nucleus. According to the aufbau principle, electrons fill lower-energy orbitals first, so potassium's configuration is [Ar]4s1 rather than [Ar]3d1.

Question 3

What are valence electrons and core electrons in an atom?

Accepted Answer

Valence electrons occupy the outermost shell orbitals with the highest principal quantum number and determine chemical properties. Core electrons occupy inner shell orbitals and correspond to noble gas configurations. For sodium, the single 3s electron is valence; the 1s2 2s2 2p6 electrons are core.

Question 4

Why do chromium and copper deviate from the aufbau principle?

Accepted Answer

Chromium and copper achieve extra stability through half-filled and completely filled d subshells. Chromium has [Ar]4s1 3d5 instead of [Ar]4s2 3d4, while copper has [Ar]4s1 3d10 instead of [Ar]4s2 3d9. Electrons shift from the 4s to 3d orbital to gain this preferred stability.

Question 5

How do orbital energies differ between the 4s and 3d subshells in transition elements?

Accepted Answer

In transition elements, the 3d and 4s subshells have similar energies, making their relative order unpredictable. The 4s subshell typically fills first due to greater penetration, but the small energy gap allows electrons to shift between orbitals. This proximity explains why exceptions to the aufbau principle occur frequently in transition metals.

Question 6

What is the electron configuration of cerium, and why is it unusual?

Accepted Answer

Cerium has the configuration [Xe]6s2 4f1 5d1 instead of the expected [Xe]6s2 4f2. This deviation occurs because the 6s, 4f, and 5d subshells are unusually close in energy. Electron-electron repulsions in the 4f orbital are larger than the small energy gaps between subshells, making the half-filled 4f and 5d configuration more stable.

Question 7

How do you use noble gas cores to abbreviate electron configurations?

Accepted Answer

Noble gas cores represent filled inner shell electrons matching the preceding noble gas in the periodic table. For any element, write the noble gas symbol in brackets followed by the valence electron configuration. For example, lithium is [He]2s1 and aluminum is [Ne]3s2 3p1, emphasizing similarities within chemical families.

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