What is the first element in the periodic table to have an electron configuration in the 4th energy level?
beryllium, carbon, potassium, or titanium?
19 K has electron configuration 1s2, 2s22p 6, 3s2 3p6 – 4s1.
20 Ca has electron configuration 1s2, 2s22p 6, 3s2 3p6 – 4s2. and
21 Sc has electron configuration 1s2, 2s22p 6, 3s2 3p6 3d1, 4s2. This is clear that K is the element that has started entering electron in 4s orbital first. Other elements with one electron in 4s orbital are Copper and Chromium. Copper (atomic number 29) has a configuration of [Ar]4s1 3d10, not [Ar]4s2 3d9 as one would expect by the Aufbau principle. Likewise, chromium (atomic number 24) has a configuration of [Ar]4s1 3d5, not [Ar]4s2 3d4 where [Ar] represents the configuration for argon.
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