To solve the problem, we need to identify the element with the electronic configuration [Ar]3d 4s .
1. Understanding the Configuration:
The configuration [Ar] represents the electron configuration of argon, which is 1s 2s 2p 3s 3p , accounting for 18 electrons. The additional 3d 4s indicates:
- 3d : 10 electrons in the 3d subshell.
- 4s : 1 electron in the 4s subshell.
Total electrons = 18 (from [Ar]) + 10 (3d) + 1 (4s) = 29 electrons.
2. Determining the Element:
The number of electrons in a neutral atom equals its atomic number. An element with 29 electrons has an atomic number of 29, which corresponds to copper (Cu) on the periodic table.
3. Verifying the Configuration:
Copper (Cu, atomic number 29) has the expected ground state configuration. The noble gas core [Ar] is followed by the filling of the 4s and 3d orbitals. Typically, for transition metals, the 4s orbital fills before 3d. However, copper is an exception:
- Expected: [Ar]4s 3d
- Actual: [Ar]4s 3d
This exception occurs because a fully filled 3d subshell (3d ) is more stable than a partially filled one, so an electron from 4s moves to 3d, resulting in [Ar]3d 4s , which matches the given configuration.
Final Answer:
The element with the electronic configuration [Ar]3d 4s is copper (Cu).