Multiple Choice
Which of the following is the correct electron configuration for a neutral bromine (Br) atom?
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10. Periodic Properties of the Elements
The Electron Configuration
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Which of the following electron configurations represents an atom of calcium (atomic number 20) in an excited state?
A
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2
B
1s^2 2s^2 2p^6 3s^2 3p^5 4s^2 3d^1
C
1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^1
D
1s^2 2s^2 2p^6 3s^2 3p^6 3d^2
Verified step by step guidance1
Step 1: Recall that the atomic number of calcium is 20, which means a neutral calcium atom has 20 electrons.
Step 2: Write the ground state electron configuration by filling orbitals in order of increasing energy using the Aufbau principle: 1s, 2s, 2p, 3s, 3p, then 4s. For calcium, this is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2$.
Step 3: Understand that an excited state occurs when one or more electrons are promoted to higher energy orbitals than in the ground state, without changing the total number of electrons.
Step 4: Analyze each given configuration to check if it has 20 electrons and if any electron is promoted from a lower energy orbital (like 4s) to a higher energy orbital (like 3d), indicating excitation.
Step 5: Identify the configuration where an electron from the 4s orbital is moved to the 3d orbital (e.g., $1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^1$), which represents an excited state of calcium.
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