### All Physical Chemistry Resources

## Example Questions

### Example Question #1 : Electron Configuration

Which of the following electron configurations indicates an atom in an excited state?

**Possible Answers:**

**Correct answer:**

An atom is considered to be in an excited state when one of the electrons has jumped to a higher energy level while a lower energy level is available. In the case of , an electron has jumped to the *2p* energy level while there is still room in the lower *2s* subshell. As a result, it is considered to be in an excited state.

### Example Question #2 : Electron Configuration

What is the complete ground state electron configuration for the magnesium atom?

**Possible Answers:**

*1s ^{2}2s^{2}2p^{6}3s^{2}*

*1s ^{2}2s^{2}2p^{6}3s^{6}*

*1s ^{4}2p^{6}3s^{2}*

*1s ^{2}2s^{2}2p^{2}3s^{2}*

*1s ^{2}2s^{2}3s^{2}*

**Correct answer:**

*1s ^{2}2s^{2}2p^{6}3s^{2}*

Magnesium has an atomic number of 12, so the total number of electrons in its configuration should add up to twelve. The maximum number of electrons in the s subshell is two. Of all the answer choices, only *1s ^{2}2s^{2}2p^{6}3s*

^{2 }fits the criteria. The sum of the exponent values is 12, matching the atomic number of magnesium, and the number of electrons in the

*s*and

*p*subshells matches the maximum amount possible.

### Example Question #3 : Electron Configuration

What is the complete ground state electron configuration for the iron atom?

**Possible Answers:**

*1s ^{2}2s^{2}2p^{6}3s^{2}4s^{2}3d^{6}*

*1s ^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{4}*

*1s ^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{6}*

*1s ^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{6}*

*1s ^{4}2s^{2}2p^{6}3s^{2}3p^{2}4s^{2}3d^{6}*

**Correct answer:**

*1s ^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{6}*

Iron has an atomic number of 26, so the total number of electrons in its configuration should add up to twenty six. The maximum number of electrons in the s subshell is two. The sum of the exponent values is 26, matching the atomic number of magnesium, and the number of electrons in the *s* and *p* subshells matches the maximum amount possible.

### Example Question #4 : Electron Configuration

In an atom or molecule, why can't two electrons have the same four electronic quantum numbers?

**Possible Answers:**

Harmonic Reaction Orders

The first law of thermodynamics

Kinetic energy operator

The Pauli Exclusion Principle

Heisenberg Uncertainty Principle

**Correct answer:**

The Pauli Exclusion Principle

The Pauli Exclusion Principle explains various phenomena such as the structure of atoms and how different atoms combine to share electrons. When you have two electrons that are located in the same orbital, the quantum numbers n, l and m_{l} are the same. However, m_{s} will be different. Two electrons cannot have the same four electronic quantum numbers because no more than two electrons may occupy an orbital, and if they do, the spin of one must cancel the spin of the other so their spins will have a zero net spin angular momentum.

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