Electron Configuration

Flame Colors Appear as Electrons Return to Lower Energy Levels

When salts such as $\mathrm{NaCl}$ or $\mathrm{KCl}$ are heated in a flame test, the visible color is not random. Heat energy can temporarily place electrons at a higher energy state. When the electrons return to a lower state, the atom or ion emits light.

The Bohr model uses discrete energy levels to explain the line spectrum of hydrogen.

The word discrete means not every energy value is allowed. Picture a staircase. You can stand on the first, second, or third step, but not in the empty space between two steps. In an atom, electrons are only allowed at certain energy levels.

Flame Colors Show Energy Differences Between Electron States

Excitation means an electron absorbs energy and temporarily moves to a higher energy level. When the electron drops back down, energy is released as light. In a flame test, the experiment must follow safety procedures and use protective equipment because it involves heating chemicals.

Electron Arrangement in the Shell Model

Electron configuration is a way to write how electrons are distributed around the atomic nucleus. Here, we use a simple shell model, where electrons are grouped into shells $\mathrm{K}$ , $\mathrm{L}$ , $\mathrm{M}$ , and $\mathrm{N}$ .

For a neutral atom:

e^- = Z

The symbol $e^-$ is the number of electrons, while $Z$ is the atomic number or the number of protons. So a neutral sodium atom with $Z = 11$ has $11$ electrons.

This simple shell model is limited to light elements up to calcium, which means $Z \le 20$ . Heavier elements require subshells and orbitals.

Interactive Electron Shell Model

Choose a neutral atom below. First read its atomic number, then see how its electrons spread from the inner shell to the outer shell.

Electron Configuration Model

Choose a neutral atom, then read the electron count in the simple

\mathrm{K}

\mathrm{L}

\mathrm{M}

, and

\mathrm{N}

shells.

It has only $1$ electron, so it is in the $\mathrm{K}$ shell.

Atomic number: $Z = 1$
Electron count: $e^- = 1$
Configuration: $1$
Outer shell: $\mathrm{K}$

Reading the Filling Order

In the simple shell model for the first elements up to calcium, electrons are filled from the shell closer to the nucleus first. The order is:

Shell	Occupation in the atoms shown here	How to read it
$\mathrm{K}$	$2$	First shell, closest to the nucleus.
$\mathrm{L}$	$8$	Filled after the $\mathrm{K}$ shell is full.
$\mathrm{M}$	$8$ for elements through calcium	Filled after the $\mathrm{L}$ shell is full.
$\mathrm{N}$	$1$ or $2$ for potassium and calcium	Used after the $2, 8, 8$ pattern.

The simple shell model can be used for the elements from hydrogen through calcium. The arrangement $2, 8, 8, 2$ is calcium's configuration. Each earlier element has the number of electrons given by its atomic number. Heavier elements require subshell filling because the $\mathrm{M}$ shell can hold more than $8$ electrons.

Sodium example:

\begin{aligned} \mathrm{Na}: Z &= 11 \\ e^- &= 11 \\ \text{configuration} &= 2, 8, 1 \end{aligned}

Chlorine example:

\begin{aligned} \mathrm{Cl}: Z &= 17 \\ e^- &= 17 \\ \text{configuration} &= 2, 8, 7 \end{aligned}

Connecting Configuration to Spectra

Electron configuration shows the arrangement of electrons in the ground state, meaning the usual stable energy state. A line spectrum appears when an electron receives energy and then returns to a lower energy level.

The energy of the emitted light can be written as:

\begin{aligned} E_{\text{photon}} &= E_{\text{high}} - E_{\text{low}} \\ &= h\nu \\ &= \frac{hc}{\lambda} \end{aligned}

The symbol $h$ is Planck's constant, $\nu$ is the frequency of light, $c$ is the speed of light, and $\lambda$ is wavelength. The exact values are $h = 6.62607015 \times 10^{-34}\ \mathrm{J\,s}$ and $c = 2.99792458 \times 10^8\ \mathrm{m\,s^{-1}}$ .

Electrons occupy specific energy levels. A transition between two levels releases light with a specific energy, while electron configuration describes how the electrons are arranged in the ground state.

Electron Count from Atomic Number

When asked to write a simple electron configuration, use this order:

Make sure whether the particle is a neutral atom or an ion.
If it is neutral, the electron count equals the atomic number.
Fill shells from inside to outside in the model being used.
Write the electron count in each shell with commas.

For neutral magnesium:

\begin{aligned} Z &= 12 \\ e^- &= 12 \\ \text{configuration} &= 2, 8, 2 \end{aligned}

The electron configuration of neutral magnesium is $2, 8, 2$ . Starting with the atomic number prevents an incorrect electron count for a neutral atom.