Electron Configuration

Flame Colors Give a Clue

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.

OpenStax explains that the Bohr model uses discrete energy levels to explain the line spectrum of hydrogen. The concept reference can be opened through openstax.org.

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 Color Points to Electron Energy

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.

What Electron Configuration Means

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-element examples up to calcium, which means $Z \le 20$ . For heavier elements, modern chemistry uses a more detailed discussion of subshells and orbitals. OpenStax discusses that deeper form in openstax.org.

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	Maximum count in this simple model	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 this early pattern	Filled after the $\mathrm{L}$ shell is full.
$\mathrm{N}$	Starts appearing around elements with $Z = 19$ to $20$ in the early pattern	Used after the $2, 8, 8$ pattern.

Important note: the $\mathrm{M}$ shell can be discussed in more detail in later chemistry. Here, the $2, 8, 8, 2$ pattern is used as a bridge for reading light elements, not as a complete rule for every element.

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:

E_{\text{photon}} = E_{\text{high}} - E_{\text{low}} = h\nu = \frac{hc}{\lambda}

The symbol $h$ is Planck's constant, $\nu$ is the frequency of light, $c$ is the speed of light, and $\lambda$ is wavelength. NIST lists the exact values $h = 6.62607015 \times 10^{-34}\ \mathrm{J\,s}$ and $c = 2.99792458 \times 10^8\ \mathrm{m\,s^{-1}}$ in its CODATA constants list. The reference can be opened through physics.nist.gov.

The main idea is enough here: electrons have certain energy levels, changes between energy levels produce specific light, and electron arrangement helps us read atomic behavior.

Reading Electrons 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}

So the electron configuration of neutral magnesium is $2, 8, 2$ . Starting from the atomic number is safer than trying to memorize a configuration list directly.