# Nakafa Learning Content > For AI agents: use [llms.txt](https://nakafa.com/llms.txt) for the site index. Markdown versions are available by appending `.md` to content URLs or sending `Accept: text/markdown`. URL: https://nakafa.com/en/subjects/chemistry/structure-matter/electron-configuration Source: https://raw.githubusercontent.com/nakafaai/nakafa.com/refs/heads/main/packages/contents/material/lesson/chemistry/structure-matter/electron-configuration/en.mdx Learn electron configuration as a way to read how electrons are distributed in atomic shells, how it connects to line spectra, and where the Bohr model is limited. --- ## 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. Visible text: When salts such as or 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's Bohr Model](https://openstax.org/books/chemistry-atoms-first-2e/pages/3-2-the-bohr-model). 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. Component: Mermaid Props: - title: Flame Color Points to Electron Energy - description: Connect electron excitation to emitted light so flame color reads as a clue about the element. ```mermaid flowchart LR A["Energy enters"] --> B["Electron is excited"] B --> C["Electron returns"] C --> D["Light is emitted"] D --> E["Line spectrum"] ``` **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 $$\mathrm{N}$$. Visible text: **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 , , , and . For a neutral atom: ```math 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. Visible text: The symbol is the number of electrons, while is the atomic number or the number of protons. So a neutral sodium atom with has 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 [Electronic Structure and Electron Configurations](https://openstax.org/books/chemistry-atoms-first-2e/pages/3-4-electronic-structure-of-atoms-electron-configurations). Visible text: This simple shell model is limited to light-element examples up to calcium, which means . For heavier elements, modern chemistry uses a more detailed discussion of subshells and orbitals. OpenStax discusses that deeper form in [Electronic Structure and Electron Configurations](https://openstax.org/books/chemistry-atoms-first-2e/pages/3-4-electronic-structure-of-atoms-electron-configurations). ## 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. Component: ElectronConfigurationLab Props: - title: Electron Configuration Model - description: Choose a neutral atom, then read the electron count in the simple{" "} $$\mathrm{K}$${", "} $$\mathrm{L}$${", "} $$\mathrm{M}$${", and "} $$\mathrm{N}$$ shells. Visible text: Choose a neutral atom, then read the electron count in the simple{" "} {", "} {", "} {", and "} shells. - labels: { chooseAtom: "Choose a neutral atom", atomicNumber: "Atomic number", electronTotal: "Electron count", configuration: "Configuration", outerShell: "Outer shell", samples: { hydrogen: { name: "Hydrogen", note: <>It has only $$1$$ electron, so it is in the $$\mathrm{K}$$ shell., }, helium: { name: "Helium", note: <>The $$\mathrm{K}$$ shell is full with $$2$$ electrons., }, carbon: { name: "Carbon", note: <>The $$\mathrm{K}$$ shell is full, then the remaining electrons enter the $$\mathrm{L}$$ shell., }, neon: { name: "Neon", note: <>Its configuration is $$2, 8$$, so the first two shells are full., }, sodium: { name: "Sodium", note: <>After $$2, 8$$, there is still $$1$$ electron in the next shell., }, magnesium: { name: "Magnesium", note: <>Its configuration is $$2, 8, 2$$ for a neutral atom., }, chlorine: { name: "Chlorine", note: <>Its outer shell has $$7$$ electrons in this simple model., }, calcium: { name: "Calcium", note: <>For elements up to calcium, the simple shell model writes $$2, 8, 8, 2$$., }, }, } Visible text: { chooseAtom: "Choose a neutral atom", atomicNumber: "Atomic number", electronTotal: "Electron count", configuration: "Configuration", outerShell: "Outer shell", samples: { hydrogen: { name: "Hydrogen", note: <>It has only electron, so it is in the shell., }, helium: { name: "Helium", note: <>The shell is full with electrons., }, carbon: { name: "Carbon", note: <>The shell is full, then the remaining electrons enter the shell., }, neon: { name: "Neon", note: <>Its configuration is , so the first two shells are full., }, sodium: { name: "Sodium", note: <>After , there is still electron in the next shell., }, magnesium: { name: "Magnesium", note: <>Its configuration is for a neutral atom., }, chlorine: { name: "Chlorine", note: <>Its outer shell has electrons in this simple model., }, calcium: { name: "Calcium", note: <>For elements up to calcium, the simple shell model writes ., }, }, } ## 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. | Visible text: | Shell | Maximum count in this simple model | How to read it | | :---- | :------------------------------- | :------------- | | | | First shell, closest to the nucleus. | | | | Filled after the shell is full. | | | for this early pattern | Filled after the shell is full. | | | Starts appearing around elements with to in the early pattern | Used after the 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. Visible text: Important note: the shell can be discussed in more detail in later chemistry. Here, the pattern is used as a bridge for reading light elements, not as a complete rule for every element. Sodium example: ```math \begin{aligned} \mathrm{Na}: Z &= 11 \\ e^- &= 11 \\ \text{configuration} &= 2, 8, 1 \end{aligned} ``` Chlorine example: ```math \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: ```math 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 [NIST's Fundamental Physical Constants](https://physics.nist.gov/cuu/Constants/Table/allascii.txt). Visible text: The symbol is Planck's constant, is the frequency of light, is the speed of light, and is wavelength. NIST lists the exact values and in its CODATA constants list. The reference can be opened through [NIST's Fundamental Physical Constants](https://physics.nist.gov/cuu/Constants/Table/allascii.txt). 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: ```math \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. Visible text: So the electron configuration of neutral magnesium is . Starting from the atomic number is safer than trying to memorize a configuration list directly.