Every element’s chemistry comes down to where its electrons sit. This guide walks through shells, subshells and the rules that decide the order they fill in, so you can write the configuration of any element in the first four periods.<\/p>\n\n\n\n
Electrons occupy shells numbered n = 1, 2, 3 and so on. Each shell splits into subshells labelled s, p, d and f. A subshell has a fixed number of orbitals, and each orbital holds at most two electrons with opposite spins.<\/p>\n\n\n\n A configuration lists the occupied subshells with the number of electrons as a superscript. Hydrogen is 1s\u00b9, helium is 1s\u00b2.<\/p>\n\n\n\n Q: How many electrons fit in a 3d subshell?<\/p>\n\n\n\n A: Ten. A d subshell has five orbitals with two electrons each.<\/p>\n\n\n\n Electrons fill the lowest-energy subshell available first. This is called the Aufbau principle. The order is not strictly by shell number: 4s fills before 3d, and 5s before 4d.<\/p>\n\n\n\n You don’t need to memorise it: the periodic table is the map. Groups 1\u20132 are the s-block, groups 13\u201318 the p-block, and the transition metals in between are the d-block. Reading the table left to right, top to bottom, gives the filling order.<\/p>\n\n\n\n Q: Which fills first, 4s or 3d?<\/p>\n\n\n\n A: 4s. That is why potassium and calcium put their outer electrons in 4s before the transition metals start on 3d.<\/p>\n\n\n\n Within a subshell, electrons spread out across the orbitals with parallel spins before any of them pair up. Compare carbon, nitrogen and oxygen:<\/p>\n\n\n\n [orbitals] C | Carbon | 1s2 2s2 2p2 | two unpaired electrons in separate 2p orbitals<\/p>\n\n\n\n [orbitals] N | Nitrogen | 1s2 2s2 2p3 | three unpaired electrons, a half-filled 2p<\/p>\n\n\n\n [orbitals] O | Oxygen | 1s2 2s2 2p4 | the fourth 2p electron has to pair up<\/p>\n\n\n\n Q: How many unpaired electrons does nitrogen have?<\/p>\n\n\n\n A: Three, one in each 2p orbital.<\/p>\n\n\n\n Long configurations repeat the core of the previous noble gas. Write that noble gas in square brackets and then only what comes after it. Sodium is [Ne] 3s\u00b9 rather than 1s\u00b2 2s\u00b2 2p\u2076 3s\u00b9.<\/p>\n\n\n\n [orbitals] Na | Sodium | 1s2 2s2 2p6 3s1 | a single valence electron in 3s<\/p>\n\n\n\n Q: What is calcium’s configuration in shorthand?<\/p>\n\n\n\n A: [Ar] 4s\u00b2.<\/p>\n\n\n\n Chromium and copper break the pattern. A half-filled or completely filled d subshell is especially stable, so one electron moves from 4s into 3d.<\/p>\n\n\n\n [orbitals] Cr | Chromium | 1s2 2s2 2p6 3s2 3p6 4s1 3d5 | six unpaired electrons<\/p>\n\n\n\n Q: Why does copper end in 4s\u00b9 3d\u00b9\u2070 rather than 4s\u00b2 3d\u2079?<\/p>\n\n\n\n A: A completely filled 3d subshell is lower in energy, which outweighs the cost of moving one electron out of 4s.<\/p>\n","protected":false},"excerpt":{"rendered":" Shells, subshells, the filling order, Hund’s rule and the exceptions \u2014 how to write the electron configuration of any element.<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_eb_attr":"","_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_hide_featured_image":false,"_jetpack_featured_image_duplicate_dismissed":0,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[50],"tags":[55,51,56],"class_list":["post-90001","post","type-post","status-publish","format-standard","hentry","category-guides","tag-beginner","tag-chemistry","tag-guide"],"jetpack_publicize_connections":[],"jetpack_shortlink":"https:\/\/wp.me\/paQrWz-npD","jetpack-related-posts":[{"id":90118,"url":"https:\/\/jlindemann.se\/homepage\/what-is-an-element\/","url_meta":{"origin":90001,"position":0},"title":"What is an element?","author":"admin","date":"November 2, 2026","format":false,"excerpt":"Everything you breathe or touch have one thing in common. They are all built from elements, the basic ingredients of everything around you. This guide will give you an overview of what an element actually is and why a single number decided which one you're looking at! Inside an atom\u2026","rel":"","context":"In "Guides"","block_context":{"text":"Guides","link":"https:\/\/jlindemann.se\/homepage\/category\/guides\/"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":2623,"url":"https:\/\/jlindemann.se\/homepage\/8-oxygen\/","url_meta":{"origin":90001,"position":1},"title":"#8 Oxygen","author":"admin","date":"August 6, 2025","format":false,"excerpt":"Carl Wilhelm Scheele, a relentless chemist of the 18th century, discovered Oxygen in 1771\u2014a gas that would prove essential not just for respiration, but for the fiery dance of combustion, corrosion, and the symphony of planetary chemistry. Today, it forms about 20.95% of Earth\u2019s atmosphere, and nearly half of the\u2026","rel":"","context":"In "Atomic"","block_context":{"text":"Atomic","link":"https:\/\/jlindemann.se\/homepage\/category\/atomic\/"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/Liquid_oxygen_in_a_beaker_4-e1754504338405.jpg?fit=799%2C450&ssl=1&resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/Liquid_oxygen_in_a_beaker_4-e1754504338405.jpg?fit=799%2C450&ssl=1&resize=350%2C200 1x, https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/Liquid_oxygen_in_a_beaker_4-e1754504338405.jpg?fit=799%2C450&ssl=1&resize=525%2C300 1.5x, https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/Liquid_oxygen_in_a_beaker_4-e1754504338405.jpg?fit=799%2C450&ssl=1&resize=700%2C400 2x"},"classes":[]},{"id":9001,"url":"https:\/\/jlindemann.se\/homepage\/getting-started-with-atomic\/","url_meta":{"origin":90001,"position":2},"title":"Getting Started with Atomic","author":"Jonatan Lindemann","date":"November 2, 2026","format":false,"excerpt":"A four-minute tour of Atomic: the periodic table, element pages, tables and tools, and the Ask Atomic chemistry assistant.","rel":"","context":"In "Guides"","block_context":{"text":"Guides","link":"https:\/\/jlindemann.se\/homepage\/category\/guides\/"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/feature-1.png?fit=804%2C603&ssl=1&resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/feature-1.png?fit=804%2C603&ssl=1&resize=350%2C200 1x, https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/feature-1.png?fit=804%2C603&ssl=1&resize=525%2C300 1.5x, https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/feature-1.png?fit=804%2C603&ssl=1&resize=700%2C400 2x"},"classes":[]},{"id":2629,"url":"https:\/\/jlindemann.se\/homepage\/9-fluorine\/","url_meta":{"origin":90001,"position":3},"title":"#9 Fluorine","author":"admin","date":"August 7, 2025","format":false,"excerpt":"This tiny titan is the lightest of the halogens and the most chemically aggressive element known. It doesn't play nicely with others \u2014 except helium, neon, and argon \u2014 and it\u2019s the absolute king when it comes to electronegativity. Fluorine exists as a diatomic molecule (F\u2082) and forms bonds that\u2026","rel":"","context":"Similar post","block_context":{"text":"Similar post","link":""},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/Fluoro_liquido_a_-196%C2%B0C_1-e1754505010545.jpg?fit=1177%2C663&ssl=1&resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/Fluoro_liquido_a_-196%C2%B0C_1-e1754505010545.jpg?fit=1177%2C663&ssl=1&resize=350%2C200 1x, https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/Fluoro_liquido_a_-196%C2%B0C_1-e1754505010545.jpg?fit=1177%2C663&ssl=1&resize=525%2C300 1.5x, https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/Fluoro_liquido_a_-196%C2%B0C_1-e1754505010545.jpg?fit=1177%2C663&ssl=1&resize=700%2C400 2x, https:\/\/i0.wp.com\/jlindemann.se\/homepage\/wp-content\/uploads\/2025\/08\/Fluoro_liquido_a_-196%C2%B0C_1-e1754505010545.jpg?fit=1177%2C663&ssl=1&resize=1050%2C600 3x"},"classes":[]},{"id":90101,"url":"https:\/\/jlindemann.se\/homepage\/why-gold-doesnt-tarnish\/","url_meta":{"origin":90001,"position":4},"title":"Why gold doesn’t tarnish","author":"Jonatan Lindemann","date":"November 2, 2026","format":false,"excerpt":"Silver darkens and iron rusts, but gold looks the same after thousands of years. The reason lies in its electrons, and in Einstein's relativity.","rel":"","context":"In "Atomic"","block_context":{"text":"Atomic","link":"https:\/\/jlindemann.se\/homepage\/category\/atomic\/"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":90102,"url":"https:\/\/jlindemann.se\/homepage\/why-mercury-is-a-liquid\/","url_meta":{"origin":90001,"position":5},"title":"Why mercury is a liquid","author":"Jonatan Lindemann","date":"November 2, 2026","format":false,"excerpt":"Mercury is the only metal that stays liquid at room temperature, even though the metals around it melt at hundreds of degrees. Here is what makes it different.","rel":"","context":"In "Atomic"","block_context":{"text":"Atomic","link":"https:\/\/jlindemann.se\/homepage\/category\/atomic\/"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]}],"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/jlindemann.se\/homepage\/wp-json\/wp\/v2\/posts\/90001","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/jlindemann.se\/homepage\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/jlindemann.se\/homepage\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/jlindemann.se\/homepage\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/jlindemann.se\/homepage\/wp-json\/wp\/v2\/comments?post=90001"}],"version-history":[{"count":2,"href":"https:\/\/jlindemann.se\/homepage\/wp-json\/wp\/v2\/posts\/90001\/revisions"}],"predecessor-version":[{"id":90117,"href":"https:\/\/jlindemann.se\/homepage\/wp-json\/wp\/v2\/posts\/90001\/revisions\/90117"}],"wp:attachment":[{"href":"https:\/\/jlindemann.se\/homepage\/wp-json\/wp\/v2\/media?parent=90001"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/jlindemann.se\/homepage\/wp-json\/wp\/v2\/categories?post=90001"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/jlindemann.se\/homepage\/wp-json\/wp\/v2\/tags?post=90001"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Subshell<\/th> Orbitals<\/th> Max electrons<\/th><\/tr><\/thead> s<\/td> 1<\/td> 2<\/td><\/tr> p<\/td> 3<\/td> 6<\/td><\/tr> d<\/td> 5<\/td> 10<\/td><\/tr> f<\/td> 7<\/td> 14<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n The filling order<\/h2>\n\n\n\n
The order to remember<\/h3>\n\n\n\n
\n
Hund’s rule<\/h2>\n\n\n\n
Noble gas shorthand<\/h2>\n\n\n\n
Exceptions<\/h2>\n\n\n\n
\n