# How Electrons Fill Orbitals How electrons fill atomic orbitals: the shapes of s, p, d and f orbitals and how many electrons each holds By Drew Crow Topic: Physics & Chemistry [Open the interactive Jig](https://ask.jig.com/j/XBs9vb96ZNnt) ## Transcript ### 1. Where electrons live Every atom has a tiny nucleus with electrons around it\. Electrons do not circle like planets\. They spread out in soft clouds called orbitals, and each kind of orbital has its own shape\. Drag the scene to look around\. The edge of each cloud is drawn where the electron is inside it about nine times out of ten\. ### 2. Two in each orbital Hydrogen has one electron, in the smallest cloud, called 1s\. Helium adds a second one\. Now the orbital is full: one orbital never holds more than two electrons, and the two always spin in opposite directions\. The up and down arrows on the ladder show the two spins\. Scientists call this two\-per\-orbital rule the Pauli exclusion principle\. ### 3. s orbitals are spheres Electrons live in layers called shells, numbered 1, 2, 3 and 4 from the nucleus outward\. Every shell has one s orbital, a round ball, and each new shell adds a bigger one: 1s, 2s, 3s and 4s, nested like layers of an onion\. One s orbital holds 2 electrons\. The cut\-open view shows the smaller spheres inside the bigger ones\. The sizes here are not to scale\. ### 4. p orbitals are dumbbells From the second shell on there are also p orbitals\. Each has two lobes on opposite sides of the nucleus, like a dumbbell\. A shell's orbitals of one shape form a group called a subshell\. The p subshell has three, pointing in three directions at right angles, so it holds 6 electrons\. The two colours show the two halves of the electron's wave, which swing in opposite ways\. They are not electric charges\. ### 5. d orbitals: five shapes From the third shell on, five d orbitals appear\. Four look like four\-leaf clovers\. The fifth is a dumbbell with a ring around its middle\. Five orbitals with two electrons each: a d subshell holds 10\. ### 6. f orbitals: seven shapes From the fourth shell on there are seven f orbitals, with even more lobes\. Seven orbitals with two electrons each: an f subshell holds 14\. The pattern is 1, 3, 5 and 7 orbitals, holding 2, 6, 10 and 14 electrons\. f electrons matter in heavy elements such as uranium, near the bottom of the periodic table\. ### 7. Filling from the bottom up Electrons take the lowest\-energy places first, like water filling a jar\. Watch neon's 10 electrons arrive\. In 2p they take one orbital each before any of them pair up, like passengers choosing empty bus seats first\. The seat rule is called Hund's rule\. That is why carbon, with 6 electrons, has two unpaired electrons in 2p\. ### 8. Build an atom Slide to add electrons, from hydrogen with 1 to krypton with 36\. Each new electron takes the lowest free place on the ladder, and its orbital lights up\. Notice that 4s fills before 3d, because it sits a little lower in energy\. Chromium \(24\) and copper \(29\) bend the rule: one 4s electron moves into 3d to make a half\-full or full d subshell\. ### 9. How much fits Add up the subshells and each shell has a limit: 2 electrons in the first shell, 8 in the second, 18 in the third and 32 in the fourth\. These limits shape the whole periodic table and how atoms join together\.