Shells and the First 20

Part of the Atomic Structure section of Coddy's Chemistry journey. Lesson 20 of 45.

Electrons sit in shells around the nucleus and fill them from the inside out. A neutral atom has as many electrons as its atomic number.

The first shell holds 2 and the second 8. For the first 20 elements the third takes 8, then the fourth starts: argon is 2, 8, 8 and potassium is 2, 8, 8, 1.

A potassium atom with its electrons in four shells: 2, 8, 8, 1

Potassium: 2, 8, 8, 1

To fill an atom's shells, count its electrons and fill 2, then 8, then 8, then the rest. Aluminum, with 13 electrons, is 2, 8, 3.

Challenge

Calcium, the element in your bones and teeth, has 20 electrons. Fill its shells.

Try it yourself

Electron shellsFill the electrons of Ca (calcium)

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Atomic Structure

1Matter and Atoms

What Everything Is Made Of Elements, Compounds, Mixtures Chemical Symbols Counting Atoms in a Formula Recap - Molecule Kit

4Electron Arrangement

Shells and the First 20Valence Electrons Ionization Energy as Evidence Light from Atoms Subshells: s, p, d Aufbau, Hund and Pauli Writing Configurations Ions and the Exceptions Recap - Configuration Race

7Practice Problems

Recap - Potassium Oxide Recap - Helium from Rocks Recap - Uranium to Lead

2Models of the Atom

Dalton and Thomson Rutherford's Gold Foil The Nuclear Atom Bohr's Shells Recap - From Ball to Nucleus

5The Periodic Table

Mendeleev's Puzzle Groups and Periods Blocks: s, p, d, f Metals, Non-metals, Metalloids Alkali Metals Halogens Noble Gases Recap - Build the Table

3Inside the Atom

Protons Decide the Element Electrons and Charge Neutrons and Mass Number Nuclide Notation Ions Isotopes Radioactive Decay Relative Atomic Mass Recap - Mystery Particles

6Periodic Trends

Atomic Radius Ionic Radius Ionization Energy Trend Electronegativity Explaining Trends Recap - Mystery Element