Learn the periodic table, one lesson at a time

Short, clear lessons on how the periodic table works, written for high school and early university chemistry. Start with what the table is and how groups and periods are arranged, then move on to valence electrons, electron configurations, periodic trends and polyatomic ions. Each lesson connects to an interactive view of the table so you can see the idea in action.

Chemistry from zero

A free course in 20 short lessons, from atoms to chemical equations. Each lesson ends with three quick questions.

Start the course

1

Atoms and elements

What everything is made of: elements, atoms and the particles inside them. · 4 lessons

1. 1 What is an element? An element is a pure substance made of only one kind of atom. It cannot be broken down into anything simpler by a chemical reaction. Gold, oxygen and carbon are elements, and each of the 118 known elements has its own box on the periodic table. 3 min read
2. 2 Inside the atom An atom is made of three kinds of even tinier particles: protons, neutrons and electrons. Protons and neutrons are packed into a small, heavy center called the nucleus, and electrons move around it. Protons are positive, electrons are negative and neutrons have no charge. 3 min read
3. 3 Atomic number and mass The atomic number is the number of protons in an atom, and it tells you which element the atom is. The mass number is the number of protons plus neutrons. With these two numbers you can count every particle: protons and electrons both equal the atomic number, and neutrons equal the mass number minus the atomic number. 3 min read
4. 4 What are isotopes? Isotopes are atoms of the same element that have different numbers of neutrons. They have the same number of protons, so they are the same element and react in almost exactly the same way, but their masses are different. Carbon-12 and carbon-14 are two isotopes of carbon. 3 min read

2

Reading the periodic table

How the table is organized: periods, groups, families, metals and nonmetals. · 5 lessons

1. 5 What is the periodic table? The periodic table is a chart that arranges all 118 known chemical elements in order of atomic number, the number of protons in each atom’s nucleus. Elements are placed in rows called periods and columns called groups, so elements with similar chemical properties line up in the same column. That pattern lets you predict a lot about an element from where it sits. 4 min read
2. 6 Groups and periods Groups are the 18 vertical columns of the periodic table, and periods are the 7 horizontal rows. Elements in the same group generally have the same number of valence electrons, so they share similar chemical properties. Elements in the same period have the same number of electron shells, and their properties change gradually from left to right. 4 min read
3. 7 Metals, nonmetals and metalloids Metals are shiny, malleable elements that conduct heat and electricity well and tend to lose electrons; nonmetals are usually dull, brittle or gaseous, conduct poorly and tend to gain or share electrons. Metalloids have properties in between. On the periodic table, metals fill the left side and center, nonmetals sit at the upper right, and metalloids lie along the zigzag staircase line that divides them. 3 min read
4. 8 Element families Element families are sets of elements with similar properties, and most periodic tables give each family its own color. This site uses ten families: alkali metals, alkaline earth metals, transition metals, post-transition metals, metalloids, reactive nonmetals, halogens, noble gases, lanthanides and actinides. You can recognize each one by its color and its place on the table. 3 min read
5. 9 Lanthanides and actinides The lanthanides (elements 57–71) and actinides (elements 89–103) are printed in two rows below the main periodic table only to keep it a practical width. They really belong inside periods 6 and 7, right after barium (56) and radium (88). Putting them in place would stretch the table to 32 columns, too wide to fit comfortably on a page or screen. 3 min read

3

Electrons

Shells, valence electrons and configurations, and the trends they explain. · 4 lessons

1. 10 Electron shells Electron shells are the energy levels where an atom’s electrons are found, arranged in layers around the nucleus. The first shell holds up to 2 electrons and the second up to 8, and for the first 20 elements the third shell also stops at 8, giving the 2-8-8 pattern. The Bohr model draws shells as circles around the nucleus, a handy but simplified picture. 3 min read
2. 11 What are valence electrons? Valence electrons are the electrons in an atom’s outermost shell, the ones that take part in chemical bonding. For main-group elements, the number of valence electrons matches the last digit of the group number: oxygen in group 16 has 6, and chlorine in group 17 has 7. Valence electrons decide which ions an element forms and how many bonds it makes. 4 min read
3. 12 How to write electron configurations An electron configuration shows how an atom’s electrons are arranged in subshells, written with a superscript for the number of electrons in each, such as 1s² 2s² 2p⁴ for oxygen. To write one, take the number of electrons from the atomic number and fill the subshells in the aufbau order (1s, 2s, 2p, 3s, 3p, 4s, 3d and so on). Noble-gas shorthand shortens long configurations, and a few elements, such as chromium and copper, break the pattern. 4 min read
4. 13 Periodic trends Periodic trends are regular patterns in element properties across the periodic table. Moving left to right across a period, atomic radius generally decreases while electronegativity and ionization energy increase; moving down a group, atomic radius increases while electronegativity and ionization energy decrease. Metallic character runs the opposite way and is greatest at the lower left of the table. 4 min read

4

Chemistry with elements

Ions, bonds, formulas, moles and equations: putting elements together. · 7 lessons

1. 14 Ions and charges An ion is an atom, or a group of atoms, that carries an electric charge because it has gained or lost electrons. Atoms that lose electrons become positive ions called cations, and atoms that gain electrons become negative ions called anions. For many elements you can predict the charge from the group number alone: sodium in group 1 forms Na⁺, and chlorine in group 17 forms Cl⁻. 3 min read
2. 15 What are polyatomic ions? A polyatomic ion is a group of two or more atoms bonded together that carries an overall electric charge and acts as a single unit in compounds. Common examples are hydroxide (OH⁻), nitrate (NO₃⁻), carbonate (CO₃²⁻), sulfate (SO₄²⁻), phosphate (PO₄³⁻) and ammonium (NH₄⁺). Most polyatomic ions are negative, and many contain oxygen. 4 min read
3. 16 Chemical bonds A chemical bond is the attraction that holds atoms together in elements and compounds. In an ionic bond, one atom transfers electrons to another and the oppositely charged ions attract; in a covalent bond, atoms share pairs of electrons; and in a metallic bond, metal atoms share a “sea” of electrons that move freely. Which type forms depends mostly on whether the atoms are metals or nonmetals. 4 min read
4. 17 How to read chemical formulas A chemical formula uses element symbols and small numbers to show which atoms are in a substance and how many of each there are. The small number after a symbol, called a subscript, counts the atoms of that element, so H₂O has 2 hydrogen atoms and 1 oxygen atom. Parentheses, coefficients and the dot in hydrates add a few more rules, but each one is just a kind of multiplying or adding. 4 min read
5. 18 What is a mole? A mole is a counting unit chemists use for atoms and molecules, just as a dozen means 12. One mole contains 6.022 × 10²³ particles, a number called Avogadro’s number. The mass of one mole of a substance in grams, its molar mass, comes straight from the atomic masses on the periodic table, so it lets you convert between grams and numbers of particles. 3 min read
6. 19 Balancing chemical equations Balancing a chemical equation means making sure there are the same number of atoms of each element on both sides of the arrow. You do it by placing whole-number coefficients in front of formulas, never by changing the subscripts. For example, hydrogen and oxygen form water in the balanced equation 2H₂ + O₂ → 2H₂O. 3 min read
7. 20 Chemical and physical changes A physical change alters the size, shape or state of a substance, but it is still the same substance afterward, as when ice melts into water. A chemical change, also called a chemical reaction, turns substances into new ones with different properties, as when iron turns into rust. The key question is always: has a new substance been made? 4 min read