Periodic Table Basics
Everything around you is built from about a hundred kinds of atoms. Learn what's inside an atom, how to read every square on the periodic table, and why its rows and columns predict how elements behave. Then build your own atoms.
Atoms and the three particles
An element is a pure substance made of only one kind of atom: hydrogen, carbon, gold. There are 118 known elements, and the periodic table lists them all. Everything else (water, salt, you) is elements joined together.
An atom is the smallest piece of an element that's still that element. It has a tiny, heavy center called the nucleus, surrounded by a cloud of electrons.
| Particle | Charge | Mass | Where |
|---|---|---|---|
| Proton | +1 | about 1 u | Nucleus |
| Neutron | 0 | about 1 u | Nucleus |
| Electron | −1 | about 1/1800 u (tiny) | Around the nucleus |
The “u” is the atomic mass unit, roughly the mass of one proton. Because electrons are so light, nearly all of an atom's mass is in its nucleus, even though the nucleus is a tiny fraction of the atom's size.
How small? If an atom were the size of a football stadium, the nucleus would be about the size of a pea on the 50-yard line. The rest is mostly empty space where the electrons move.
Atomic number, mass number, isotopes and ions
Three simple rules tell you almost everything about an atom:
- Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12 has 6 neutrons, carbon-14 has 8. Same element, different mass. Some isotopes, like carbon-14, are radioactive.
- Ions are atoms that gained or lost electrons. Lose electrons and you get a positive ion (Na⁺). Gain electrons and you get a negative ion (Cl⁻). Changing electrons never changes the element.
- 17 protons → atomic number 17 → chlorine
- Mass number = 17 + 18 = 35 → chlorine-35
- Charge = 17 − 18 = −1
- A chloride ion, Cl⁻, the “chloride” in sodium chloride (table salt)
How to read a periodic table square
Every element gets one box. Layouts vary a little between tables, but almost all show the same four things.
Why is the atomic mass a decimal, when protons and neutrons come in whole numbers? Because it's an average of all the element's natural isotopes, weighted by how common each is. Natural carbon is mostly carbon-12 with a little carbon-13, so the average is just over 12. To estimate an atom's neutrons, round the atomic mass and subtract the atomic number: 12 − 6 = 6.
Odd symbols? Some come from Latin names. Na is sodium (natrium), K is potassium (kalium), Fe is iron (ferrum), Au is gold (aurum).
Groups and periods
The table's shape isn't random. It's arranged so elements with similar behavior line up.
Groups (1 to 18)
Elements in the same column have the same number of outer electrons, so they act alike. Sodium and potassium both react violently with water.
Periods (1 to 7)
The row number tells you how many electron shells the atom has. Each new row starts a new shell.
| Group | Family | Known for |
|---|---|---|
| 1 | Alkali metals (Li, Na, K…) | Soft, very reactive metals. Hydrogen sits here too but is a nonmetal. |
| 2 | Alkaline earth metals (Be, Mg, Ca…) | Reactive, but less than group 1. Calcium is in your bones. |
| 3–12 | Transition metals (Fe, Cu, Au…) | Classic metals: strong, shiny, good conductors. |
| 17 | Halogens (F, Cl, Br, I) | Very reactive nonmetals that form salts with metals. |
| 18 | Noble gases (He, Ne, Ar…) | Barely react at all. Neon signs, helium balloons. |
The two rows floating below the table, the lanthanides and actinides, actually belong in periods 6 and 7. They're pulled out so the table fits on a page.
A bit of history. Russian chemist Dmitri Mendeleev published his periodic table in 1869. He left gaps for elements nobody had found yet and predicted their properties. When gallium and germanium were discovered, they fit his predictions.
Metals, nonmetals and metalloids
A zigzag “staircase” line runs down the right side of the table, starting near boron. It splits the elements into three big types.
| Metals | Nonmetals | Metalloids | |
|---|---|---|---|
| Where | Left of the staircase (most elements) | Upper right, plus hydrogen | Along the staircase |
| Look | Shiny | Dull, often gases | A mix |
| Electricity | Conduct well | Poor conductors | Semiconductors |
| Bend? | Bend and stretch | Brittle if solid | Brittle |
| Electrons | Tend to lose them (+ ions) | Tend to gain them (− ions) | In between |
| Examples | Iron, copper, aluminum | Oxygen, carbon, sulfur | Silicon, boron |
About three quarters of all elements are metals. Silicon, a metalloid, is the semiconductor at the heart of computer chips. At room temperature, most elements are solids; mercury and bromine are the only liquids, and 11 are gases.
Don't try this at home. Pure sodium and potassium catch fire or explode in water, and chlorine and fluorine gases are poisonous. Leave the demos to videos and trained teachers.
Electron shells and valence electrons
Electrons fill shells (energy levels) around the nucleus, inner shells first. For the first 20 elements a simple rule works: the first shell holds 2, the second holds 8, and the third holds 8 before the fourth shell starts.
- Oxygen (8 electrons): 2, 6
- Sodium (11): 2, 8, 1
- Chlorine (17): 2, 8, 7
- Calcium (20): 2, 8, 8, 2
The electrons in the outer shell are the valence electrons. They're the ones that do the chemistry: bonding, reacting, forming ions. For groups 1–2 and 13–18, there's a shortcut:
- Group 1 → 1 valence electron, group 2 → 2
- Groups 13 to 18 → subtract 10: group 13 has 3, group 17 has 7, group 18 has 8 (helium is the exception with 2)
Atoms are most stable with a full outer shell, usually 8 electrons (the octet rule). Noble gases already have one, which is why they barely react. Everyone else gains, loses or shares electrons to get there.
- Sodium (2, 8, 1) has 1 extra electron. Losing it leaves a full shell → Na⁺
- Chlorine (2, 8, 7) is 1 short. Gaining one fills its shell → Cl⁻
- Opposite charges attract
- Na⁺ + Cl⁻ = NaCl, table salt
Spot the pattern. Metals on the left have few valence electrons and give them away, forming positive ions. Nonmetals on the right are close to full and take electrons, forming negative ions. Nonmetals bonding with each other share electrons instead, like the two hydrogens and one oxygen in water.
FAQCommon questions
How many elements are there?
118 are known and named. The newest, up to oganesson (element 118), were made in labs and last only fractions of a second.
What's the difference between atomic number and mass number?
Atomic number is the number of protons and tells you the element. Mass number is protons plus neutrons for one specific atom. The decimal atomic mass on the table is an average over all natural isotopes.
Why is hydrogen in group 1 if it isn't a metal?
It has 1 valence electron like the alkali metals, so it's placed above them. But it's a gas and behaves like a nonmetal under normal conditions.
Can an atom change into a different element?
Only if its number of protons changes, which takes a nuclear reaction such as radioactive decay. Chemical reactions only move electrons around, so the elements stay the same.
Why don't noble gases react?
Their outer shell is already full, so they have no need to gain, lose or share electrons. A few heavy ones can be forced to react in a lab.
What are the rows at the bottom of the table?
The lanthanides and actinides. They belong in periods 6 and 7 but are shown separately so the table isn't too wide.