The periodic table hangs on the wall of nearly every chemistry classroom, and at first glance it looks like a wall of random letters and numbers. It is actually one of the most useful cheat sheets ever made. Once you know how to read the periodic table, a single box tells you how an atom is built and roughly how it will behave.
This guide explains what each number in a box means, what the rows and columns tell you, and the trends that show up on almost every chemistry test.
Quick answer
To read the periodic table, start with one element’s box. The whole number is the atomic number, the count of protons. The one or two letters are the chemical symbol, and the decimal is the average atomic mass in atomic mass units. The table runs in order of atomic number. Its 7 horizontal rows, called periods, show how many electron shells an atom has. The 18 vertical columns, called groups, collect elements with the same number of outer electrons, so elements in a column react in similar ways. Metals sit on the left, nonmetals on the right.
Key takeaways
- Atomic number = protons: it is the element’s ID number and never changes.
- Periods (rows): the period number equals the number of electron shells.
- Groups (columns): same outer electrons, similar chemistry.
- Three zones: metals on the left, nonmetals on the right, metalloids along the staircase.
- 118 elements are confirmed and named today.
What Do the Numbers in Each Box Mean?
Every box describes one element. Layouts vary a little between textbooks, but most show the same four things. Here is sodium as an example.
| Part of the box | Sodium shows | What it tells you |
|---|---|---|
| Atomic number | 11 | 11 protons in every sodium atom (and 11 electrons in a neutral atom) |
| Symbol | Na | Short name, from the Latin “natrium” |
| Name | Sodium | The element’s English name |
| Atomic mass | 22.990 | Average mass of its atoms, in atomic mass units (u) |
Finding neutrons: a worked example
The box does not list neutrons directly, but you can work them out for the most common isotope:
- Round the atomic mass to the nearest whole number to get the mass number. Sodium: 22.990 rounds to 23.
- Subtract the atomic number: 23 − 11 = 12.
- A typical sodium atom has 12 neutrons.
The atomic mass is a decimal because it averages all the natural isotopes of an element, atoms with the same protons but different numbers of neutrons. Chlorine’s 35.45 is a mix of chlorine-35 and chlorine-37.
Why Do Some Symbols Not Match the Name?
Most symbols are the first one or two letters of the English name: H for hydrogen, C for carbon, Ca for calcium. A handful come from Latin names that chemists used centuries ago:
- Na, sodium (natrium)
- K, potassium (kalium)
- Fe, iron (ferrum)
- Cu, copper (cuprum)
- Ag, silver (argentum)
- Au, gold (aurum)
- Pb, lead (plumbum, the root of “plumber”)
Only the first letter of a symbol is ever a capital. Co is cobalt, while CO is carbon monoxide, a compound of carbon and oxygen.
What Do Periods (Rows) Tell You?
The table has 7 horizontal rows called periods. The period number tells you how many electron shells, or energy levels, the atoms have. Hydrogen and helium in period 1 have one shell. Sodium in period 3 has three.
Moving left to right across a period, each element has one more proton and one more electron than the one before. The two rows floating below the main table, the lanthanides and actinides, actually belong in periods 6 and 7. They are pulled out only so the table fits on a page.
What Do Groups (Columns) Tell You?
The 18 vertical columns are called groups. Elements in the same group have the same number of electrons in their outer shell, called valence electrons. Because outer electrons do the reacting, elements in a group behave like family members.
| Group | Family name | Valence electrons | Typical behavior |
|---|---|---|---|
| 1 | Alkali metals | 1 | Soft, very reactive, react with water |
| 2 | Alkaline earth metals | 2 | Reactive, but less than group 1 |
| 3 to 12 | Transition metals | Varies | Hard, shiny, good conductors (iron, copper, gold) |
| 17 | Halogens | 7 | Very reactive nonmetals, form salts |
| 18 | Noble gases | 8 (helium has 2) | Full outer shell, almost never react |
For main-group elements, the last digit of the group number gives the valence electrons: group 13 has 3, group 16 has 6. Hydrogen sits in group 1 but is a gas and behaves differently from the metals below it.
Metals, Nonmetals and Metalloids
Look for the zigzag “staircase” line that starts near boron (B) and steps down to the right. It divides the table into three zones.
- Metals, left of the line: about three quarters of all elements. Shiny, conduct heat and electricity, can be bent or hammered.
- Nonmetals, right of the line (plus hydrogen): dull, poor conductors, often gases at room temperature, like oxygen and nitrogen.
- Metalloids, along the line: boron, silicon, germanium, arsenic, antimony and tellurium. Their mixed properties make silicon perfect for computer chips.
Periodic Trends You Can Predict
The real power of the table is prediction. These three trends show up again and again on tests.
- Atomic radius (size) gets smaller from left to right, because more protons pull the electrons in tighter. It gets larger going down a group, because each row adds a shell.
- Electronegativity, how strongly an atom pulls on shared electrons, rises toward the top right. Fluorine has the highest value of all.
- Metal reactivity increases going down groups 1 and 2. Lithium fizzes in water; cesium reacts explosively.
The Russian chemist Dmitri Mendeleev published the first widely used table in 1869. He trusted these patterns so much that he left gaps for undiscovered elements and predicted their properties. Gallium and germanium were found within about 20 years and fit his predictions closely.
Common Mix-Ups About the Periodic Table
Myth: Elements are arranged by atomic mass. Mendeleev mostly used mass, but the modern table is ordered by atomic number. That is why argon (39.95) comes before potassium (39.10).
Myth: Atomic mass is the number of protons plus neutrons. That sum is the mass number of a single isotope, always a whole number. Atomic mass is a weighted average, so it is usually a decimal.
Myth: Hydrogen is an alkali metal. It sits in group 1 because it has one valence electron, but under normal conditions it is a nonmetal gas.
Myth: The table is finished. Elements 113, 115, 117 and 118 were named only in 2016, and labs are still trying to make element 119.
Frequently Asked Questions
How many elements are on the periodic table?
There are 118 confirmed elements. About 90 occur naturally on Earth; the rest have been made in laboratories, usually in tiny amounts that last only fractions of a second.
What is the difference between a period and a group?
A period is a horizontal row and tells you the number of electron shells. A group is a vertical column and collects elements with the same number of valence electrons.
Why are noble gases unreactive?
Their outer electron shell is already full. They have no need to gain, lose or share electrons, so they rarely form compounds.
Which element is the heaviest?
Oganesson, element 118, has the highest atomic number and mass. Only a handful of its atoms have ever been made.
How do I memorize the first 20 elements?
Learn them in groups of five, say them out loud, and quiz yourself daily for a week. Spaced practice with flashcards works far better than rereading a list.
Quick Recap
- Each box gives the atomic number (protons), symbol, name and average atomic mass.
- Neutrons ≈ rounded atomic mass minus atomic number.
- Periods (rows) show electron shells; groups (columns) show shared chemistry.
- Metals left, nonmetals right, metalloids on the staircase; atoms shrink left to right and grow top to bottom.
PubChem’s interactive periodic table from NIH lets you click any element for its full data, and NIST publishes a printable periodic table with atomic data. Explore more on our chemistry page and our guide to studying effectively, then build element cards with the free Flashcard Maker.
