How to Read the Periodic Table: Atomic Number, Groups, Periods and Practice Questions

Quick answer: The periodic table organizes chemical elements by increasing atomic number, which is the number of protons in an atom’s nucleus. Horizontal rows are called periods; vertical columns are groups or families. The modern standard table displays 7 periods and 18 numbered groups, and it includes 118 confirmed chemical elements. An element’s square commonly shows its atomic number, chemical symbol, name and an atomic-weight value.

This U.S. middle-school and early high-school chemistry lesson explains how to read the table, compare elements, understand important families and distinguish atomic number, mass number and atomic weight. The official NIST Periodic Table of the Elements is an authoritative companion reference.

Learning goals

• Explain why elements are arranged by atomic number.
• Identify periods (rows), groups (columns), chemical symbols and atomic-number values.
• Distinguish an atom’s protons, neutrons and electrons.
• Calculate neutrons in a specified isotope using its mass number.
• Recognize key groups such as alkali metals, halogens and noble gases.
• Answer practice questions without confusing symbols or masses.

What is an element?

An element is a substance whose atoms have the same number of protons. Hydrogen atoms always have one proton, carbon atoms have six, and oxygen atoms have eight. An element can appear as single atoms, molecules or in compounds; the essential identifying feature is its proton number. The table lists elements, not every molecule or compound that can be made from them.

How to read one box on the periodic table

Consider oxygen. On the NIST table, you can identify:

• Atomic number: 8. Every oxygen atom has 8 protons.
• Symbol: O. The official one-letter chemical abbreviation is uppercase O.
• Name: Oxygen. The element’s English name.
• Atomic-weight value: approximately 15.999 on many educational tables. This is not the number of neutrons in every oxygen atom.
• Location: period 2, group 16 on the common 1–18 group numbering system.

Exact atomic-weight displays vary by table and isotope information. To avoid errors, use the value and units printed on your assigned reference chart.

Atomic number vs. atomic mass vs. mass number

Atomic number = number of protons

Every neutral atom also has the same number of electrons as protons. An ion has gained or lost electrons and therefore may have a different electron count, but its atomic number does not change.

Mass number = protons + neutrons

The mass number belongs to a particular isotope. For example, sodium-23 has atomic number 11 and mass number 23, so it contains 11 protons and 12 neutrons. A neutral sodium-23 atom also has 11 electrons. The calculation is neutrons = mass number − atomic number = 23 − 11 = 12.

Atomic weight is generally not an isotope’s mass number

The atomic-weight values printed on many classroom periodic tables reflect weighted information about naturally occurring isotopes; they can be decimals. For example, the commonly displayed atomic-weight value of chlorine is approximately 35.45. That does not mean an individual chlorine atom contains 35.45 protons or exactly 18.45 neutrons. For problems asking for neutrons, you need a specified isotope’s integer mass number, not simply a rounded periodic-table weight unless your teacher explicitly directs that approximation.

What are groups and periods?

A group is a vertical column; a period is a horizontal row. Elements in the same group often share patterns in their electron arrangements and chemical properties, but there are exceptions and more complex behavior among transition metals. For beginner study, locating a group and a period correctly is more important than assuming every group member reacts identically.

Group 1: alkali metals

Lithium (Li), sodium (Na), and potassium (K) belong to the alkali-metal family. These metals are chemically reactive. Hydrogen is placed above Group 1 in many periodic tables but is not normally classified as an alkali metal. Do not attempt chemical reactions with sodium or potassium outside a supervised and equipped laboratory; they can react dangerously with water.

Group 17: halogens

Fluorine (F) and chlorine (Cl) are examples of halogens. These reactive nonmetals often form salts when they combine with suitable metals. Chlorine gas can be dangerous; an educational explanation is not an invitation to produce or handle it.

Group 18: noble gases

Helium (He), neon (Ne), and argon (Ar) belong to Group 18. Noble gases tend to be less reactive than many other elements, but it is too broad to claim that all noble gases never form compounds. The behavior depends on the element and conditions.

Metals, nonmetals and metalloids

Most listed elements are classified as metals; many conduct electricity well, but their properties vary. Common nonmetals include carbon, nitrogen, oxygen, fluorine and sulfur. Metalloids, such as silicon, show a mixture of properties associated with metals and nonmetals. Exact metalloid lists sometimes differ across educational references, so follow the classification key on your class chart.

Six worked examples from the official NIST table

1. Hydrogen (H): atomic number 1, so a neutral atom has one proton and one electron. It is in period 1.
2. Carbon (C): atomic number 6, period 2, group 14. Carbon is central to organic compounds.
3. Oxygen (O): atomic number 8, period 2, group 16.
4. Sodium (Na): atomic number 11, period 3, group 1. Its symbol comes from the Latin-derived name natrium.
5. Chlorine (Cl): atomic number 17, period 3, group 17. Note that the second letter in the symbol is lowercase l.
6. Iron (Fe): atomic number 26, period 4, group 8. The symbol comes from ferrum, not the English word “iron.”

Why does the table repeat patterns?

As atomic number increases, the arrangement of electrons changes. Recurring aspects of outer-electron configurations lead to recurring chemical properties. That pattern is known as periodicity. It helps explain why elements in the same family can behave similarly. The periodic table is not ordered by increasing atomic mass: some neighboring elements illustrate why increasing atomic number is the more reliable modern organizing rule. Read NIST’s article on Henry Moseley for historical context.

Element, molecule or compound: how are they different?

Oxygen (O) is an element, but ordinary oxygen gas typically consists of O₂ molecules, each containing two oxygen atoms. Water (H₂O) is a compound because it contains hydrogen and oxygen atoms chemically bonded in a fixed ratio. Table salt (NaCl) is a compound formed from sodium and chloride ions in a crystal lattice. Do not confuse NaCl with a single separate “element” in the table.

Safe study method: four steps to read any element

1. Find the element name or symbol on the official NIST periodic table.
2. Record the atomic number and use it to identify the proton count.
3. Identify the group and period, then compare neighbors carefully.
4. If the problem asks for neutrons, first find the specific isotope’s mass number; do not confuse it with average atomic weight.

Look at laboratory materials only with an instructor and proper protective equipment. The accompanying photograph shows a real U.S. Geological Survey chemist conducting elemental chemical analysis at a laboratory bench. It does not show an actual periodic-table diagram or a student-friendly experiment to reproduce.

Eight periodic-table practice questions

1. What does an element’s atomic number tell you?
2. Are periods horizontal or vertical?
3. Which group contains neon (Ne)?
4. How many protons does a sodium atom have?
5. What is the symbol for iron?
6. How many neutrons are in sodium-23?
7. Which is a compound: oxygen (O) or water (H₂O)?
8. Is hydrogen normally classified as an alkali metal simply because of its position above Group 1?

Answer key

1. The number of protons.
2. Horizontal rows.
3. Group 18 (noble gases).
4. 11.
5. Fe.
6. 23 − 11 = 12 neutrons.
7. Water (H₂O).
8. No. Hydrogen is not normally classified as an alkali metal.

Common misunderstandings

• “Atomic number counts protons and neutrons.” It counts protons only.
• “Atomic weight is the same as mass number.” Atomic weight often refers to an isotope-weighted value; mass number is integer nucleon count for a specific isotope.
• “H and He have the same group because both are in the first period.” They share a row, not a group.
• “Everything in a group has identical properties.” Group patterns are useful but not absolute.

Related study topics

Read more in Chemistry & Periodic Table; review Biology & Life Science for elements in photosynthesis, and browse Physics & Motion for physical science fundamentals.

Official scientific references and image source

• NIST — Periodic Table of the Elements (2024 reference table)
• NIST — Atomic Number by Element
• NIST — Henry Moseley and the Periodic Table
• USGS — Scientist Conducting Manual Total Mercury Analysis, Public Domain Photo

Photo: USGS scientist performing a real chemical analysis, public domain. The laboratory scene is illustrative; the numerical values and element examples come from NIST’s periodic-table resources.