The periodic table tells you what every element is made of, how it behaves, and how it relates to other elements. Each square contains an element’s identity (its number of protons), its average mass, and its symbol. But the real power of the table is in its layout: where an element sits reveals how many electrons it has, how it bonds with other elements, how large its atoms are, and whether it’s a metal, nonmetal, or something in between.
Atomic Number and Mass
Every element on the table has an atomic number, which is simply the number of protons in its nucleus. This number defines the element. All carbon atoms have 6 protons, all oxygen atoms have 8, and all gold atoms have 79. Change the proton count and you change the element entirely.
Below or beside the symbol, you’ll find the atomic mass. This number isn’t as clean as the atomic number because it accounts for isotopes, which are versions of the same element with different numbers of neutrons. Carbon, for instance, almost always has 6 neutrons, but a small fraction of carbon atoms have 7 or 8. The atomic mass listed on the table is a weighted average of all naturally occurring isotopes, which is why it’s rarely a round number. These values are periodically updated by the international body that governs chemistry standards. The most recent revision, published in 2024, adjusted the values for gadolinium, lutetium, and zirconium.
What Columns (Groups) Reveal
The vertical columns are called groups, and they’re the key to understanding how an element behaves chemically. Elements in the same group share the same number of valence electrons, which are the outermost electrons that participate in bonding. Group 1 elements have one valence electron. Group 2 elements have two. This pattern continues across the main groups, all the way to Group 18, whose members have full outer shells.
Because chemical reactions are fundamentally about electrons being shared, donated, or accepted, elements in the same column react in similar ways. Sodium (Group 1) and potassium (Group 1) both react violently with water for the same reason: each has a single valence electron it readily gives up. Fluorine and chlorine (Group 17) are both highly reactive because each needs just one more electron to complete its outer shell. This pattern of similar behavior within a column is the entire reason the table is called “periodic.”
Group 18, the far-right column, contains the noble gases: helium, neon, argon, krypton, xenon, and oganesson. These elements already have full outer electron shells, so they have little incentive to react with anything. This stability is the basis of the octet rule, which states that atoms tend to form bonds in ways that give them eight valence electrons, mimicking the configuration of a noble gas.
What Rows (Periods) Reveal
The horizontal rows are called periods, and each one tells you how many electron shells an element’s atoms have. Hydrogen and helium sit in period 1 because their electrons occupy a single energy level. Elements in period 2 (lithium through neon) have two energy levels. Period 3 elements have three, and so on down the table. The more energy levels an atom has, the farther its outermost electrons sit from the nucleus, which directly affects the atom’s size and how tightly it holds onto those electrons.
Predictable Trends Across the Table
The table’s layout creates several reliable patterns that let you predict an element’s physical and chemical properties just from its position.
Atomic size gets smaller as you move left to right across a row. This seems counterintuitive since you’re adding protons and electrons, but the extra protons pull the electron cloud inward more tightly. Moving down a column, atoms get larger because each new row adds another electron shell, pushing the outer electrons farther from the nucleus.
Ionization energy is the energy needed to strip an electron from an atom. It increases from left to right across a period because atoms with more protons hold their electrons more tightly. It decreases as you move down a group because the outermost electrons are farther from the nucleus and easier to remove. This is why metals on the lower left of the table lose electrons so easily, making them highly reactive.
Electronegativity, or how strongly an atom attracts electrons during bonding, follows the same directional pattern. It increases from left to right and decreases from top to bottom. Fluorine, sitting in the upper right (excluding noble gases), is the most electronegative element on the table.
Metals, Nonmetals, and Metalloids
The table is roughly divided into two large territories. Metals occupy the left side and center, making up the vast majority of elements. They conduct electricity, bend without breaking, and tend to lose electrons in reactions. Nonmetals cluster on the upper right side. They’re often gases or brittle solids and tend to gain electrons.
Between these two zones sits a diagonal staircase of six elements called metalloids: boron, silicon, germanium, arsenic, antimony, and tellurium. These elements look metallic but don’t conduct electricity as well as true metals. That partial conductivity makes them semiconductors, and it’s the reason silicon is the backbone of modern electronics. Transistors, computer chips, and solar cells all depend on silicon’s ability to conduct electricity under some conditions but not others.
The Block Structure
If you zoom out and look at the table’s shape, you’ll notice it has distinct rectangular regions. These correspond to which type of electron orbital is being filled as you move across a row.
The two tall columns on the far left (Groups 1 and 2, plus helium) form the s-block, where electrons fill the simplest type of orbital. The six columns on the right (Groups 13 through 18) make up the p-block. The wide middle section of ten columns (Groups 3 through 12) is the d-block, home to the transition metals like iron, copper, and gold. The two detached rows typically printed below the main table are the f-block, containing the lanthanides and actinides.
This block structure means you can use the periodic table as a map for figuring out any element’s electron configuration. Start at hydrogen and trace across each row: you fill two s-block positions, then (from period 4 onward) ten d-block positions, then six p-block positions. The number of elements in each block matches the maximum number of electrons each orbital type can hold.
How Many Elements Are on the Table
The table currently contains 118 confirmed elements. The most recent addition, oganesson (element 118), was officially named in November 2016. It sits in Group 18 with the noble gases, though it’s so unstable that only a few atoms have ever been produced, and its actual chemical behavior remains largely predicted rather than observed. Its expected boiling point is roughly 80°C, which would make it surprisingly easy to condense compared to lighter noble gases like helium or neon.
Elements 1 through 94 occur naturally, though some only in trace amounts. Elements 95 through 118 have been synthesized in laboratories. The atomic weights listed on the table are maintained and revised by IUPAC, the international authority on chemical nomenclature, with the most recent comprehensive update based on 2021 data and subsequent revisions through 2024.