Guides · 01
How to Read the Periodic Table: From a Single Cell to the Block Structure
A beginner's guide to the periodic table: atomic number, symbol and atomic weight, groups and periods, s/p/d/f blocks, and the metal–nonmetal border.
The periodic table is not a list of 118 names to memorize. It is closer to a folded map that shows how the elements resemble one another and how they differ. Like any map, it becomes useful once you know the legend: what is printed inside a single cell, what the rows and columns mean, and what the colored regions stand for. With that legend in hand you can make surprisingly good guesses about an element you have never met before.
What one cell tells you
Almost every periodic table puts at least three pieces of information in each cell.
- Atomic number: the number of protons in the nucleus. Hydrogen is 1, carbon is 6, iron is 26. The atomic number defines the element. Any atom with 26 protons is iron, no matter how many neutrons or electrons it has.
- Chemical symbol: a one- or two-letter code used worldwide. The first letter is always a capital, the second always lowercase. Co (cobalt) and CO (carbon monoxide) mean completely different things, so the capitalization matters. Many symbols come from Latin names, which is why sodium is Na (natrium), potassium is K (kalium) and iron is Fe (ferrum).
- Atomic weight: the average mass of the element's atoms relative to one-twelfth of a carbon-12 atom. Because it is weighted by the natural mix of isotopes, it is rarely a whole number. Chlorine, for example, is a blend of isotopes with mass numbers 35 and 37, which gives an atomic weight of 35.45.
Some cells show a value in square brackets, such as technetium [97] or plutonium [244]. These elements have no stable isotopes, so there is no natural isotope mix to average. The bracketed number is the mass number of the longest-lived isotope. The atomic weights on this site follow the abridged standard atomic weights published by the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW) in 2024.
Richer tables add electron configuration, electronegativity, melting point or the state of the element at room temperature. However crowded a cell looks, the key number is the atomic number, because the whole table is filled in strictly by atomic number, one cell at a time.
Rows are periods, columns are groups
A horizontal row is a period. The modern table has seven, numbered 1 to 7. The period number matches the outermost electron shell that is occupied in a ground-state atom of that element. Period 1 holds only hydrogen and helium; periods 2 and 3 hold eight elements each; periods 4 and 5 hold eighteen; periods 6 and 7 hold thirty-two. The reason the rows have different lengths comes from the orbital structure described below.
A vertical column is a group. IUPAC recommends numbering them 1 through 18 from left to right. Older textbooks used labels such as IA or IIB, but the letters were applied differently in Europe and North America, which caused confusion, so the 1 to 18 system is now the standard.
Elements in the same group have similar outer-electron arrangements, and therefore similar chemistry. Lithium, sodium and potassium in group 1 all react vigorously with water and readily form +1 ions. Fluorine, chlorine, bromine and iodine in group 17 all tend to gain one electron and form −1 ions. Helium, neon and argon in group 18 barely react at all. So the fastest way to size up an element is to ask which group it sits in.
The s, p, d and f blocks: why the table is not a rectangle
The table has two tall columns on the left, a lower central section, a wide block on the right and two rows floating underneath. That shape comes from grouping elements by the type of orbital their last electron enters. There are four such blocks.
| Block | Location | Elements per period | Examples |
|---|---|---|---|
| s-block | groups 1–2 (plus helium) | 2 | hydrogen, sodium, calcium |
| p-block | groups 13–18 | 6 | carbon, oxygen, chlorine, neon |
| d-block | groups 3–12 | 10 | iron, copper, gold |
| f-block | lanthanides and actinides | 14 | neodymium, uranium |
An s subshell holds up to 2 electrons, a p subshell 6, a d subshell 10 and an f subshell 14. In period 2 only s and p fill, so there are 2 + 6 = 8 elements. From period 4 a d subshell joins in, giving 2 + 10 + 6 = 18. From period 6 the f subshell adds another 14, for 2 + 14 + 10 + 6 = 32. The sequence of period lengths, 2, 8, 8, 18, 18, 32, 32, follows directly from those capacities.
Helium is a small exception to the tidy picture. Its configuration is 1s², which makes it an s-block element, yet its chemistry is that of a noble gas, so nearly every table places it at the top of group 18. Blocks and groups do not always line up perfectly.
Why the lanthanides and actinides sit below the table
Two separate rows hang beneath the main table. The upper one is the lanthanides (atomic numbers 57 to 71) and the lower one the actinides (89 to 103). They really belong inside periods 6 and 7, next to group 3.
They are pulled out purely for layout. Inserting the fourteen f-block columns into the body of the table would make it 32 columns wide, which is awkward on a page or a screen. So the f-block is cut out and placed underneath, and a marker such as an asterisk or "57–71" is left in the gap. The full 32-column "long form" is also used, and it has the advantage that atomic numbers run without a break. Both layouts contain exactly the same information.
There is also a long-running discussion about which elements belong under scandium and yttrium in group 3: lanthanum and actinium, or lutetium and lawrencium. IUPAC has run a project on the question, and different textbooks still show different versions. It is enough to know that the variation exists.
Metals, nonmetals and metalloids
If you color the table by type, the largest region by far is metal. Roughly three-quarters or more of all elements are metals, filling the left side and the middle. Metals tend to be shiny, conduct heat and electricity well, can be hammered or drawn into shape, and lose electrons to form positive ions.
The nonmetals cluster in the upper right: hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur, the halogens and the noble gases. Many are gases at room temperature, and the solids tend to be brittle and poor conductors, although graphite is a notable exception.
Between the two regions runs a staircase-shaped border that starts at boron and steps down through silicon, germanium, arsenic, antimony and tellurium. Elements along this line are called metalloids. They may look metallic yet behave chemically like nonmetals, and their electrical conductivity sits between that of metals and insulators. That in-between behavior is exactly why silicon and germanium became the foundation of the semiconductor industry. Whether polonium and astatine count as metalloids depends on the criteria used, so different tables color them differently.
A reading routine
When you meet an unfamiliar element, work through these steps.
- Read the atomic number to get the proton count and the position in the table.
- Note the period to estimate how many electron shells are occupied.
- Note the group and think of its better-known neighbors.
- Identify the block to see which kind of orbital receives the last electron.
- Check whether it lies in the metal, metalloid or nonmetal region.
Once these five habits are automatic, the table stops being something to memorize and becomes a tool for reasoning. Suppose you had never heard of rubidium. Knowing only that it sits in group 1, period 5, you could predict that it is a soft metal that reacts with water even more violently than sodium or potassium, and you would be right. Try clicking through the elements on the table and checking their group, period, block and category for yourself.