How to Read the Periodic Table, With Iron as a Worked Example

The periodic table lists all 118 known elements in order of atomic number, the number of protons in the nucleus. Each row is a period and each column is a group. Elements in the same group have the same arrangement of outer electrons, so they react in similar ways. Once you know where an element sits, you can predict a lot about it before you read a single number.

This guide explains what each part of a tile means and how the layout works, then walks through iron in the interactive periodic table so you can see every value the tool gives you.

Why is the table shaped the way it is?

Dmitri Mendeleev published the first widely used table in 1869. He sorted the elements by mass and left gaps for elements nobody had found yet. Today the order is by atomic number, and the shape comes from how electrons fill their shells.

The modern layout has 7 periods and 18 groups, numbered 1 to 18 from left to right as IUPAC recommends. A new period starts each time a new electron shell begins to fill. That is why period 1 holds only hydrogen and helium, while periods 6 and 7 hold 32 elements each.

Those two long periods would make the table far too wide. So the lanthanides (57 to 71) and actinides (89 to 103) are lifted out and shown as two rows underneath. In the tool, group 3 of periods 6 and 7 shows a dashed placeholder reading "57–71" and "89–103" to mark where they belong.

What do s, p, d and f blocks mean?

The blocks name the subshell that the last electron goes into:

• s-block: groups 1 and 2, plus helium. These are reactive metals such as sodium and calcium.
• p-block: groups 13 to 18. This block holds the metalloids, most nonmetals, the halogens and the noble gases.
• d-block: groups 3 to 12, the transition metals.
• f-block: the lanthanides and actinides in the two bottom rows.

The Block filter in the tool highlights one block at a time. It is a quick way to see that the f-block really does slot in between groups 2 and 3.

Worked example: looking up iron

Type Fe, iron or 26 into Search elements. Iron gets an outline and the other tiles fade. Its tile shows the atomic number 26, the symbol Fe, the name and the mass 55.845. Click it and the details panel lists:

Field	Value
Atomic mass	55.845 u
Period / Group / Block	4 / 8 / d-block
Category	Transition metal
Electron configuration	[Ar]4s2 3d6
Electronegativity (Pauling)	1.83
Melting point	1811 K (1537.85 °C)
Boiling point	3134 K (2860.85 °C)
Density	7.874 g/cm³
State at room temperature	Solid
Discovered	Ancient times

Here is how to read it. [Ar] is shorthand for argon's 18 electrons. Iron then adds two electrons in the 4s subshell and six in 3d, for 26 in total. The six d electrons put iron in the d-block and in group 8. An electronegativity of 1.83 is middling: iron gives up electrons to oxygen fairly easily, which is why it rusts. The panel converts temperatures from kelvin by subtracting 273.15, so 1811 K becomes 1537.85 °C.

At the top of the panel, three chips copy the Symbol, Atomic mass and Electron config. with one click, which is handy for lab reports.

Why are some masses in brackets?

Most atomic masses in the table are standard atomic weights from IUPAC's Commission on Isotopic Abundances and Atomic Weights. Each one is an average over the element's natural isotopes, weighted by how common each isotope is on Earth.

Some elements have no stable isotope, so there is no natural mix to average. For those, the table shows the mass number of the longest-lived isotope in square brackets. Technetium shows [97] and oganesson shows [294]. The details panel adds "(longest-lived isotope)" so you don't mistake it for a measured average.

The heaviest elements bring a second kind of uncertainty. Only a handful of atoms of oganesson have ever been made. So its electron configuration is marked "(predicted)", its state reads "Gas (expected)", and its melting point, boiling point and density read "Unknown".

How do the filters and views help?

Five dropdowns narrow the table: Period, Group, Block, Category and State at 20 °C. You can combine them. For example, Period 4 plus Transition metal shows the ten metals from scandium to zinc. The legend under the table works as a shortcut too: click a category name, such as Halogen, to filter by it, and click it again to clear it. A Reset button appears whenever a search or filter is active.

Search matches part of a name, or an exact symbol or atomic number. So ar lights up argon, and also carbon, arsenic, barium and a few others because their names contain those letters. Type the full name or the atomic number when you want exactly one tile.

Set State to Liquid and only two tiles stay lit, mercury and bromine. Gas tiles show density in grams per liter at 0 °C and 1 atm instead of g/cm³. Chlorine, for example, reads 3.214 g/L.

On a narrow phone screen, scroll the table sideways or press ☰ List view for a card list of the current matches. With a keyboard, Tab onto a tile and use the arrow keys to move to its neighbors, then press Enter to open it. Screen readers hear each tile's name, symbol, atomic number and category.

All the element data ships with the page, taken from PubChem's public-domain periodic table. Browsing and searching happen in your browser, and nothing is looked up online.

Where to go next

To get the mass of a compound rather than one element, the molar mass calculator adds up the atomic weights in a formula for you. For converting a melting point between kelvin, Celsius and Fahrenheit, use the temperature converter.

FAQ

What is the difference between atomic number and atomic mass?

The atomic number counts the protons and defines the element. Every iron atom has 26. Atomic mass also includes neutrons and is averaged over isotopes, so it isn't a whole number.

Why is hydrogen in group 1 if it isn't a metal?

It has one outer electron, like the alkali metals, so it sits above them. Chemically it behaves as a nonmetal, and the tool labels it Nonmetal.

Which elements were named most recently?

Nihonium (113), moscovium (115), tennessine (117) and oganesson (118). IUPAC approved their names in 2016, which completed period 7.

Look up any element in the interactive periodic table.