Periodic Table Lookup

The periodic table is the central reference tool of chemistry, organising all known elements by their atomic number and chemical properties. This lookup tool lets you search any element by name, symbol, or atomic number to instantly retrieve its key properties, electron configuration, and common uses.

How to Use the Periodic Table Lookup

1. Type an element name (e.g., carbon), symbol (e.g., C), or atomic number (e.g., 6) into the search field.
2. The tool displays the element's full property card instantly.
3. Use the group and period filters to browse related elements.
4. Click any element in the interactive table to view its details.
5. Use the data in chemistry calculations, material science research, or study.

The Formula

Each element is defined by three key numbers. The atomic number (Z) is the number of protons in the nucleus and uniquely identifies the element. The mass number (A) is the sum of protons and neutrons: A = Z + N, where N is the neutron count. The atomic mass shown on the table is the weighted average of all naturally occurring isotopes.

Electron configuration follows the Aufbau principle: electrons fill orbitals in order of increasing energy (1s, 2s, 2p, 3s, 3p, 4s, 3d...). The configuration determines the element's chemical behaviour, reactivity, and position in the periodic table. Elements in the same group (column) share the same number of valence electrons and therefore have similar chemical properties.

Real-World Example

Looking up Iron (Fe):

• Atomic number (Z) = 26
• Symbol = Fe (from Latin: ferrum)
• Atomic mass = 55.845 g/mol
• Period = 4, Group = 8 (transition metals)
• Electron configuration = [Ar] 3d6 4s2
• Common oxidation states = +2 (ferrous) and +3 (ferric)

Iron's two common oxidation states explain why it forms two distinct series of compounds, such as iron(II) chloride (FeCl2) and iron(III) chloride (FeCl3), with different colours and properties.

How the Periodic Table Is Organised

The table is arranged in 7 horizontal periods and 18 vertical groups. Period number indicates the number of electron shells. Group number (using the IUPAC 1-18 numbering) indicates valence electron count for main group elements. The table is divided into blocks based on the subshell being filled: s-block (groups 1-2), p-block (groups 13-18), d-block (transition metals, groups 3-12), and f-block (lanthanides and actinides). Elements increase in atomic number from left to right and top to bottom.

Frequently Asked Questions

What is the difference between atomic number and mass number? The atomic number is the number of protons and defines the element; changing the proton count creates a different element entirely. The mass number is the total count of protons and neutrons in a specific isotope. Different isotopes of the same element have the same atomic number but different mass numbers.

Why are lanthanides and actinides placed separately at the bottom of the table? These f-block elements are placed below the main table purely for layout convenience. Inserting them in their proper position between groups 2 and 3 would make the table extremely wide (32 columns). They belong in periods 6 and 7 respectively.

What are noble gases and why are they unreactive? Noble gases (Group 18: helium, neon, argon, krypton, xenon, radon) have completely full outer electron shells (eight electrons, or two for helium). This configuration is highly stable, giving them very little tendency to gain, lose, or share electrons. Under normal conditions they do not form chemical bonds.

How do I use the periodic table to predict an element's chemical behaviour? Elements in the same group have the same number of valence electrons and form similar types of compounds. Electronegativity increases from left to right across a period and decreases down a group. Metallic character increases from right to left and top to bottom. These trends allow you to predict reactivity, bond type, and likely oxidation states.

The iron card taken apart into three numbers

A property card shows several numbers together, and iron is the clearest case for separating them, because the page already uses it as its worked example.

The first three rows are exact whole numbers. The fourth is not, and the reason it is not is the point of the section below.

Where the standard atomic weight comes from

Natural iron is a mixture of four stable isotopes, and the atomic weight printed on the table is the abundance-weighted average of their masses rather than the mass of any single atom. NIST publishes both the masses and the abundances.

The abundances sum to 100.000 percent, and the weighted average of the four masses works out at 55.8451, which is the 55.845 the card shows once the last digit is rounded. Fe-56 alone accounts for 91.754 percent of natural iron, which is why the atomic weight sits so close to 56 rather than in the middle of the range from 53.9 to 57.9. A sample of iron enriched in Fe-57 would have a different atomic weight, and this is the reason CIAAW reports standard atomic weights for normal materials rather than for every sample.

The weighted average also explains why no single atom of iron weighs 55.845 units. The card's fourth row is a property of the mixture, while the first three rows describe one atom.

Mass number against atomic mass across four elements

The two numbers are easy to run together, and placing them side by side makes the difference plain. The nominal mass number used here is the mass number of the element's dominant naturally occurring isotope.

Hydrogen is the one row where the neutron count reaches zero, because protium is a single proton with no neutron and it makes up almost all natural hydrogen. Uranium behaves the opposite way: Z is 92 and the dominant isotope has a mass number of 238, so the nucleus carries 146 neutrons against 92 protons.

The last column is the standard atomic weight from CIAAW. Hydrogen, carbon and oxygen all carry interval values rather than single figures, because their isotope ratios vary measurably between sources. Iron carries a single value with a stated uncertainty, 55.845 with a tolerance of 0.002.

Counting the electrons from a configuration

The configuration of an element has to add up to its atomic number, and that addition is a fast check on any card. The noble gas core in brackets supplies a known count, the argon core in each row below contributing 18 electrons.

Every total in the last column matches the element's atomic number, which is the whole test. A configuration that does not add up to Z is either misread or misprinted. The 4s subshell filling before 3d in the iron row is what the Aufbau principle the page describes produces, and it is worth reading the row again if the order looks wrong.

What the group number predicts

Group position gives the valence count for the main group elements, and the valence count is what drives the chemistry.

Helium is the single exception in group 18, carrying two valence electrons rather than eight, and it still forms no bonds under ordinary conditions. The d-block elements between groups 3 and 12 do not follow this table, which is why the page describes group number as predicting valence count for main group elements and not for the transition series as a whole.

A molar mass worked from the card

The atomic weight on the card is the figure that feeds every stoichiometry calculation, and iron shows the difference between an element's two oxidation states in a way that is easy to check.

The iron share is the mass of the iron atoms in the formula divided by the compound's molar mass. Fe2O3 carries two iron atoms, so its share of 69.94 percent uses 2 x 55.845. FeO carries one, and its 77.73 percent is the highest of the four rows. The two chlorides are the compounds the page's example names as ferrous and ferric, and their molar masses differ by 35.45, one chlorine atom.

The same arithmetic runs in the other direction. Dividing a mass by the molar mass gives a count of moles: 100 g of iron is 100 / 55.845 = 1.7907 mol, and two moles of iron weigh 111.690 g. Those two operations cover most of what a lookup card is used for.

Limits of the lookup card

The card is a reference and it is worth reading with its boundaries in mind.

• The standard atomic weight is the value for normal materials. An enriched or isotopically altered sample needs the mass of the specific isotope instead.
• The atomic mass is an average. Working with neutrons or with mass spectrometry means working with an isotope mass, not the weighted figure on the card.
• The configuration shown is the ground state of the neutral atom. Ions and excited states have different configurations, and the transition metals have several low-lying arrangements.
• The nominal mass number is a reference point, not a measurement of the sample in front of you.
• Group trends hold well across the main group elements and less well across the transition series, where oxidation state depends on the specific element and its ligands.
• Positions on the table are settled by IUPAC and are rarely revised, but new element names and symbols have been added within living memory, so a card older than a few years is worth checking against a current source.

Also try these free tools:

• Molar Mass Calculator
• Chemical Equation Balancer
• Density Calculator