It shows the periodic table of all 118 elements and, next to it, a molar mass calculator for any chemical formula. Click an element, or move across the table with the arrow keys, to see its atomic number, standard atomic weight, group and period, block, electron configuration, state at 25 °C, Pauling electronegativity, melting and boiling points, and when and by whom it was discovered. Search by name, symbol or number ("iron", "Fe" or "26"; Vietnamese names such as "sắt" work too), or narrow the table to one category, block or state; elements that do not match are dimmed so you can still see where the rest sit.
The molar mass calculator reads formulas the way they are written in a textbook: H2O, Ca(OH)2, Al2(SO4)3, K4[Fe(CN)6], and hydrates with a dot such as CuSO4·5H2O (you can type ·, * or a full stop). It lists every element with its number of atoms, its share of the mass and the percentage composition, which is the table you need for empirical-formula exercises.
Where the numbers come from
Atomic weights are the standard atomic weights of IUPAC's Commission on Isotopic Abundances and Atomic Weights (CIAAW), including the 2024 revisions for gadolinium, lutetium and zirconium. Most values are shown with their uncertainty in the last digits: 55.845(2) for iron means 55.845 ± 0.002. Fourteen elements, carbon among them, have a weight that varies in nature (its isotopes are not mixed in the same proportion everywhere). For those, CIAAW gives an interval such as [12.0096, 12.0116] and a conventional value for everyday calculations, 12.011; the tool shows both and calculates with the conventional value. That is why water comes out at 2 × 1.008 + 15.999 = 18.015 g/mol, the value in most textbooks.
Elements with no stable isotope and no characteristic isotopic composition, such as technetium, promethium and everything from polonium on except thorium, protactinium and uranium, have no standard atomic weight. The table shows the mass number of the longest-lived isotope in brackets, [97] for technetium, as the IUPAC table does. A mass number is not an average mass, so the calculator refuses those elements instead of returning a misleading number.
Electron configurations are generated from the filling order (the Madelung or aufbau rule) and replaced by the measured ground state for the twenty elements where nature disagrees, such as chromium ([Ar] 3d⁵ 4s¹) and copper ([Ar] 3d¹⁰ 4s¹); those are marked. From element 104 on, configurations are predictions. Melting and boiling points are in kelvin and degrees Celsius at 1 atm; carbon and arsenic sublime instead of melting, and where no reliable measurement exists the tool says "not established" rather than printing an estimate.
The layout follows the IUPAC recommendation of 2021: lutetium and lawrencium sit in group 3 under scandium and yttrium, and lanthanum to ytterbium and actinium to nobelium form the two f-block rows.
How to compute a molar mass
Multiply each element's atomic weight by the number of its atoms and add everything up. Brackets multiply everything inside them: in Ca(OH)2 there are 1 Ca, 2 O and 2 H, so 40.078 + 2 × 15.999 + 2 × 1.008 = 74.092 g/mol. In a hydrate the dot means "plus": CuSO4·5H2O is one CuSO4 (159.602) plus five waters (5 × 18.015 = 90.075), 249.677 g/mol, usually rounded to 249.68. Sodium chloride is 22.990 + 35.45 = 58.44 g/mol.
The calculator does the same, without rounding until the end, and keeps the full precision when you copy the result.
Common mistakes
- Case matters. Co is cobalt; CO is carbon monoxide. NO is nitric oxide, No is nobelium. The tool reads exactly what you type.
- A subscript after a bracket applies to the whole bracket, not only to the last element: (NH4)3PO4 has 3 N and 12 H.
- Do not add charges. Write SO4 rather than SO4^2-; the mass of an ion differs from the neutral formula only by its electrons, about 0.0005 g/mol per charge.
- Rounded weights add up. Using H = 1 and O = 16 gives 18 for water instead of 18.015; on a 2-mole calculation the error is already 0.03 g. Use the values your course asks for, and the full values when precision matters.
To convert grams to moles or between other units, the unit converter and the scientific calculator are next door.
