A periodic table is a dense piece of information design. Each box packs an element's identity, its weight and, through its position, a fair amount of its chemistry. Most of the time you need it for one job: turning a formula into a molar mass so that grams can become moles. This article explains how to read the table, why some of the weights look strange, and then works molar masses step by step with the numbers chemists actually use today. You can follow along in the periodic table and molar mass calculator.
What is in one box
Take iron. Its box shows:
- 26, the atomic number: the number of protons. This, and only this, makes the atom iron. Elements are ordered by it.
- Fe, the symbol, from the Latin ferrum. Symbols are one capital letter, or a capital followed by a small letter. The case carries meaning: Co is cobalt, CO is carbon monoxide.
- 55.845(2), the standard atomic weight. It is the average mass of iron atoms as they occur on Earth, relative to one twelfth of a carbon-12 atom, and numerically the mass in grams of one mole. The (2) is the uncertainty in the last digits: 55.845 ± 0.002.
What the position tells you
Rows are periods. An element in period 4 has its outermost electrons in the fourth shell. Columns are groups, numbered 1 to 18, and elements in a group share a similar outer electron arrangement and so behave alike: lithium, sodium and potassium in group 1 all react with water to give hydrogen and an alkaline solution; fluorine, chlorine, bromine and iodine in group 17 all form salts with metals.
The table is also cut into blocks named after the subshell being filled: the s-block is groups 1 and 2, the p-block groups 13 to 18, the d-block (transition metals) groups 3 to 12, and the f-block the two separate rows at the bottom. Those rows are pulled out only to keep the table from being 32 columns wide. The current IUPAC recommendation (2021) puts lutetium and lawrencium in group 3 under scandium and yttrium, and starts the f-block rows with lanthanum and actinium. Older tables do it differently, which is why textbooks disagree about what sits under yttrium.
Electron configurations follow from the position. Iron is in period 4, group 8, the sixth element of its d-block row: [Ar] 3d⁶ 4s². A few elements break the simple filling rule because a half-full or full d subshell is more stable: chromium is [Ar] 3d⁵ 4s¹ and copper [Ar] 3d¹⁰ 4s¹. Those are the two exceptions every chemistry course expects you to know.
Why some weights are intervals or in brackets
If your table is recent, hydrogen may show [1.00784, 1.00811] instead of 1.008. Since 2009 IUPAC's Commission on Isotopic Abundances and Atomic Weights (CIAAW) has given a growing list of elements, fourteen today, including hydrogen, carbon, nitrogen, oxygen, sulfur and chlorine, an interval, because their isotope mix measurably differs between sources: hydrogen in seawater is slightly heavier than in natural gas. For calculations CIAAW publishes a single conventional value: H 1.008, C 12.011, N 14.007, O 15.999, S 32.06, Cl 35.45.
A number in square brackets with no decimals, such as [97] for technetium or [226] for radium, is something else entirely. These elements have no stable isotopes and no fixed natural composition, so they have no standard atomic weight. The number is the mass number of the longest-lived isotope. You cannot compute a meaningful molar mass with it; you need to know which isotope you have.
Molar mass, step by step
The molar mass of a compound is the sum of the atomic weights of all the atoms in its formula, in grams per mole.
Water, H2O. Two hydrogens and one oxygen: 2 × 1.008 + 15.999 = 18.015 g/mol.
Sodium chloride, NaCl. 22.990 + 35.45 = 58.44 g/mol. So 0.25 mol of salt weighs 0.25 × 58.44 = 14.61 g.
Calcium hydroxide, Ca(OH)2. The 2 after the bracket doubles everything inside it, giving 1 Ca, 2 O and 2 H:
| Element | Atoms | × weight | = mass |
|---|---|---|---|
| Ca | 1 | 40.078 | 40.078 |
| O | 2 | 15.999 | 31.998 |
| H | 2 | 1.008 | 2.016 |
| Total | 74.092 g/mol |
Copper(II) sulfate pentahydrate, CuSO4·5H2O. The dot in a hydrate means "plus", and the 5 multiplies the whole water molecule. CuSO4 is 63.546 + 32.06 + 4 × 15.999 = 159.602; five waters are 5 × 18.015 = 90.075; together 249.677 g/mol, usually rounded to 249.68. Water makes up 90.075 ÷ 249.677 = 36.08% of the crystal's mass, which is why heating blue copper sulfate leaves noticeably less white powder behind.
Percentage composition comes from the same table. In water, hydrogen is 2.016 ÷ 18.015 = 11.19% of the mass and oxygen 88.81%. The calculator lists these percentages for every formula.
Common mistakes
- Wrong case. "CO" and "Co" are different substances. "NaCL" is not a formula; there is no element "L".
- Applying a bracket subscript to one atom only. (NH4)3PO4 contains 3 nitrogens and 12 hydrogens, not 3 nitrogens and 4 hydrogens.
- Forgetting the water of crystallisation. Weighing out "copper sulfate" for a solution means weighing the pentahydrate if that is what is in the jar: 249.68 g/mol, not 159.60.
- Rounding too early. H = 1, O = 16 gives 18 for water. That 0.015 is small, but it compounds over several steps and gets marked wrong when the question expects 18.02.
- Including charges. The molar mass of sulfate is that of SO4, 96.06 g/mol; the two extra electrons add about 0.001 g/mol, below what any school balance can see.
- Using a mass number as an atomic weight. [226] for radium is a single isotope's mass number, not an average to use in a formula.
Try it
Open the periodic table, type CuSO4·5H2O into the calculator and compare the breakdown with the table above, then click chromium and copper to see their exceptional configurations. For unit conversions around your answer, the unit converter is next door.