Reading the Interactive Periodic Table Tool
Almost every stoichiometry, molar-mass and solution-concentration question starts with the same step: looking up an atomic mass. The interactive periodic table in the calculator suite lists all 118 elements with their atomic numbers and atomic masses, laid out in the standard rows (periods) and columns (groups) you already know from your textbook. This guide covers what to read off it and how to use those numbers correctly.
What each element box tells you
- Atomic number — a small whole number, equal to the number of protons (and, in a neutral atom, electrons).
- Symbol — the one- or two-letter chemical symbol.
- Atomic mass — a decimal number in g/mol, the weighted average mass of the element's naturally occurring isotopes. This is the number you need for molar-mass calculations — not the atomic number, and not a rounded whole-number "mass number".
- Position (period and group) — the row and column the element sits in, which tells you its general chemical family (alkali metal, halogen, noble gas, transition metal, and so on).
Worked example 1 — molar mass of potassium permanganate, KMnO₄
Look up: K = 39.098, Mn = 54.938, O = 15.999 (all g/mol).
K: 1 × 39.098 = 39.098
Mn: 1 × 54.938 = 54.938
O: 4 × 15.999 = 63.996
M(KMnO₄) = 39.098 + 54.938 + 63.996 = 158.032 g/mol
Worked example 2 — molar mass of calcium carbonate, CaCO₃
Look up: Ca = 40.078, C = 12.011, O = 15.999 (all g/mol).
Ca: 1 × 40.078 = 40.078
C: 1 × 12.011 = 12.011
O: 3 × 15.999 = 47.997
M(CaCO₃) = 40.078 + 12.011 + 47.997 = 100.086 g/mol
Worked example 3 — molar mass of a hydrated salt, MgSO₄·7H₂O
Look up: Mg = 24.305, S = 32.06, O = 15.999, H = 1.008 (all g/mol).
Anhydrous MgSO₄: Mg 24.305 + S 32.06 + O (4 × 15.999 = 63.996) = 24.305 + 32.06 + 63.996 =
120.361
Water of crystallisation: 7 × (2 × 1.008 + 15.999) = 7 × 18.015 = 126.105
M(MgSO₄·7H₂O) = 120.361 + 126.105 = 246.466 g/mol
This is very close to the accepted molar mass of Epsom salt — a good sign that the atomic masses were read correctly and the water of crystallisation was multiplied in fully.
Worked example 4 — using position to predict a common ion charge
Magnesium (Mg, atomic number 12) sits in Group 2 of the periodic table. Elements in Group 2 (the alkaline earth metals) characteristically lose their two outermost electrons to form a stable, noble-gas-like electron arrangement.
Predicted common ion: Mg²⁺ — which is exactly the ion found in compounds like MgSO₄ and MgCl₂.
This kind of prediction works reliably for the main-group metals on the left of the table (Groups 1 and 2). It becomes unreliable for the transition metals in the middle block, which commonly show more than one oxidation state — do not extend the same shortcut to them without checking.
Common mistakes that cost marks
- Reading the atomic number where the atomic mass was needed — the atomic number is a small whole number; the atomic mass has decimals and is always larger (except for hydrogen, where they are close).
- Using a rounded mass number instead of the true atomic mass — for example, using Cl = 35 instead of the correct weighted average, 35.45 g/mol.
- Forgetting to multiply the water of crystallisation by its own subscript in a hydrated-salt formula, as in Example 3.
- Assuming every element predicts its ion charge from group number alone — true for most main-group elements, not reliable for transition metals.
Where this is tested
| Exam / topic | Typical use |
|---|---|
| CBSE Class 10 — Periodic Classification of Elements | Reading element position and predicting basic properties |
| CBSE Class 11 — Classification of Elements and Periodicity | Deeper trend-based reasoning using period and group position |
| Stoichiometry and molar-mass problems at every level | Looking up atomic masses quickly and accurately |
| GATE / CSIR-NET numerical-heavy papers | Fast, reliable atomic-mass lookup under time pressure |
Look it up as you work. The Interactive Periodic Table lists all 118 elements with their atomic numbers and atomic masses, ready to use in any molar-mass or stoichiometry calculation.
Open the Interactive Periodic Table →