Elements 11–20 on the periodic table

What changes and what stays fixed

The atomic number defines the element through the number of protons in its nucleus. Isotopes have different numbers of neutrons but remain the same element. An ion has gained or lost electrons without changing the proton count. Those distinctions explain why the sequence stays stable even when an atom’s mass or charge changes. Atomic weights and electron configurations are useful extra properties, but they are not alternative keys for sorting this particular ten-element interval.

The third period and the start of the fourth

Sodium through argon form the third period of the table. Potassium and calcium begin the next period. The row break is part of the periodic organization rather than a gap in atomic numbering. The sequence contains metals, nonmetals and a noble gas, showing why adjacent atomic numbers do not imply identical chemical behaviour. Silicon’s position also provides a reminder that everyday material names and elemental identities need to be distinguished: compounds containing an element are not the pure element itself.

Names and symbols in the game

Sodium uses Na and potassium uses K, so guessing a symbol by taking the first English letter would not work reliably. The game accepts the standard symbols and common English spelling variants, including aluminium and aluminum. Sulfur and sulphur likewise identify one element rather than separate answers. IUPAC’s table is the reference for official symbols and atomic numbers. Use the element number printed with the symbol, rather than the weight value nearby. The interval stops at calcium; a later element may be familiar, but familiarity does not put it inside the defined atomic-number range.

Sources & further reading

• IUPAC: May 2022 periodic table
• Royal Society of Chemistry: periodic table

Explanation updated Thursday, October 1, 2026. The puzzle’s original data date is listed below.