One-Sentence Summary
The first 20 elements, covering hydrogen through calcium across three short periods, form the foundational dataset for teaching atomic structure and periodicity in chemistry.
Dataset: Atomic Numbers 1–20
| Atomic number | Symbol | Element name | Standard atomic weight (u) | Group | Period |
|---|---|---|---|---|---|
| 1 | H | Hydrogen | 1.008 | 1 | 1 |
| 2 | He | Helium | 4.0026 | 18 | 1 |
| 3 | Li | Lithium | 6.94 | 1 | 2 |
| 4 | Be | Beryllium | 9.0122 | 2 | 2 |
| 5 | B | Boron | 10.81 | 13 | 2 |
| 6 | C | Carbon | 12.011 | 14 | 2 |
| 7 | N | Nitrogen | 14.007 | 15 | 2 |
| 8 | O | Oxygen | 15.999 | 16 | 2 |
| 9 | F | Fluorine | 18.998 | 17 | 2 |
| 10 | Ne | Neon | 20.180 | 18 | 2 |
| 11 | Na | Sodium | 22.990 | 1 | 3 |
| 12 | Mg | Magnesium | 24.305 | 2 | 3 |
| 13 | Al | Aluminium | 26.982 | 13 | 3 |
| 14 | Si | Silicon | 28.085 | 14 | 3 |
| 15 | P | Phosphorus | 30.974 | 15 | 3 |
| 16 | S | Sulfur | 32.06 | 16 | 3 |
| 17 | Cl | Chlorine | 35.45 | 17 | 3 |
| 18 | Ar | Argon | 39.948 | 18 | 3 |
| 19 | K | Potassium | 39.098 | 1 | 4 |
| 20 | Ca | Calcium | 40.078 | 2 | 4 |
Field Descriptions
- Atomic number: the number of protons in the nucleus, which determines the element.
- Symbol: the internationally used element symbol, derived from the element name (e.g., Na comes from the Latin name Natrium).
- Standard atomic weight: the weighted average atomic mass of naturally occurring isotopes as published by IUPAC, in units of u (daltons).
- Group: numbered 1–18 per IUPAC; elements in the same group share similar chemical properties.
- Period: the horizontal row an element occupies; the period number equals the number of electron shells.
Key Notes on the Dataset
- Period 1 contains only two elements, hydrogen and helium; Periods 2 and 3 each contain eight.
- Groups 1–2 (alkali/alkaline earth metals) and Groups 13–18 (main group) exhibit the typical main-group patterns.
- Atomic weights do not strictly increase with atomic number (e.g., Ar 39.95 > K 39.10), due to differences in isotopic abundance.
- This dataset lists standard atomic weights; individual elements (such as lithium) have a normal range of variation because of differences in isotopic sources.
Sources and References
- International Union of Pure and Applied Chemistry (IUPAC) standard atomic weights: ciaaw.org
- U.S. National Institute of Standards and Technology (NIST) element data: physics.nist.gov
- PubChem periodic table: pubchem.ncbi.nlm.nih.gov