---
id: science/atoms-elements-periodic-table
title: "Atoms, elements and the periodic table"
license: CC BY 4.0
license_url: https://creativecommons.org/licenses/by/4.0/
attribution: "Content adapted from Chemistry (1st edition) by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, OpenStax, CC BY 4.0. Access for free at https://openstax.org/books/chemistry/pages/1-introduction"
condensed_by: "iinventedlight (Grok Bot, AI-assisted)"
source: https://openstax.org/books/chemistry/pages/preface
source_license: CC BY 4.0
publisher: OpenStax (Rice University)
checked_on: 2026-10-24
updated: 2026-10-24
date: 2026-10-24
status: reviewed
reviewed_on: 2026-10-24
category: science
gap: "demand map 2026-10-23: basic science explainer (atoms, elements, periodic table)"
license_gate: "passed: OpenStax Chemistry (1st edition) preface states CC BY 4.0 International, checked 2026-10-24; source approved by Cole 2026-10-24: \"Yes use the OpenStax chem textbook\""
sources:
  - id: S1
    title: "Chemistry (1st edition), Ch. 2 sections 2.1, 2.3 and 2.5"
    authors: "Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson (OpenStax)"
    publisher: OpenStax (Rice University)
    url: https://openstax.org/books/chemistry/pages/1-introduction
    accessed: 2026-10-24
    license: "CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)"
    attribution: "Content adapted from Chemistry (1st edition) by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, OpenStax, CC BY 4.0. Access for free at https://openstax.org/books/chemistry/pages/1-introduction"
  - id: P4
    title: "Periodic table (Wikipedia)"
    publisher: Wikimedia Foundation
    url: https://en.wikipedia.org/wiki/Periodic_table
    accessed: 2026-10-24
    license: "CC BY-SA 4.0 - link only, not condensed"
---
# Atoms, elements and the periodic table

**Source and license:** Content adapted from Chemistry (1st edition) by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, OpenStax, CC BY 4.0. Access for free at [openstax.org/books/chemistry/pages/1-introduction](https://openstax.org/books/chemistry/pages/1-introduction). Licensed [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/). This page is an AI-assisted summary by iinventedlight in its own words, not the textbook itself.

## Summary
All matter is built from atoms. An atom is a tiny, dense, positive nucleus of protons and neutrons with electrons spread through the much larger space around it. The number of protons fixes which element an atom is. The periodic table lists the elements in order of that number, and its rows and columns group elements whose behaviour repeats.

## Facts
- id: F1
  claim: Greek thinkers such as Leucippus and Democritus proposed indivisible particles of matter around the fifth century BC, but only as a philosophical idea. Experimental atomic theory began with John Dalton in the early 1800s.
  where: https://openstax.org/books/chemistry/pages/2-1-early-ideas-in-atomic-theory
  sources: [S1]
  license: CC BY 4.0
- id: F2
  claim: "Dalton's main ideas: each element is made of one kind of atom with its own characteristic mass; compounds join atoms of different elements in small whole-number ratios; and chemical reactions rearrange atoms without creating or destroying them."
  where: https://openstax.org/books/chemistry/pages/2-1-early-ideas-in-atomic-theory
  sources: [S1]
  license: CC BY 4.0
- id: F3
  claim: Dalton's picture explains why a given compound always has the same proportions of its elements by mass (definite proportions), and why two elements forming several compounds do so in simple whole-number mass ratios (multiple proportions).
  where: https://openstax.org/books/chemistry/pages/2-1-early-ideas-in-atomic-theory
  sources: [S1]
  license: CC BY 4.0
- id: F4
  claim: An atom holds positive protons and neutral neutrons in its nucleus, with negative electrons around it. The nucleus carries nearly all the mass, while the electrons take up nearly all the volume. A nucleus is about 100,000 times narrower than its atom (roughly 10^-15 m versus 10^-10 m).
  where: https://openstax.org/books/chemistry/pages/2-3-atomic-structure-and-symbolism
  sources: [S1]
  license: CC BY 4.0
- id: F5
  claim: The atomic number Z counts the protons and defines the element. Every carbon atom has 6 protons, for example. The mass number A is protons plus neutrons, so the neutron count is A minus Z.
  where: https://openstax.org/books/chemistry/pages/2-3-atomic-structure-and-symbolism
  sources: [S1]
  license: CC BY 4.0
- id: F6
  claim: Atoms of the same element with different numbers of neutrons are isotopes, such as carbon-12 and carbon-13. They have nearly identical chemistry but different masses.
  where: https://openstax.org/books/chemistry/pages/2-3-atomic-structure-and-symbolism
  sources: [S1]
  license: CC BY 4.0
- id: F7
  claim: A neutral atom has as many electrons as protons. Gaining electrons gives a negative ion (anion), and losing them gives a positive ion (cation). The proton count, and so the element, stays the same.
  where: https://openstax.org/books/chemistry/pages/2-3-atomic-structure-and-symbolism
  sources: [S1]
  license: CC BY 4.0
- id: F8
  claim: Atomic masses are given in atomic mass units, where one amu is defined as exactly one twelfth of the mass of a carbon-12 atom (about 1.66 x 10^-24 g). The atomic mass shown for an element is an average over its naturally occurring isotopes, weighted by how common each one is.
  where: https://openstax.org/books/chemistry/pages/2-3-atomic-structure-and-symbolism
  sources: [S1]
  license: CC BY 4.0
- id: F9
  claim: Mendeleev (1869) and Meyer (1870) each arranged the known elements by mass and saw their properties repeat. Mendeleev left gaps and predicted elements that had not yet been found. Gallium and germanium, discovered later, matched his predictions.
  where: https://openstax.org/books/chemistry/pages/2-5-the-periodic-table
  sources: [S1]
  license: CC BY 4.0
- id: F10
  claim: "The modern periodic law: when elements are listed by increasing atomic number, their properties recur at regular intervals. Horizontal rows are periods, and the 18 vertical columns are groups. Elements in the same group behave alike."
  where: https://openstax.org/books/chemistry/pages/2-5-the-periodic-table
  sources: [S1]
  license: CC BY 4.0
- id: F11
  claim: Most elements are metals, which are shiny and conduct heat and electricity well and sit on the left and middle of the table. Nonmetals sit on the upper right. Metalloids, along the staircase between the two, have in-between properties.
  where: https://openstax.org/books/chemistry/pages/2-5-the-periodic-table
  sources: [S1]
  license: CC BY 4.0
- id: F12
  claim: "Some groups have common names: alkali metals (group 1 except hydrogen), alkaline earth metals (group 2), halogens (group 17) and noble gases (group 18). Groups 1, 2 and 13-18 are the main-group elements. Groups 3-12 are the transition metals, and the two rows usually printed below the table are the inner transition metals (lanthanides and actinides)."
  where: https://openstax.org/books/chemistry/pages/2-5-the-periodic-table
  sources: [S1]
  license: CC BY 4.0

## Pointers
- answers: "Overview and history of the periodic table"
  where: https://en.wikipedia.org/wiki/Periodic_table
  access: free (CC BY-SA 4.0; link only)
- answers: "Related library science notes"
  where: https://iinventedlight.com/library/reference/us-gov/why-sky-is-blue
  access: free (iinventedlight library)

## Licence
Text in this note: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), condensed by iinventedlight from Chemistry (1st edition) by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, OpenStax, https://openstax.org/books/chemistry/pages/1-introduction, CC BY 4.0. Changes: facts restated and condensed; no text, figures or problems copied.