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According to Dalton's atomic theory, atoms are the smallest units of matter and are, in essence, indivisible. This means that during chemical reactions, atoms are not broken down into smaller pieces like protons, neutrons, or electrons. Dalton envisioned them as solid, hard spheres like marbles that cannot be further split into smaller parts. The word "indivisible" is crucial because, during Dalton's time, the idea of splitting an atom into smaller subatomic particles was unheard of. While modern science has shown that atoms are indeed made of smaller particles, Dalton's concept was pioneering at the time.

• Atoms were thought to be the most basic form of matter.
• All materials are composed of these indivisible atoms.
• Atoms retain their identity during chemical changes.

Dalton's atomic theory proposed that atoms of a specific element are identical in terms of their mass and chemical properties. This assumption suggested that, for example, all carbon atoms would be the same in weight and in the way they reacted with other elements. This was a groundbreaking idea because it explained why substances composed of the same element consistently showed the same physical and chemical behaviors.
While modern chemistry recognizes that there can be isotopes—atoms of the same element with different masses due to varying numbers of neutrons—Dalton's original concept helped lay the groundwork for understanding the consistency in the behavior of elements.

• Atoms of the same element are uniform in mass and properties.
• This uniformity explains the predictable reactions of elements.
• Isotopes are an exception, not identified in Dalton's original theory.

Dalton also explained how atoms participate in chemical reactions. At the core of his theory is the idea that compounds are created when atoms of different elements combine in specific and fixed ratios, such as water always consisting of two hydrogen atoms and one oxygen atom.
This concept helps explain why chemical reactions result in products with new properties rather than individual changes to the atoms themselves. The key is that although atoms combine or recombine into different structures, they do not transform into other types of atoms.

• Compounds are formed from specific ratios of different atoms.
• Atoms rearrange but don't change during chemical reactions.
• This idea explains the conservation of mass during reactions.