History of the Periodic Table

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Triads Noun

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[trai-ads]

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Triads

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Groups of three chemically similar elements where the middle element's properties are an average of the other two.

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Example: This image incorrectly shows a musical triad on a piano, which is a group of three notes, instead of a chemical triad from the periodic table.

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Atomic Mass Noun

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[uh-tom-ik mas]

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Atomic Mass

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The mass of an atom, used by early scientists as the primary organizing principle for the periodic table.

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Example: This diagram shows where to find the atomic mass on a periodic table tile, using Oxygen as an example. The atomic mass is 15.999.

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Law of Octaves Noun

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[law uv ok-tivz]

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Law of Octaves

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The principle that chemical properties repeat for every eighth element when they are arranged by increasing atomic weight.

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Example: This table shows John Newlands' Law of Octaves, which arranged elements by atomic weight and found that their properties repeated every eighth element, like musical octaves.

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Valency Noun

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[vey-luhn-see]

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Valency

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The combining power of an element, which determines how it bonds with other elements in chemical compounds.

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Example: This diagram shows a Sodium atom, pointing to the single electron in its outermost shell. This is a valence electron, and its presence determines the element's valency.

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Atomic Number Noun

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[uh-tom-ik num-ber]

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Atomic Number

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The number of protons in an atom's nucleus, serving as the fundamental organizing principle of the modern periodic table.

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Example: This diagram of an oxygen atom shows 8 protons in the nucleus, which defines its atomic number as 8.

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Periodic Law Noun

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[peer-ee-od-ik law]

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Periodic Law

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The principle that the physical and chemical properties of elements are a repeating function of their atomic number.

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Example: This diagram shows how atomic size (radius) changes in a predictable pattern on the periodic table, which is a key example of the Periodic Law.

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Periodic Adjective

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[peer-ee-od-ik]

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Periodic

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A term describing a repeating pattern or recurrence of properties that happens at regular, predictable intervals.

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Example: This image shows a repeating wave pattern, a type of periodic behavior, but it is unrelated to the periodic properties of chemical elements.

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Valence Electrons Noun

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[vey-luhns ih-lek-trons]

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Valence Electrons

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The electrons located in the outermost energy level of an atom that determine its chemical properties and reactivity.

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Example: This Bohr model of a Sodium atom shows its electrons in shells. The single electron in the outermost shell is its valence electron.

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Periods Noun

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[peer-ee-uhds]

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Periods

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The horizontal rows of the periodic table, where all elements within a row have the same number of principal energy levels.

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Example: This diagram shows the basic outline of the periodic table, identifying the horizontal rows as 'Periods' to explain the table's fundamental structure.

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Principal Energy Levels (PEL) Noun

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[prin-suh-puhl en-er-jee lev-uhls]

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Principal Energy Levels (PEL)

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The main energy shells occupied by an atom's electrons, with the number of levels corresponding to an element's period.

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Example: This diagram shows electrons orbiting an atom's nucleus in distinct paths called electron shells, which represent the principal energy levels.