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  2. Periodic table - Wikipedia

    en.wikipedia.org/wiki/Periodic_table

    The chemical elements are what the periodic table classifies and organizes. Hydrogen is the element with atomic number 1; helium, atomic number 2; lithium, atomic number 3; and so on. Each of these names can be further abbreviated by a one- or two-letter chemical symbol; those for hydrogen, helium, and lithium are respectively H, He, and Li. [6]

  3. Atomicity (chemistry) - Wikipedia

    en.wikipedia.org/wiki/Atomicity_(chemistry)

    Atomicity is the total number of atoms present in a molecule of an element. For example, each molecule of oxygen (O 2) is composed of two oxygen atoms.Therefore, the atomicity of oxygen is 2.

  4. Abundance of the chemical elements - Wikipedia

    en.wikipedia.org/wiki/Abundance_of_the_chemical...

    Hydrogen is set to an abundance of 12 on this scale. The Sun's photosphere consists mostly of hydrogen and helium; the helium abundance varies between about 10.3 and 10.5 depending on the phase of the solar cycle; [13] carbon is 8.47, neon is 8.29, oxygen is 7.69 [14] and iron is estimated at 7.62. [15]

  5. Helium - Wikipedia

    en.wikipedia.org/wiki/Helium

    Hydrogen: Helium: Lithium: ... monatomic gas and the first in the noble gas group in the periodic table ... it moves at 20 meters per second at 1.8 K through helium ...

  6. Period (periodic table) - Wikipedia

    en.wikipedia.org/wiki/Period_(periodic_table)

    Most helium was formed during the Big Bang, but new helium is created through nuclear fusion of hydrogen in stars. [5] On Earth, helium is relatively rare, only occurring as a byproduct of the natural decay of some radioactive elements. [6] Such 'radiogenic' helium is trapped within natural gas in concentrations of up to seven percent by volume ...

  7. List of chemical elements - Wikipedia

    en.wikipedia.org/wiki/List_of_chemical_elements

    Like the periodic table, the list below organizes the elements by the number of protons in their atoms; it can also be organized by other properties, such as atomic weight, density, and electronegativity. For more detailed information about the origins of element names, see List of chemical element name etymologies.

  8. Alkali metal - Wikipedia

    en.wikipedia.org/wiki/Alkali_metal

    After 1869, Dmitri Mendeleev proposed his periodic table placing lithium at the top of a group with sodium, potassium, rubidium, caesium, and thallium. [28] Two years later, Mendeleev revised his table, placing hydrogen in group 1 above lithium, and also moving thallium to the boron group.

  9. Period 2 element - Wikipedia

    en.wikipedia.org/wiki/Period_2_element

    Lithium is the first alkali metal in the periodic table, [note 1] and the first metal of any kind in the periodic table. [note 2] At standard temperature and pressure, lithium is a soft, silver-white, highly reactive metal. With a density of 0.564 g⋅cm −3, lithium is the lightest metal and the least dense solid element. [6]