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  2. Helium - Wikipedia

    en.wikipedia.org/wiki/Helium

    Helium (from Greek: ἥλιος, romanized: helios, lit. 'sun') is a chemical element; it has symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table.

  3. List of chemical elements - Wikipedia

    en.wikipedia.org/wiki/List_of_chemical_elements

    A chemical element, often simply called an element, is a type of atom which has a specific number of protons in its atomic nucleus (i.e., a specific atomic number, or Z). [ 1 ] The definitive visualisation of all 118 elements is the periodic table of the elements , whose history along the principles of the periodic law was one of the founding ...

  4. Periodic table - Wikipedia

    en.wikipedia.org/wiki/Periodic_table

    Elements are placed in the periodic table according to their electron configurations, [38] the periodic recurrences of which explain the trends in properties across the periodic table. [39] An electron can be thought of as inhabiting an atomic orbital, which characterizes the probability it can be found in any particular region around the atom.

  5. Helium atom - Wikipedia

    en.wikipedia.org/wiki/Helium_atom

    A helium atom is an atom of the chemical element helium. Helium is composed of two electrons bound by the electromagnetic force to a nucleus containing two protons along with two neutrons, depending on the isotope , held together by the strong force .

  6. Helium compounds - Wikipedia

    en.wikipedia.org/wiki/Helium_compounds

    The helium atom is small with the radius of the outer electron shell at 0.29 Å. [2] Helium is a very hard atom with a Pearson hardness of 12.3 eV. [3] It has the lowest polarizability of any kind of atom, however, very weak van der Waals forces exist between helium and other atoms.

  7. 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. [1] In older contexts, atomicity is sometimes equivalent to valency. Some authors also use the term to refer to the maximum number of valencies observed ...

  8. Atom - Wikipedia

    en.wikipedia.org/wiki/Atom

    The number of protons in an atom (which Rutherford called the "atomic number" [27] [28]) was found to be equal to the element's ordinal number on the periodic table and therefore provided a simple and clear-cut way of distinguishing the elements from each other. The atomic weight of each element is higher than its proton number, so Rutherford ...

  9. List of elements by atomic properties - Wikipedia

    en.wikipedia.org/wiki/List_of_elements_by_atomic...

    This is a list of chemical elements and their atomic properties, ordered by atomic number (Z). Since valence electrons are not clearly defined for the d-block and f-block elements, there not being a clear point at which further ionisation becomes unprofitable, a purely formal definition as number of electrons in the outermost shell has been used.