Valence electrons in the context of Pauling scale

A block of the periodic table is a set of elements unified by the atomic orbitals their valence electrons or vacancies lie in. The term seems to have been first used by Charles Janet. Each block is named after its characteristic orbital: s-block, p-block, d-block, f-block and g-block.

The block names (s, p, d, and f) are derived from the spectroscopic notation for the value of an electron's azimuthal quantum number: sharp (0), principal (1), diffuse (2), and fundamental (3). Succeeding notations proceed in alphabetical order, as g, h, etc., though elements that would belong in such blocks have not yet been found.

Carbon-based life is the concept that carbon is the core element for all known forms of life, and a core component of all complex biological molecules with carbon representing approximately 45–50% of all dry biomass. Complex biological molecules consist of carbon atoms bonded with other elements, in particular oxygen and hydrogen but frequently also nitrogen, phosphorus, and sulfur (collectively known as CHNOPS). In these molecules carbon's four valence electrons and subsequent propensity to form four bonds lends to its function as the "skeleton" of organic molecules. In addition to molecular level properties of carbon, life on Earth also relies on large scale processes known as the carbon cycle, involving geological and atmospheric processes.

While is it widely believed that any life found elsewhere in the universe will most likely also be carbon-based, silicon and boron have been discussed as alternatives because of their abilities to form 4 or 5 bonds respectively. These possibilities have resulted in science-fiction literature stories that include life based silicon or boron or other alternative elements.

Electronegativity, symbolized as χ, is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the distance at which its valence electrons reside from the charged nucleus. The higher the associated electronegativity, the more an atom or a substituent group attracts electrons. Electronegativity serves as a simple way to quantitatively estimate the bond energy, and the sign and magnitude of a bond's chemical polarity, which characterizes a bond along the continuous scale from covalent to ionic bonding. The loosely defined term electropositivity is the opposite of electronegativity: it characterizes an element's tendency to donate valence electrons.

On the most basic level, electronegativity is determined by factors like the nuclear charge (the more protons an atom has, the more "pull" it will have on electrons) and the number and location of other electrons in the atomic shells (the more electrons an atom has, the farther from the nucleus the valence electrons will be, and as a result, the less positive charge they will experience—both because of their increased distance from the nucleus and because the other electrons in the lower energy core orbitals will act to shield the valence electrons from the positively charged nucleus).