By Kevin Carr – Updated Mar 24, 2022
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Atoms connect through chemical bonds by sharing electrons, a process governed by each element’s outermost electron count. The number of electrons available for sharing in the outer shell is what chemists call the valence.
Hydrogen, along with all elements in the first group of the periodic table, has a valence of one.
Valence electrons are the electrons residing in the highest energy level that can participate in chemical bonding. In covalent chemistry, these electrons are shared between atoms to satisfy the octet rule, which states that a stable configuration usually requires eight electrons in the outer shell. When an atom already has eight electrons, it behaves like a noble gas and remains inert. Atoms with fewer than eight electrons will share or transfer electrons to complete their octet.
Hydrogen is a special case: its 1s orbital can accommodate only two electrons. The single electron in this orbital is its valence electron, so hydrogen’s valence number is one. It can share this lone electron with another atom to fill its shell, or it can lose it to form a proton (H+) in acidic solutions. Because of this versatility, hydrogen bonds with many elements. For instance, four hydrogen atoms bond with one carbon atom (valence four) to form methane (CH4), and three hydrogen atoms combine with one nitrogen atom (valence five) to produce ammonia (NH3).
Hydrogen’s ability to share or donate its valence electron allows it to form both covalent and ionic bonds. It can donate its electron to electronegative atoms such as fluorine or chlorine, creating ionic species like HCl+. Hydrogen can also pair with itself to produce H2 molecules. In aqueous solution, hydrogen often loses its electron, becoming the hydron (H+) that contributes to a solution’s acidity.
Group 1 elements—including lithium, sodium, and potassium—also have a valence of one. Group 2 elements (beryllium, magnesium, calcium, strontium, barium) carry a valence of two. Transition metals (groups 3–12) display a range of valences from one to seven, depending on their electron configuration and oxidation state.
Group 13 atoms (boron, aluminum) have a maximum valence of three. Group 14 atoms (carbon, silicon, germanium) can reach a valence of four. Group 15 atoms (nitrogen, phosphorus, arsenic) max out at five. Group 16 atoms (oxygen, sulfur, selenium) attain up to six. Group 17 atoms (fluorine, chlorine, bromine) can reach seven. Noble gases (group 18) possess eight valence electrons but, because they are already stable, are considered to have a valence of zero.
