Two hydrogen atoms form a bond with each other to share electrons so that each has the more stable electronic configuration of helium with 2 electrons. A bond where two atoms share electrons is called a covalent bond. The elements in the second row, lithium through fluorine, also form bonds so that they can attain the stable, filled-shell electron configuration of neon with 8 electrons.
Representing the Electronic ConfigurationThe electronic configuration of the second row (n = 2) elements is in the table. We can represent the valence electrons, those in the outermost shell, with dots. The filled shell orbital (1s2 for these elements) can't be shared and it is not involved in any chemical bonds. Each of the second row elements has 4 valence orbitals: 2s, 2px, 2py, and 2pz.
Lithium has a single electron in its 2s orbital (represented by the dot). Beryllium has 2 electrons in its 2s orbital, represented by 2 dots close together. Carbon has a pair of electrons in its 2s orbital (2 dots together) and 1 electron each in 2 of its 2p orbitals (2px and 2py, for example) represented by single dots.
Because there are 4 valence orbitals, some of the elements have another electron configuration that is only slightly higher in energy.
Bonds to HydrogenHydrogen can combine with all of the second role elements except neon to make molecules. The half-filled hydrogen 1s orbital can combine with any other half-filled orbitals of these elements. Neon doesn't have any half-filled orbitals. When electrons are shared between two atoms, they are included in the count of valence electrons for each.
Polar and Non-Polar BondsThe electrons in the bond between two atoms are shared but, in bonds between different atoms, they are not necessarily shared equally. In going from left to right across the second row of the periodic table, the elements have an increasingly greater tendency to pull the bonding electrons in to themselves. This tendency is called electronegativity.
Numbers are assigned to elements that indicate their relative electronegativity. Electronegativity is a periodic property that increases from left to right across a row and increases from bottom to top of a column.
When two bonded atoms differ in electronegativity by more than 0.4 units, we say that the bond is polar. The element with the lower electronegativity number will be more electron poor than the element with the higher number.