Without knowing the electronegativity values, in what situation can you be absolutely sure that the bonding between two atoms will be purely covalent?

Electronegativity is a chemical property describing how strongly an atom can attract or hold onto electrons when forming a bond. The most widely used scale for electronegativity was devised by Linus Pauling, in which values range from around 0.7 for elements like cesium to 4.0 for fluorine, the most electronegative element.

Understanding electronegativity is crucial because it helps predict how atoms will bond together. When there is a large difference in electronegativity between two atoms, an ionic bond is more likely to form. Conversely, a small difference, or no difference at all, indicates a covalent bond where electron sharing will occur. This understanding of electronegativity is what allows us to solve problems where we must identify if a bond is purely covalent without knowing the exact values.

Covalent bonding is a fundamental chemical interaction where atoms share pairs of electrons to achieve stability. This type of bond generally happens between non-metal atoms with similar electronegativity values since their 'desire' to attract electrons to themselves is about the same.

A perfectly covalent bond, also known as a nonpolar covalent bond, happens when the electronegativities are equal, leading to an equitable sharing of electrons. The concept is instrumental in understanding molecules' behavior and properties, such as in water (H2O) where the oxygen and hydrogen atoms are held together chiefly by polar covalent bonds.

When identical atoms bond together, such as in oxygen (O2) or nitrogen (N2) gas molecules, the bond is always purely covalent. This happens because each atom has the exact same electronegativity; thus, they exert an equal force on the shared electrons. There is no 'tug-of-war' over the shared electrons, which leads to an equal distribution of electron density across the molecule.

This equal distribution is critical in understanding properties such as why these elemental gases do not usually react with other elements or compounds unless provided with an extra push, such as high energy or a catalyst. These pure covalent bonds make the molecules relatively stable and nonreactive.

Electron sharing is the core mechanism at play in covalent bonding. Atoms share their electrons to fill their outermost shells, gaining stability similar to that of noble gases. The shared electrons occupy what is known as molecular orbitals that belong to both bonding atoms.

When two atoms with different electronegativities form a bond, the sharing is not equal—the more electronegative atom will have a greater electron density around it. Consequently, this leads to a polar covalent bond. However, in a pure covalent bond, the electron sharing is perfectly even, resulting in a nonpolar molecule where electrons are equally distributed.