Question

Why are some bonds ionic and some covalent?

Short answer

The type of bond formed between atoms depends on their relative electronegativities. When there is a large electronegativity difference, an ionic bond forms through the transfer of electrons, typically between metals and non-metals. If the electronegativity difference is small, a covalent bond forms through the sharing of electrons, usually between non-metal elements with similar electronegativities.

Step-by-step solution

Introduction to Bonds

Atoms can interact with each other in order to achieve a more stable state with a full outer electron shell, and this interaction can either involve the transfer of electrons (forming ionic bonds) or the sharing of electrons (forming covalent bonds). The type of bond that forms between atoms depends largely on their respective electronegativities.

Electronegativity

Electronegativity is the measure of the attraction an atom has for electrons in a chemical bond. In general, atoms with larger electronegativities tend to strongly attract electrons, while atoms with smaller electronegativities tend to have weaker attraction for electrons.

Ionic Bonds

Ionic bonds occur when there is a large electronegativity difference (\(\Delta EN\)) between the two bonding atoms. This causes one atom to "steal" electrons from the other, resulting in a transfer of electrons and the formation of charged species called ions. The positively charged ion (cation) and negatively charged ion (anion) are then attracted to each other, forming an ionic bond. Typical ionic bonds occur between metals (with lower electronegativities) and non-metals (with higher electronegativities).

Covalent Bonds

Covalent bonds occur when the electronegativity difference between the two bonding atoms is small, which means that both atoms have a similar attraction for electrons. In this case, sharing electrons between the atoms allows both to achieve a full outer electron shell, resulting in a more stable bond. Covalent bonds usually occur between non-metal elements with similar electronegativities.

Recap

To summarize, the type of bond formed between atoms depends on their relative electronegativities. If there is a large electronegativity difference between the atoms, an ionic bond will form through the transfer of electrons. If the electronegativity difference is small, a covalent bond will likely form through the sharing of electrons.