Atoms are a lot like people. Sometimes they're happy being loners, but most of the time they prefer to bond with others. So why do most atoms prefer to bond rather than ride solo?
Atoms want to reach the most stable or lowest energy state they can. Every atom of a pure unbonded element is electrically neutral because it has the same number of protons as it does electrons. Usually, this isn't their most stable arrangement.
Atoms are stable When their atomos shell, called their valence shell, is completely filled with electrons. For many atoms, it takes eight electrons to fill it. This is called the octet rule. Some exceptions to the octet rule are hydrogen and helium.
They only need two. If atoms don't have a full valence shell, they'll get one by losing, gaining, or sharing valence electrons through chemical bonds. In an ionic bond, there is a transfer of electrons from a metal to a non-metal, creating oppositely charged ions. The electrostatic attraction that occurs between these oppositely charged ions is called an ionic bond.
Table salt, sodium chloride, is held together by ionic bonds. Sodium, a metal, has one valence electron. Chlorine, a nonmetal, has seven.
Nonmetals have higher electronegativity values, so they pull away the metal's valence electrons. It's an unfair game of tug of war, where the nonmetal dominates every time. Chlorine is a stronger opponent, so it pulls away sodium's valence electron to fill its own valence shell. Sodium's second shell, which already has eight valence electrons, now acts as its outermost shell.
Now the sodium atom becomes a positively charged ion called a cation, the chlorine atom becomes a negatively charged ion called an anion, and since opposite charges attract, the atoms bond together and form the ionic compound sodium chloride. Unlike covalent molecules, ionic compounds don't exist as individual units. They form these repeating three-dimensional structures called crystal lattices. This attraction between ions is so strong that they can bond into larger structures, like the beans of selenite found in Mexico's Cave of Crystals.
Covalent bonds are formed when atoms share electrons. The pair of shared electrons forms a new orbital that extends around the nuclei of both atoms. producing a molecule.
Covalent bonds occur between non-metals since they have the same or similar electronegativity values. In an H2 molecule, both atoms have the same electronegativity values. It's like hydrogen is facing a clone of itself in a tug-of-war match, so it ends in a tie. The shared pair of electrons split their time evenly between the valence shell of each hydrogen atom, giving both atoms a valence shell that's complete. When the electrons are shared evenly, it's called a nonpolar covalent bond.
In an HCl molecule, chlorine has the higher electronegativity value, making it the stronger opponent. Chlorine isn't quite strong enough to completely pull the electron over to its side, but the electron pair spends more of its time on chlorine's side. This uneven sharing of the electron pair causes chlorine to carry a partial negative charge and hydrogen to carry a partial positive charge.
When the electrons are shared unequally, it's called a polar covalent bond. Different types of atoms can form different numbers of covalent bonds. Hydrogen can form one, oxygen two, nitrogen three, and carbon four. The number of bonds a single atom can form is pretty small.
However, they can link together with other atoms to form more complex molecules, like the caffeine in your coffee, which gives you that extra boost in the morning. Thanks to the relationships that form between atoms, an amazing array of compounds are formed from just about 90 naturally occurring elements. Compounds that are essential to the existence of life itself, and the incredible diversity found on our planet and potentially beyond.