molecular orbital
/ mo-LEK-yoo-ler OR-bih-tal /
You may already picture electrons in a single atom as living in fuzzy clouds called atomic orbitals — the 1s, the 2p, and so on. But what happens to those clouds when two atoms come close enough to bond? They do not just sit side by side. They merge into new clouds that wrap around the whole molecule. A molecular orbital is one of those new clouds: a region, shared by two or more nuclei, where a molecule's electrons are likely to be found.
More precisely, a molecular orbital is a mathematical function describing the wave-like behavior of an electron spread over a whole molecule rather than belonging to one atom. Just like atomic orbitals, each molecular orbital has a definite energy and can hold at most two electrons, with opposite spins. The electrons fill the molecular orbitals from lowest energy upward, exactly the way they fill atomic orbitals in a single atom. The result tells you how strongly the atoms are held together and what the molecule will do.
This idea is the heart of the most powerful general model of bonding in chemistry. Where the older Lewis-dot picture draws a bond as a static pair of dots stuck between two atoms, molecular orbital theory lets electrons spread out, delocalize, and occupy levels of different energy. That extra honesty is what lets it explain things Lewis structures get plainly wrong — most famously, why ordinary oxygen gas is magnetic.
In the simplest molecule, H2, the two 1s atomic orbitals combine into two molecular orbitals: a low-energy one that blankets both nuclei (the bonding orbital) and a high-energy one with a gap between the nuclei (the antibonding orbital). The two electrons drop into the lower one, and that shared, spread-out cloud is the H-H bond.
Two atomic orbitals always make two molecular orbitals — one stabilizing, one destabilizing.
A molecular orbital is a mathematical function, not a physical pipe or tube; the 'cloud' is just where the electron is likely to be, and the number of molecular orbitals always equals the number of atomic orbitals you started with.