Molecular solids may consist of single atoms, diatomic, and/or polyatomic molecules. Although a diatomic molecule, HF forms relatively strong intermolecular hydrogen bonds. Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules. In such hydrides, hydrogen can exist as either atomic or diatomic entities. In the vapour phase, beryllium oxide is present as discrete diatomic molecules. Oxygen is an example of a paramagnetic diatomic. Phosphorus is also known as diphosphorus (P), an unstable diatomic allotrope. This is typical of the infrared spectra of heteronuclear diatomic molecules. The lowest excited state of the diatomic oxygen molecule is a singlet state. A diatomic molecular orbital diagram is used to understand the bonding of a diatomic molecule. It is highly electronegative and non-metallic, usually diatomic, gas down to very low temperatures. Unlike its lighter congener iodine, evidence for diatomic astatine is sparse and inconclusive. Under ordinary conditions on Earth, elemental hydrogen exists as the diatomic gas, H. The Morse potential is used to approximate the vibrational structure of a diatomic molecule. The bond in a homonuclear diatomic molecule is non-polar.