Some elements— for example, helium and neon–consist of single atoms that are not connected to each other— that is, they are monatomic elements. In contrast, oxygen, in its most common form, contains two atoms in each molecule, connected to each other by a chemical bond. We write the formula for an oxygen molecule as O2, with the subscript showing the number of atoms in the molecule. Six other elements also occur as diatomic molecules (that is, they contain two atoms of the same element per molecule): hydrogen (H2), nitrogen (N2),fluorine (F2), chlorine (Cl2), bromine (Br2), and iodine(I2). It is important to understand that under normal conditions, free atoms of O, H, N, F, Cl, Br, and I do not exist. Rather, these seven elements occur only as diatomic elements.