1) A particle diagram with two circles representing two different elements, each with a double bond between the atoms.
2) A particle diagram with two circles representing the same element, each with a double bond between the atoms.
In both cases, the double bond represents the covalent bond between the two atoms, and the different circles represent the different elements. For example, a particle diagram with two circles, one red and one blue, each with a double bond, would represent a mixture of oxygen (O2) and nitrogen (N2) gases.
