The molecules that contain an atom with an expanded octet are:
a. SeO2F2
b. ONF3
c. XeF2
An expanded octet occurs when an atom in a molecule has more than eight electrons in its valence shell. Let's analyze each molecule:
SeO2F2 - Selenium (Se) is in Group 16 and can have an expanded octet. In this molecule, selenium has 10 electrons around it (two lone pairs and three bonding pairs). So, it contains an atom with an expanded octet.
ONF3 - Oxygen (O) is in Group 16 and usually does not exceed an octet. Nitrogen (N) is in Group 15 and can exceed an octet. In this molecule, nitrogen has 10 electrons around it (one lone pair and four bonding pairs). So, it contains an atom with an expanded octet.