In reality, there is no chemical compound with the formula Se2O4. SeO2 (Selenium Dioxide) and SeO3 (Selenium Trioxide) are two common chemicals that should be well understood.
Specifically, SeO2 is commonly known as Selenium Dioxide, its English name is also Selenium Dioxide. The atomic mass of Selenium is 78.96 (g/mol), and for Oxygen, it is 15.999 (g/mol). Therefore, the molecular mass of SeO2 is 78.96 + 2*15.999 = 110.958 (g/mol). The molecular structure of SeO2 consists of 1 Selenium atom and 2 Oxygen atoms.
In terms of properties, SeO2 is a white solid with a distinctive smell. Under standard conditions, this substance can turn into pale yellow steam. Selenium Dioxide has a neutral pH and is a strong oxidizer.
SeO2 is often prepared in the lab by heating Selenium in air.
Industrially, SeO2 is usually produced by burning Selenium or synthesized from Selenite or Selenide.
For SeO3, the formula is similar, except that there are 3 Oxygen atoms. SeO3 is very toxic and easily flammable. It can be synthesized from SeO2 by reacting with O2.
However, if you see the formula Se2O4 somewhere, please check the source of information before using it.