How many of the following are Lewis acids?CCl4, SiF4, Se, Mg2+,BF3, SO2, SO42-, C2H4

To determine which of the given substances are Lewis acids, we need to understand what a Lewis acid is. A Lewis acid is a chemical species that can accept a pair of electrons.

1. CCl4: Carbon tetrachloride is not a Lewis acid because it does not have any vacant orbitals to accept electron pairs.

2. SiF4: Silicon tetrafluoride is a Lewis acid because silicon can expand its octet and accept a pair of electrons in its vacant 3d orbital.

3. Se: Selenium is not a Lewis acid because it does not have any vacant orbitals to accept electron pairs.

4. Mg2+: Magnesium ion is a Lewis acid because it has a positive charge and can accept a pair of electrons to achieve a stable electron configuration.

5. BF3: Boron trifluoride is a Lewis acid because boron has a vacant orbital which can accept a pair of electrons.

6. SO2: Sulfur dioxide is not a Lewis acid because it does not have any vacant orbitals to accept electron pairs.

7. SO42-: Sulfate ion is not a Lewis acid because it has a negative charge and does not have any vacant orbitals to accept electron pairs.

8. C2H4: Ethene is not a Lewis acid because it does not have any vacant orbitals to accept electron pairs.

So, out of the given substances, SiF4, Mg2+, and BF3 are Lewis acids.