According to the Lewis concept, a base is a species that can donate an electron pair .

• $BF_3$: The central boron atom has only 6 electrons in its valence shell (electron-deficient). Thus, it acts as a Lewis acid .
• $CF_4$: The central carbon atom has a complete octet and no lone pair of electrons. Hence, it behaves neither as a Lewis acid nor as a Lewis base.
• $SiF_4$: The central silicon atom has a complete octet but also possesses vacant d-orbitals. It can accept electron pairs to expand its octet, acting as a Lewis acid.
• $PF_3$: The central phosphorus atom has a complete octet and possesses one lone pair of electrons. Therefore, it can donate this lone pair and behave as a Lewis base.