Three electron(s) are removed from a silicon atom. Enter the chemical symbol of the element.

Na (sodium)

Explanation:

An atom, which is the smallest indivisible particle of an element, contains three sub-atomic particles namely: proton, electron, and neutron. An atom is said to be neutrally charged if it contains a charge 0.

In a neutral atom, the number of electrons and the number of protons in an element are equal. These numbers are called the ATOMIC NUMBER. That is, in a neutral atom;

proton no. = electron no. = Atomic no.

Hence, in this case where Silicon atom has an electron number of 14. If three of this electrons are removed, there will be 11 electrons left, hence, this atom becomes SODIUM element, which has 11 electrons in its atom, with chemical symbol Na.

pure substance: iron,copper

impure substance: water,charcoal

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.