The valence electrons of representative (“a” groups) elements are

The number of valence electrons in any atom of a Representative Element corresponds to its Group Number. All elements within a Group or Family, have the same number of electrons in their outermost energy levels. This outermost energy level is referred to as an element's valence shell,

Explanation:

the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.