Predict the valency of A atomic number 5

atomic number = number of protons

number of protons = number of electrons

So, number of electrons = 5

So, electron configuration = 2, 3

So, its valency = 3

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

each chemical family is represented as a group or column on the periodic table. based on chemical properties, there are 6 types of chemical families. group 1 represents alkali metals, group 2 represents alkaline earth metals, groups 3 to 12 represent transition metals, groups 13 to 16 represent non-metals and other metals, group 17 represents halogens and group 18 represents noble gases.