Which statement about atoms of elements in the same group of the periodic table is true?
they have similar chemical properties and the same number of valence electrons. they have the same number of protons and the same mass.
they have the same number of energy levels and the same amount of reactivity. they have the same number of electrons and the same atomic radius size.

Option (a) is the correct answer.

Explanation:

Chemical properties are defined as the properties which tend to show change in chemical composition of a substance.

For example, toxicity, reactivity, combustion etc are all chemical properties.

Since, elements which belong to same period show similar chemical properties.

This is because elements that lie in the same group have same number of valence electrons. Hence, they tend to show same type of reactivity which makes them exhibit similar chemical properties.

Thus, we can conclude that the statement they have similar chemical properties and the same number of valence electrons, is true about atoms of elements in the same group of the periodic table.

answer: the enthalpy of the overall chemical reaction is +87.88 kj or +88 kj.

according to hess's law, the overall enthalpy of a reaction is the difference between the sum of enthalpy values of all the products and the sum of enthalpy values of all the reactants.

δh(reaction) = ∑nδh(products) - ∑nδh(reactants)

δh(reaction) = [δh +δh] - [δh]

δh(reaction) = [1(-287.02) + 1(0) ] - [1(-374.9)] kj = +87.88 kj

here, δh (pcl3) = -287.02 kj, δh(cl2) = 0, δh(pcl5) = -374.9 kj

therefore, enthalpy of the overall reaction is +87.88 kj or +88 kj.