Regarding the representative elements: (a) What does knowing the group number tell you about an element's valence electrons? (b) What does knowing the period number tell you about an element's valence electrons?

Before diving into the exercise's questions, let's briefly understand the concept of valence electrons and the periodic table. The valence electrons are the electrons in an atom's outermost shell, which contribute to the chemical properties of the element. The periodic table is a tabular arrangement of the chemical elements organized based on their atomic number, electron configurations, and recurring chemical properties. Now, let's discuss how the group number and period number relate to an element's valence electrons.

The group number in the periodic table represents the vertical columns. All elements in a group of representative elements or main groups have the same number of valence electrons. The group number of representative elements (1A to 8A or 1 to 18) provides information about the number of valence electrons in those elements. For example, for Group 1A (1) or alkali metals, all elements in this group have one valence electron in their outermost shell. Similarly, elements in Group 2A (2) or alkaline earth metals have two valence electrons, and so on until Group 8A (18) or noble gases have eight valence electrons (except helium, which has only two valence electrons). So, knowing the group number tells us about the number of valence electrons an element possesses.

The period number in the periodic table represents the horizontal rows. Elements in the same period have the same number of electron shells, where valence electrons reside. As the period number increases from 1 to 7, elements in the same period have the same number of electron shells (or energy levels). Valence electrons always occupy the outermost shell of the atom. So, knowing the period number tells us about the energy level in which valence electrons are present. In summary, the period number informs us about which electron shell the valence electrons are found in an atom.