Which of the following sets of elements have the same number of valence electrons? P, S, Cl because they are in the same period

B, Al, Ga because they are in the same group

Na, Ca, Ba because they are in the first two groups

Ag, Cd, Ar because they are transition elements and a noble gas

The following sets of elements have the same number of valence electrons : B, Al, Ga because they are in the same group

In an atom, there are levels of energy in the shell and subshell  

This energy level is expressed in the form of electron configurations.  

Valence electrons are electrons used in a chemical bond , and in an atom's outermost energy level  

The periodic system of elements is an arrangement of elements classified based on similar properties and their atomic number order.  

The periodic system is divided into groups (vertical order) and periods (horizontal order)  

Tn the periodic system

In one group will have the same number of valence electrons (except the He element in group VIIIA and the elements in the transition group), while in one period will have the same number of shells

B, Al, Ga because they are in the same group

Electron configuration :

₅B = 2 3

₁₃Al = 2 8 3

₃₁Ga = 2 8 18 3

₄₉In = 2 8 18 18 3

₈₁Tl = 2 8 18 32 18 8 3

The configuration above shows that the valence electrons from group 3 are the same, i.e. 3 electrons

altitude, density, pressure

air gets less dense the higher the altitude and also the air pressure is less as altitude rises.

the seeing the electronic configuration we can say that the element is fluorine.

fluorine is the most electronegative element in the periodic table.

but by only seeing the configuration we cannot say about its reactivity because we don't know if its configuration after excitation or by addition of electrons.

but if not then, it is the lightest halogen and exists as a highly toxic pale yellow diatomic gas at standard conditions. as the most electronegative element, it is extremely reactive: almost all other elements, including some noble gases, form compounds with fluorine.