The periodic table of elements is structured in increasing atomic number. Atomic number of an element refers to the number of protons in an atom of that element. Each element is followed by the next element with one more proton than itself.

Increasing numbers of protons often correspond to atomic mass, but this trend does not hold true in all cases due to variation in isotope variation and neutron addition. For example, the atomic mass of nickel is slightly less than that of cobalt and the atomic mass of hassium is slightly less than that of bohrium.

The periodic table of elements contains the elements that have been deemed as elements by scientists as a collective whole. Although there are atoms with higher masses than , none of them have been stable enough to be considered elements. , however, is not completely stable, and complete stability is not a requirement to be considered an element.

The atomic radius decreases as we move from left to right across a period (row) in the periodic table for a couple reasons. Each time we move from one element to the right, we add a proton and an electron (and a certain number of neutrons depending on the isotope). Because there are no new electron shells present, this extra electron must be added to the same valence shell of this atom. The added positive charge from the proton in the nucleus and the added negative charge from the added electron are opposites, and are thus attracted to each other. The greater the number of each, the greater their pull on one another, and thus the smaller the radius of the atom.

The noble gases have a stable octet of electrons in their outer shell and tend to neither lose nor gain electons. They tend to not react with other chemicals.

Alkali metals and halogens are highly reactive; alikali metals need to lose a single electron to reacha a stable octet, while halogens need to gain a single electron. Alkaline earth metals are reactive as well, needing to lose two electrons. Transition metals vary in their valence electron numbers and organizations, but are still considered more reactive than the noble gases.

As the atomic number increases in a period, the number of protons increases and creates a stronger electrostatic pull on the electrons. The positive protons in the nucleus attract the negative electrons in the electron cloud. As electrons are added across a pariod, they are added into the same orbital shell (as determined by the principle quantum number of the atom). Since the shell does not change, the radius of the atom does not increase. Rather, the attractive force of the protons acts to decrease the radius of the electron cloud.

The amount of protons in an element is represented by its atomic number. The periodic table is listed in order of atomic number. A smaller atomic number equates to a fewer amount of protons in an element. It is possible to identify the amount of protons for all of the elements listed with the use of a periodic table. 

Calcium has a total of two electrons in the outer electron shell. It is possible to determine this by looking at the column on the periodic table in which calcium is located. Calcium is in group two, and all elements located in group two have two electrons in the outer electron shell. Of the elements listed as answers, only barium shares the same group. All elements in the same group have the same amount of electrons in the outer shell.