In the following we shall use this radius as a measure for the size of the real two-electron atom. The classical boundary of such an atom is given by We have shown that the best atomic orbitals for a two- electron atom in the ground state are equal to the Is orbital of a fictitious one- electron atom with nuclear charge +Zese. Putting all these facts together gives us the requested order S Se Fe K Rb. Selenium is the rightmost of the three, so it will be the smallest, followed by iron and then potassium. Finally, potassium, iron, and selenium are in the fourth period. Similarly, the positions of rubidium and potassium in Group 1 tell us that potassium is the smaller of the two. Sulfur and selenium are both in Group 16 because sulfur is above selenium, it will be smaller. Ītomic radius increases as we go from top to bottom or right to left in the periodic table. Then use the general trends for atomic size to place them in the requested order. Begin by finding each element in the periodic table. We want to arrange the elements from smallest to largest. Using only the periodic table, rank the following elements in order of increasing size Fe, K, Rb, S, and Se. We have already discussed the concept of The trend in moving across a row is less intuitive. ![]() ![]() The size of the valence orbitals increases with n, so the size of the atom increases as we go down the period. As we go down a group in the table, the first of these factors is at work. Two factors are important (1) the shell in which the valence electrons are found and (2) the strength of the interaction between the nucleus and the valence electrons. The size of the atom is determined largely by its valence electrons because they occupy the outermost orbitals.
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