Similarly, as we proceed across the row, the increasing nuclear charge is not effectively neutralized by the electrons being added to the 2 s and 2 p orbitals. Consequently, beryllium is significantly smaller than lithium. This means that the effective nuclear charge experienced by the 2 s electrons in beryllium is between +1 and +2 (the calculated value is +1.66). (More detailed calculations give a value of Z eff = +1.26 for Li.) In contrast, the two 2 s electrons in beryllium do not shield each other very well, although the filled 1 s 2 shell effectively neutralizes two of the four positive charges in the nucleus. Thus the single 2 s electron in lithium experiences an effective nuclear charge of approximately +1 because the electrons in the filled 1 s 2 shell effectively neutralize two of the three positive charges in the nucleus. Although electrons are being added to the 2 s and 2 p orbitals, electrons in the same principal shell are not very effective at shielding one another from the nuclear charge. All have a filled 1 s 2 inner shell, but as we go from left to right across the row, the nuclear charge increases from +3 to +10. The atoms in the second row of the periodic table (Li through Ne) illustrate the effect of electron shielding. Contrast this with the atomic number, which is simply the number of protons. Mass number is often denoted using a capital letter A. In other words, it is the sum of the number of nucleons in an atom. The greater the effective nuclear charge, the more strongly the outermost electrons are attracted to the nucleus and the smaller the atomic radius.Ītomic radii decrease from left to right across a row and increase from top to bottom down a column. Mass number is an integer (whole number) equal to the sum of the number of protons and neutrons of an atomic nucleus. For all elements except H, the effective nuclear charge is always less than the actual nuclear charge because of shielding effects. \( \newcommand\)) experienced by electrons in the outermost orbitals of the elements.
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