Effective nuclear charge elements

FIGURE 1.47 The periodic variation in the atomic radii of the elements. The variation across a period can be explained in terms of the effect of increasing effective nuclear charge that down a group by the occupation of shells with inc reasing principal quantum number. 

SOLUTION The smaller member of a pair of isoelectronic ions in the same period will be an ion of an element that lies farther to the right in a period, because that ion has the greater effective nuclear charge. If the two ions are in the same group, the smaller ion will be the one that lies higher in the group, because its outermost electrons are closer to the nucleus. Check your answer against the values in Appendix 2C. 

All the elements in a main group have in common a characteristic valence electron configuration. The electron configuration controls the valence of the element (the number of bonds that it can form) and affects its chemical and physical properties. Five atomic properties are principally responsible for the characteristic properties of each element atomic radius, ionization energy, electron affinity, electronegativity, and polarizability. All five properties are related to trends in the effective nuclear charge experienced by the valence electrons and their distance from the nucleus. 

Any covalent bond between atoms of different elements is polar to some extent, because each element has a different effective nuclear charge. Each element has a characteristic ability to attract bonding electrons. This ability is called electronegativity and is symbolized by the Greek letter chi. When two elements have different electronegativity values, a bond between their atoms is polar, and the greater the difference (A. the more polar the bond. 

Based upon Slater s rules, we have found that the effective nuclear charge increases sharply between periods one and three and then stays at 2.20 for the rest of the alkali metal group. You may recall that the ionization energy for an element can be calculated by using the equation ... 

How does the effective nuclear charge affect the ionization energies of the elements in a period on the periodic table ... 

For most elements, subtracting the total number of inner-shell electrons from the nuclear charge provides a convenient estimate of the effective nuclear charge, as Figure 5.30 illustrates. 

Moving down any group, the effective nuclear charge generally stays the same. The effective nuclear charge for all group 1 elements, for example, is... 

An electron in the outermost occupied shell of which element experiences the greatest effective nuclear charge ... 

The same effects felt by the group 1A elements when a single electron is lost are felt by the group 2A elements when two electrons are lost. For example, loss of two valence-shell electrons from an Mg atom (Is2 2s2 2p6 3s2) gives the Mg2+ cation (Is2 2s2 2p6). The smaller valence shell of the Mg2+ cation and the increase in effective nuclear charge combine to cause a dramatic shrinkage. In the same way, a similar shrinkage is encountered whenever any of the metal atoms on the left-hand two-thirds of the periodic table is converted into a cation. 

---