Atomic number effective nuclear charge

As well as being attracted to the nucleus, each electron in a many-electron atom is repelled by the other electrons present. As a result, it is less tightly bound to the nucleus than it would be if those other electrons were absent. We say that each electron is shielded from the full attraction of the nucleus by the other electrons in the atom. The shielding effectively reduces the pull of the nucleus on an electron. The effective nuclear charge, Z lle, experienced by the electron is always less than the actual nuclear charge, Ze, because the electron-electron repulsions work against the pull of the nucleus. A very approximate form of the energy of an electron in a many-electron atom is a version of Eq. 14b in which the true atomic number is replaced by the effective atomic number ... 

The equations for nuclear reactions are balanced using the same methods developed for chemical reactions. Charge, mass, and atomic number are conserved. The electron is assigned an effective atomic number Z = — 1 for this purpose. 

Increases atomic radius, nuclear charge, ionic size, shielding effect, atomic number (number of protons, number of electrons), covalent character of halides, acidity of oxides... 

So, let us concentrate on the matter of the fitting of mathematical expressions to the numerical data for the radial distributions for the electrons in many-electron atoms. Maybe it is appropriate to use an effective atomic number, Zes, rather than the full Z in the equations in Table 1.1. Such a change is consistent with the view that the other electrons screen some, but not all, of their nuclear protonic charges in many-electron atoms. 

The effect of varying nuclear charge on ionic radii is seen in the variation in radius in an isoelectronic series of ions. The term isoekctronic means that flie ions possess the same number of electrons. For example, each ion in the series 0, F , Na, Mg, and Al has 10 electrons. The nuclear chaige in this series increases steadily in the order listed. (Recall that the charge on the nucleus of an atom or monatomic ion is given by the atomic number of the element.) Because the number of electrons remains constant, the radius of the ion decreases with... 

SOLUTION Each subshell in Ne experiences a different amount of shielding and therefore has a different characteristic Zeff. The effective atomic number represents the apparent shift in the nuclear charge felt by one electron because of the shielding of the other electrons. Therefore, the effective nuclear charge is... 

The actual atomic number is 10. The Is electrons are only slightly shielded from the nuclear charge, whereas the n = 2 electrons experience substantial shielding. These values underestimate the effective atomic number because we have over-represented the electron-electron repulsion. 

The Z-1 value is the so-called effective atomic number. The point is that one electron kicked out of the K level left a second one untouched. The latter takes part in the screening of the positive nuclear charge by the negative electron charge. 

The decrease in atomic radius moving across the periodic table can be explained in a similar manner. Consider, for example, the third period, where electrons are being added to the third principal energy level. The added electrons should be relatively poor shields for each other because they are all at about the same distance from the nucleus. Only the ten core electrons in inner, filled levels (n = 1, n = 2) are expected to shield the outer electrons from the nucleus. This means that the charge felt by an outer electron, called the effective nuclear charge, should increase steadily with atomic number as we move across the period. As effective nuclear charge increases, the outermost electrons are pulled in more tightly, and atomic radius decreases. 

See Standard oxidation voltage See Standard reduction voltage Effective nuclear charge Positive charge felt by the outermost electrons in an atom approximately equal to the atomic number minus the number of electrons in inner, complete levels, 154 Efflorescence Loss of water by a hydrate, 66 Effusion Movement of gas molecules through a pinhole or capillary,... 

The nuclear charge and the electrons it attracts primarily determine the ways in which atoms behave toward other atoms. Mass differences cause only minor chemical effects. Since the isotopes of an element have the same nuclear charge and the same number of electrons per neutral atom, they react in the same ways. Thus we can speak of the chemistry of oxygen without specifying which one of the three stable isotopes is reacting. Only the most precise measurements will indicate the very slight chemical differences among them. 

Fig. 1-16. Moseley plot for Ka2 lines. The curvature at high Z is due to a change in the effective nuclear charge (Z — 1). The insert shows the atomic number Z to be more fundamental than the atomic weight M. X-rays made possible the first experimental determinations of Z. Crosses = atomic weight dots = atomic number.

---