The effective nuclear charge often symbolized as or is the net positive charge experienced by an electron in a polyelectronic atom the term effective is used because the shielding effect of negatively charged electrons prevents higher orbital electrons from experiencing the full nuclear charge of the nucleus due to the repelling effect of inner layer electrons. Except for hydrogen z eff is always less than z and z eff increases from left to right as you go across a row. Relationship between the effective nuclear charge z eff and the atomic number z for the outer electrons of the elements of the first three rows of the periodic table.
The shielding effect explains the trend in atomic size on the periodic table and also why valence electrons are readily removed from an atom. The shielding effect is the name given to the balance between the attraction between valence electrons and protons and the repulsion between valence and inner electrons. The valence electrons added to counterbalance the increasing nuclear charge screen one another ineffectively.
Although the number of core electrons stays the same across the period the number of protons increases. The effective nuclear charge increases from left to right across any period of the periodic table. Therefore using the equation for effective nuclear charge z eff z σ we see that bromine has a greater effective nuclear charge than potassium and that this trend is expected across the whole periodic table.
Bromine has 35 protons. Image showing periodicity of the chemical elements for effective nuclear charge clementi 6p in a 3d spiral periodic table column style. Image showing periodicity of the chemical elements for effective nuclear charge clementi 6p in a spiral periodic table heatscape style.
Atoms and the periodic table let s first remind ourselves about the atom. We can think of effective nuclear charge as the positive charge felt by the outermost electrons in an atom. Therefore as we go from left to right on the periodic table the effective nuclear charge of an atom increases in strength and holds the outer electrons closer and tighter to the nucleus.
We can see from equation ref 4 that the effective nuclear charge of an atom increases as the number of protons in an atom increases figure pageindex 2. In general for any many electron atom any particular electron will always be screened from the nucleus to some extent by the remaining electrons. The difference between the full nuclear charge z and the screening effect of the inner two electrons is called the effective nuclear charge or z eff.
