Magnesium (Mg) has an atomic number of 12, so the effective nuclear charge felt by its valence electrons would be +12. Silicon (Si) has an atomic number of 14, so the effective nuclear charge felt by its valence electrons would be +14. Bromine (Br) has an atomic number of 35, so the effective nuclear charge felt by its valence electrons would be +35.
The effective nuclear charge felt by valence electrons is determined by electron shielding. If the core electrons were 100% effective at shielding the valence electrons and there's no valence electron shielding, the effective nuclear charge felt by valence electrons in Mg,Si,Br would be determined by the atomic number of each element.
The effective nuclear charge felt by valence electrons can be determined by considering electron shielding. In this scenario, if the core electrons were 100% effective at shielding the valence electrons from the nuclear charge and the valence electrons provided no shielding for each other, the effective nuclear charge felt by valence electrons in the compound MgSiBr would be determined by the number of protons in the nucleus (atomic number).
For example, Magnesium (Mg) has an atomic number of 12, so the effective nuclear charge felt by its valence electrons would be +12. Silicon (Si) has an atomic number of 14, so the effective nuclear charge felt by its valence electrons would be +14. Bromine (Br) has an atomic number of 35, so the effective nuclear charge felt by its valence electrons would be +35.