which of the following results in a decrease in zeff, given that the number of protons remains the same?

When Mendeleev arranged the known chemical elements according to their similarity in chemical properties, it was noticed that their arrangement according to atomic mass led to some differences. iodine and tellurium; If they were arranged according to the atomic mass, they would be in the wrong place, but when they were arranged according to the atomic number (the number of protons) this led to their chemical properties matching the arrangement.

which of the following results in a decrease in zeff, given that the number of protons remains the same?

The atomic number is 1 for hydrogen as its nucleus contains 1 proton.
Helium has an atomic number of 2, and in its nucleus there are 2 protons and 2 neutrons.
Then comes lithium in the periodic table and there are in its nucleus 3 protons (atomic number 3) in addition to 4 neutrons,
Thus, the atomic number of the elements increases, even heavy elements such as lead (atomic number 82) and uranium (atomic number 92).
Besides the atomic number Z, which is the number of protons, in chemistry and physics we are also interested in the atomic mass A, which is the sum of the protons and neutrons in the atomic nucleus. A is called the mass number.

It has been observed that the equal numbers of protons and neutrons stabilize the nucleus, especially for the first 20 elements in the periodic table. After that, the nucleus needs more neutrons to maintain the stability of the nucleus in order to balance the increasing repulsion force between the protons (the neutrons are electrically neutral). If the nucleus is not stable, it is radioactive. Among the different isotopes of an element, we may find what is stable or unstable, for example, carbon-12, which has 6 protons + 6 neutrons, is stable, and carbon-14, which has 6 protons + 8 neutrons, is radioactive and unstable.