The correct answer choice is an accurate explanation of effective nuclear charge. Since electrons are negatively charged, they are attracted to protons, which are positively charged. Protons are located in the nucleus of an atom.

Rn is radon, a noble gas, which is not a metal. As a rule, elements to the left are metals, elements to the right are non-metals. The barrier would be the elements that are immideately to the right of the transition metals

Non-metals are on the right side of the periodic table—past the metaloids and metals are on the left

As and Nb are the only combination that is a metal and non-metal

The metals are found on the left side and the middle of the periodic table, and include the active metals, transition metals, and lanthanide and actinide series of elements. They easily give up electrons to form cations, since they have LOW ionization energies.

Selenium is on the right side of the periodic table, so it is not a metal or metalloid. It is not in column 7 or 8, so it is not a halogen or noble gas. Thus, it must be a nonmetal.

Electricity is produced and conducted by the movement of electrons, and thus the most important property that allows them to be used for wiring is their valence electrons that can move freely. 

Ionic bonds occur between metals and non-metals.  The non-metal species are often halogens, very electronegative atoms which will completely take one of the non-metal's electrons.  An ionic bond is not as much a bond as it is an association between positive and negatively charged atoms (ions).  This association leads to the formation of rigid lattice structures of ionic compounds. Which makes them very stable, as seen by their high boiling points.  When looking for ionic bonding, look for bonding between non-metals and metals, and atoms that are from opposite ends of the periodic table. 

While many transition metals will lose an electron from the s shell before the d shell, many transition metals will lose only one electron from the s shell or none at all. The reason for this is that there is an enhanced stability when the transition metal's d shell is half filled, , so trasition metals will lose or not lose from the s shell in order to obtain a more stable electron configuration.

Metals are on the left side of the periodic table, nonmetals on the right, and metalloids follow a diagonal line from boron down to polonium. Silicon is one of these.

In general, metallic character decreases from the left of the period to the right, with the far right of the table representing the noble gases. Metalloids represent the junction between the metals and nonmetals within the period.

Metals are very large atoms that tend to lose electrons and become cations in solution. They will easily gain a positive charge, rather than a negative charge.

Metals typically have the following characteristics: they are malleable, ductile, and are excellent conductors for electricity. Metals are found on the left side of the periodic table.