The chemistry of elements in a group are the most similar due to their valence electron configuration being identical. Both phosphorus and arsenic have 5 valence electrons, therefore,  is most chemically similar to . Phosphate is the polyatomic ion .

First, we have to remember that the more negative the electron affinity (EA) is, the easiest will be for an atom to grab an extra electron. A positive EA means that we have to supply an atom with energy for it to accept an extra electron. The general trend of EA is to increases from bottom to top in a group and from left to right in a period. Hence, sodium is on top of potassium it should have larger affinity. However, calcium has a smaller EA than potassium. Elements of the group 2A do not follow the general trend. They have a semi-closed shell electronic structure with paired electrons in the outermost s orbitals. Such structure requires extra energy to be broken.

Transition metals are found in the "middle" of the table

Na and Sr are metals

Cl and Po are non-metals

In this question, we're asked to determine a period trend. Specifically, we're asked to determine which answer choice increases as one moves down the periodic table.

Electronegativity describes the pull an atom has on the electrons shared in a covalent bond. The greater the effective nuclear charge, the greater the attraction these electrons will have for the positively charged nucleus. When moving from left to right on the table, electronegativity increases. Also, moving from bottom to top on the table corresponds to an increase in electronegativity.

The trend in electron affinity is similar to the trend in electronegativity for much the same reasons. Electron affinity describes how willing an atom is to accept an additional electron. When this happens, a certain amount of energy is released. The more energy that is released, the more stable that atom has become. Electron affinity increases as one moves from left to right on the table, and also when one moves from top to bottom.

Ionization energy is the amount of energy that is needed in order to remove an electron from an atom or ion. Once again, for much the same reasons as discussed above, the trend of ionization energy will increase when moving from left to right across the periodic table, and also when moving from bottom to top.

Lastly, let's consider atomic radius. This term describes roughly the distance between the center of an atom and its outer electron shell. As we move from left to the right, the effective nuclear charge increases as each atom gains an additional proton (a more positively charged nucleus will exert a greater pull on the outer electrons, causing the radius to decrease). Also, as we move down the periodic table, additional energy shells (energy levels) are added. Since each subsequent shell represents valence electrons that are farther away from the nucleus, the radius of the atom increases. Thus, as we move down the table, atomic radius is the property that is expected to increase.

Hydrogen does not have enough space for 8 electrons, since it has only one s-subshell, which holds a maximum of 2 electrons.