One of the biggest factors that can affect the bond angles in a molecule is the presence of lone pairs. A general rule is that each lone pair will decrease the predicted bond angle by about two degrees. Methane, for example, has no lone pairs on the central carbon, so its bond angles are 109.5 degrees. Ammonia, however, has one lone pair, which makes its angles closer to 107.5 degrees. Finally, water has 2 lone pairs, giving it a bond angle of about 105 degrees.

A trigonal pyramidal geometry is witnessed when molecules have a central atom bonding to three additional atoms and one lone pair. The nitrogen atom in ammonia has three hydrogens attached, as well as a lone pair, in order to satisfy its octet. This gives ammonia a trigonal pyramidal geometry.

Methane has a tetrahedral geometry, carbon dioxide is linear, and boron trifluoride is trigonal planar.

An sp³d² hybridized atom contains six degenerate orbitals. Hund's Rule states that all degenerate orbitals must be singly occupied before any orbital becomes doubly occupied. This molecule contains eight valence electrons, which results in two doubly occupied hybrid orbitals (two lone pairs) and four singly occupied hybrid orbitals to which the ligands bind. Thus, the molecule contains four bonds and two lone pairs, which corresponds to a square planar geometry.  

From the periodic table, we find that the atomic number, i.e., the number of protons in the nucleus of  is 15. Given that the mass number is 32, and recalling the formula which relates mass number to the number of neutrons and protons: , we find that the number of neutrons in the nucleus of one atom of  is 17.

Transition metals extend from groups 3 through 12 and periods 4 through 7. Ru, in its neutral state, has 44 electrons. Therefore, when it becomes a  cation, it will have 42 electrons. 

The atomic number is equal to the number of protons in the element, so from the periodic table, we find that the element with atomic number 34 is selenium. Since , we calculate that the mass number is 70. Lastly, there are two more electrons than protons, so the charge will be .

Covalent bonds are by far the strongest, requiring  to be broken. 

Next are hydrogen bonds, which require between  to be broken.

Next are dipole-dipole interactions, which require  to be broken.

Finally, van der Waals forces are the weakest of those listed, requiring  to be broken.

An atom is considered to be in an excited state when one of the electrons has jumped to a higher energy level while a lower energy level is available. In the case of , an electron has jumped to the 2p energy level while there is still room in the lower 2s subshell. As a result, it is considered to be in an excited state.

Magnesium has an atomic number of 12, so the total number of electrons in its configuration should add up to twelve. The maximum number of electrons in the s subshell is two. Of all the answer choices, only 1s²2s²2p⁶3s² fits the criteria. The sum of the exponent values is 12, matching the atomic number of magnesium, and the number of electrons in the s and p subshells matches the maximum amount possible. 

Iron has an atomic number of 26, so the total number of electrons in its configuration should add up to twenty six. The maximum number of electrons in the s subshell is two. The sum of the exponent values is 26, matching the atomic number of magnesium, and the number of electrons in the s and p subshells matches the maximum amount possible.