E=hf, or E=hc/λ

h is Planck's constant with a value of 6.62606957 × 10^-34 m² kg / s. Converting 475nm to m gives 4.75x10^-7m. c is the speed of light with 3x10⁸m/s. Putting these variables into the second equation you get 4.174x10^-19J

The atomic number is equal to the number of protons, because no two elements have the same number of protons. It is however possible for different elements to have the same number of electrons (through the loss or gain of electrons) or neutrons (isotopes). 

Electronegativity is defined as the tendency of an atom to attract an electron in a bond that it shares with another atom. Because oxygen wants to receive two elctrons, while both sodiums wish to lose one electron, oxygen has a higher electronegativity than sodium. Typically, electronegativity can be seen as increasing as you go to the top right of the periodic table. For example, fluorine has a higher electronegativity than nitrogen.

This is the correct definition of ionization energy. By removing an electron from an atom, a cation is produced.

Atoms in the third period and above have d-orbitals that can hold up to 10 elecrons. This is what allows for atoms to exceed the octet rule.

Metallic compounds have unique properties due to their electron motility. In metals, electrons are able to move freely around and between atoms. In non-metals, electrons are more tightly bound to the nucleus due to increased nuclear positive charge and decreased atomic radius. The fluidity of electrons in metals allows them to conduct electric charge.

Metals are generally non-brittle. Metals are solids at room temperature and conduct electricity in the liquid phase, but they share these properties with a number of non-metal compounds as well.

Transition metals have only partially filled valence bands. Thus, the electrons can move among the d orbitals, and this electron flow allows the transition metals to be good conductors of electricity. 

The number of neutorns is the only thing out of the answer choices that does not impact an element's atomic radius. Since neutrons have no charge, they do not impact the attractive forces between electrons and protons.

B (boron) is one of the atoms known for making fewer than 4 covalent bonds, and therefore not filling its octet. C (carbon) always makes 4 bonds, while O (oxygen) and Cl (chloride) are also known for having full octets.

Strontium is in the same group as calcium in the periodic table, which means that they both have the same number of valence electrons; they are both alkaline earth metals with two valence electrons. This is one of the most important chemical properties, as it dictates how an element will react. Like calcium, strontium will lose its two valence electrons easily to form ionic compounds.