Alkali metals are much less dense than other metals due to their large radii, which results from having a single loosely bound valence electron. Some of the alkali metals have such low densities that they can float on water.

Alkaline earth metals are found in the second group of the periodic table and include beryllium, magnesium, calcium, strontium, barium, and radium. These compounds are not as reactive as the alkali metals (found in group 1), but still participate in many reactions due to their electron configuration. Alkaline earth metals carry two valence electrons, located in the s orbital. Loss of these two electron leaves the alkaline earth metals with a full octet, giving them a stable oxidation state of +2. In contrast, the alkali metals have a stable oxidation state of +1. Both compounds have very low first ionization energies, but the second ionization energies of the alkaline earth metals are much lower than those of corresponding alkali metals. Removing a second electron from an alkali metal removes it from a stable octet, while removing an additional electron from an alkaline earth metal results in a stable octet.

While all alkali metal salts are soluble, the alkaline earth metals result in several exceptions to the solubility rules. For example, is not soluble in aqueous solutions.

Halogens are in the group next to the noble gasses. They have seven valence electrons, and therefore have a high electronegativity. The addition of only a single electron (production of an anion) generates a full valence octet.

Their diameters vary within the group. The diameter can be very small, like fluorine, or large, like iodine. They do not conduct electricity well, as they are non-metals.

The halogens are the second to last column in the periodic table, meaning that they have an affinity for a single additional electron. Halogens would be most likely to react with alkali metals, which contain only one loosely bound electron in the valence shell. Alkali metals have very low ionization energy, readily losing an electron, while halogens have very high electronegativity, readily gaining an electron. This interaction allows the alkali metals to form ionic bonds with the halogens.

The question states that a stronger acid will possess a weaker bond between the hydrogen atom and the acid. To have a weak bond, it is essential to have a non-metal atom that will repel electrons from the hydrogen. This will enable the hydrogen atom to distance itself from the acid, which will make the bond weaker (it will make it easier to remove the hydrogen atom).

Recall that electronegativity of an atom is defined as the ability of the atom to pull electrons towards itself. An atom with a high electronegativity will have high attraction for electrons, whereas an atom with low electronegativity will have low attraction for electrons; therefore, to have a strong acid, the non-metal atom must have a low electronegativity. In the periodic table, the electronegativity decreases as you go from right to left and top to bottom. This means that iodine will have a lower electronegativity than fluorine and will form the strongest acid.

Remember that fluorine is the most electronegative atom in the entire periodic table. Every halogen, except fluorine, forms a strong acid. , , and  are all strong acids ( is the strongest), whereas  is a weak acid due to the strength of the bond between hydrogen and fluorine.

Neutral halogens are found in group 17 of the periodic table. These elements contain seven valence electrons and have electron configurations that end with , where  is the outermost shell number and corresponds with the period (row) of the halogen.

The outermost shell contains seven valence electrons (two in the  orbital and five in the  orbital). If the electron configuration ended in  then the halogen has an extra electron, has a complete octet, and is not neutral (has a charge of ).

Noble gases are found in group 18 of the periodic table. The key characteristic of noble gases is their eight valence electrons (complete octet) in the ground state; therefore, noble gases have one more valence electron than a neutral halogen in the same row. Alkali metals are found in group 1 of the periodic table. They only have one valence electron. The easiest way for them to complete an octet is to lose an electron. Halogens, on the other hand, have seven valence electrons and gain an electron to complete octet. This means that halogens have more attraction (affinity) for electrons than the alkali metals.

The electron configuration of a neutral halogen ends in . There are a total of three  orbitals in a shell, and each  orbital can contain two electrons. This means that  orbitals can contain a total of six electrons. In the n^th shell of a neutral halogen, the  orbitals only contain five electrons. Two  orbitals will contain two paired electrons, and one  orbital will only contain one, unpaired electron; therefore, a neutral halogen will have a  orbital with an unpaired electron. 

Halogens are elements found in group 17 of the periodic table. They are characterized by their seven valence electrons. To complete an octet, halogens only need to gain a single electron. Since they gain electrons, halogens are usually reduced and serve as good oxidizing agents. Recall that when a substance is reduced it is also classified as an oxidizing agent because it can oxidize another atom (remove an electron from another atom). 

Halogens gain an electron to complete an octet because they have seven valence electrons. The elements in the oxygen group (group 16) gain two electrons to complete an octet because they have six valence electrons.

Halogens are highly electronegative. Electronegativity is defined as the ability of an atom to pull electrons towards itself. Since they need only one electron to complete an octet, halogens have high attraction towards electrons and, consequently, have high electronegativity values. Note that electronegativity increases as you go from left to right on the periodic table; therefore, halogens have the highest electronegativity of any group. Noble gases (group 18) are inert and do not have any attraction for electrons (low electronegativity).

The correct answer is cobalt, since it is the only metal among the answer choices. Metals have all the properties described (malleability, ductility, and conductivity). Sulfur, boron, and silicon do not exhibit these properties to the same extent.

The question states that the element is a solid at room temperature and conducts electricity. These are characteristics of a metal; therefore, the element is likely a metal. Recall that metals are usually found on the left side of the periodic table. The groups that are classified as metals include the alkali metals (group 1), alkaline earth metals (group 2), the aluminum family (group 3), and the transition metals (D block).

The answer choices state that it could be either sodium or silicon. Silicon is in group 14 and is considered a non-metal; therefore, the element has to be sodium. Metals are good electrical conductors and solid at room temperature. They are also ductile, which means that a metal can be stretched to create thin wires. 

Metals are good reducing agents, not oxidizing agents. Recall that metals usually have one, two, or three valence electrons. Since they only have a few valence electrons, metals prefer to lose their valence electrons to complete an octet. When an element loses electrons it is considered to be oxidized and can act as a reducing agent; therefore, sodium is not an oxidizing agent.

Metals are found on groups on the left side of the period table (groups 1, 2, and 3, and transition metals). The groups on the left side have fewer valence electrons than the groups on the right side. Groups 1, 2, and 3 have one, two, and three valence electrons, respectively. Metals thus have fewer valence electrons than non-metals.

Electronegativity is a chemical property that is defined as the ability of an element to attract electrons towards itself. Since they have fewer valence electrons, metals find it easier to lose electrons to generate a complete octet (have eight valence electrons). Rather than attract electrons to fill octet, metals give them away. On the other hand, it is easier for non-metals to gain electrons to complete octet because they have larger amounts of valence electrons in their ground state. This means that non-metals have higher attraction for electrons and, consequently, have higher electronegativities.