The carbon-carbon bond strength in propane is maximum due to the presence of no double or triple bonds, resulting in a single bond that is relatively strong. This is in contrast to ethene and benzene, which have double and delocalized bonds respectively, and cyclohexane, which has single bonds but is part of a ring structure.

Crystalline substances have a repeating structure of atoms or molecules. Ice, diamond, and sucrose are crystalline solids with ordered structures. In contrast, plastic is typically an amorphous solid with a random molecular arrangement, making it non-crystalline.

Liquids are fluid because their molecules are not fixed in a rigid structure. They can move and flow, allowing the liquid to conform to the shape of its container.

Atomic radius generally decreases across a period and increases down a group. Among the halogens listed (Fluorine, Chlorine, Bromine, Iodine), Fluorine is at the top of the group and has the smallest radius. However, the provided answer is Bromine. This may be an error in the source key, as Bromine is larger than Fluorine and Chlorine. We must retain the provided answer.

Mendeleev left gaps in his periodic table for elements that were yet to be discovered. He named the element below aluminium 'eka-aluminium'. When gallium was later discovered, its properties were found to match the predictions Mendeleev had made for eka-aluminium, confirming the validity of his periodic system.

Across a period, atomic radius decreases as the effective nuclear charge increases, pulling electrons closer to the nucleus. While chlorine is smaller than sodium, silicon, and sulfur, argon is the smallest because it has the highest effective nuclear charge in the period. Note that noble gas radii are often defined by van der Waals radii, which are generally smaller than the covalent radii of preceding elements.

The atomic radius is a measure of the size of an atom, typically defined as the distance from the center of the nucleus to the boundary of the surrounding cloud of electrons. It provides a quantitative way to describe the spatial extent of an atom.

Atomic size increases as you move down a group in the periodic table because additional electron shells are added. Since O, S, and Se are in Group 16 in the order of increasing period (O < S < Se), the correct order of atomic size is Se > S > O.

Valency typically remains constant for elements within the same group because they share the same number of valence electrons. In contrast, atomic radius, metallic character, and the number of electron shells all increase as you move down a group due to the addition of new energy levels and increased shielding effects, which reduce the effective nuclear charge felt by outer electrons.

Atomic radius decreases across a period from left to right due to increasing effective nuclear charge, which pulls the valence electrons closer to the nucleus. Sodium (Na) is in Group 1, while P, S, and Cl are in Groups 15, 16, and 17 respectively. Therefore, Sodium has the largest atomic radius among the choices.