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.

As you move from left to right across a period, the number of protons in the nucleus increases, which increases the effective nuclear charge. Since the electrons are being added to the same valence shell, the increased nuclear attraction pulls the electron cloud closer to the nucleus. Consequently, the atomic radius decreases across the period from sodium to chlorine.

As you move down a group in the periodic table, each subsequent element has an additional principal energy level or electron shell. This increase in the number of shells increases the distance between the nucleus and the outermost electrons, thereby increasing the atomic radius despite the increase in nuclear charge.

The modern periodic table is based on the Periodic Law, which states that the physical and chemical properties of elements are periodic functions of their atomic numbers. Atomic number represents the number of protons in the nucleus, which determines the electronic configuration and chemical behavior of the element.