The modern periodic table is organized into 18 vertical columns known as groups and 7 horizontal rows known as periods. Elements within the same group share similar chemical properties due to their identical valence electron configurations.

In the periodic table, elements in the same period share the same number of principal energy levels, or electron shells. As you move from left to right across a period, electrons are added to the same outer shell, while the number of protons and electrons increases by one for each successive element.

Mendeleev's Periodic Law states that the physical and chemical properties of elements are a periodic function of their atomic weights (masses). This was the foundational principle used to organize the first widely accepted periodic table, predating the modern law based on atomic number.

Mendeleev predicted the existence and properties of several elements. Specifically, Scandium (Sc), Gallium (Ga), and Germanium (Ge) were the elements that filled the gaps he left for 'eka-boron', 'eka-aluminium', and 'eka-silicon', respectively, providing strong evidence for his periodic classification system.

Group 18 elements are known as noble gases or inert gases. They possess a full valence shell (eight electrons, except for helium which has two), making them chemically stable and largely unreactive under standard conditions. This electronic configuration results in very high ionization energies and low electron affinities, distinguishing them from other groups in the periodic table.

Electronegativity decreases down a group because the atomic radius increases, leading to a greater distance between the nucleus and the valence electrons, which reduces the effective attraction for shared electrons.

Metallic character increases down a group because the ionization energy decreases as the valence electron is further from the nucleus. For Group 1 elements, the metallic character increases in the order Li < Na < K < Rb < Cs, as the atomic size increases and the outer electron is more easily lost.

Elements in the same group of the periodic table share the same number of valence electrons. This similarity in electronic configuration is the primary reason why elements in the same group exhibit similar chemical properties.

Metallic character increases as you move from right to left across a period and from top to bottom down a group. Rubidium (Rb) is an alkali metal located in Group 1 of the fifth period. As an alkali metal, it has the lowest ionization energy in its period, making it the most electropositive and thus the most metallic element in that row of the periodic table.

Statement 1 is generally accepted as reactivity decreases toward the middle of a period and increases toward the ends. Statement 2 is false because non-metal reactivity decreases down a group due to increased atomic size and decreased electron affinity, making it harder to attract electrons.