The modern periodic table is organized into 18 vertical columns, which are referred to as groups. These groups are numbered from 1 to 18, and elements within the same group typically exhibit similar chemical properties due to having the same number of valence electrons.

John Newlands proposed the Law of Octaves in 1864, which stated that elements exhibit repeating properties every eighth element, similar to musical octaves. At the time of his proposal, he had identified 56 elements. While his law was limited to lighter elements, it was a significant early attempt to organize elements based on periodic properties.

In the fourth period of the periodic table, the elements Germanium (Ge) and Arsenic (As) are classified as metalloids. These elements exhibit properties intermediate between metals and non-metals, distinguishing them from the surrounding metallic and non-metallic elements in the same period.

The periodic table is organized such that metals are found on the left side and in the center (transition metals). Alkali metals (Group 1) and alkaline earth metals (Group 2) occupy the far-left columns. Non-metals are generally located on the upper right side of the table. This arrangement reflects the trend of decreasing metallic character and increasing electronegativity as one moves from left to right across a period.

Metallic character is defined by the ease with which an atom loses electrons. Across a period from left to right, effective nuclear charge increases, making it harder to lose electrons, thus metallic character decreases. Down a group, the atomic radius increases and the valence electrons are further from the nucleus, making them easier to lose, which increases metallic character. Therefore, metallic character decreases across a period and increases down a group.

An element with an atomic number of 17 is Chlorine. Its electronic configuration is 2, 8, 7. Having 7 valence electrons, it belongs to Group 17, which is the halogen group. Halogens are highly reactive non-metals, and Chlorine exists as a diatomic gas under standard conditions.

Electronegativity is defined as the tendency of an atom to attract a shared pair of electrons in a covalent bond. It differs from electron affinity, which is the energy change when an electron is added to a neutral gaseous atom, and ionization enthalpy, which is the energy required to remove an electron.

The Modern Periodic Law, proposed by Henry Moseley, states that the physical and chemical properties of elements are periodic functions of their atomic numbers. Unlike Mendeleev's original table, which was based on atomic mass, the modern table uses atomic number (the number of protons in the nucleus) to arrange elements, which resolves inconsistencies and better reflects the electronic structure of atoms.

Elements in the same group (vertical column) of the periodic table have the same number of valence electrons in their outermost shell. This similarity in valence electron configuration is the primary reason why elements in the same group exhibit similar chemical properties and reactivity. For example, all alkali metals in Group 1 have one valence electron, leading to their characteristic high reactivity.

The electronic configuration 2, 8, 8 corresponds to Argon (atomic number 18). Since the valence shell is completely filled with eight electrons, it is a noble gas. Noble gases are located in Group 18 of the modern periodic table, characterized by their chemical inertness and stable octet configuration.