John Alexander Reina Newlands proposed the Law of Octaves in 1865. He observed that when elements are arranged in increasing order of their atomic weights, the properties of every eighth element are a repetition of the first, similar to the octaves in music. This was a significant early attempt at organizing elements based on periodicity, though it only worked well for lighter elements.

While others like Dobereiner and Newlands also used atomic mass, Mendeleev is most famously credited for his comprehensive periodic table based on atomic mass. He organized elements into groups and periods, demonstrating the periodic nature of their properties more effectively than his predecessors.

Chlorine has an atomic number of 17. Its electronic configuration is 2, 8, 7. The presence of three electron shells places it in Period 3, and having seven valence electrons places it in Group 17 (the halogens).

Noble gases are elements in Group 18 of the periodic table, characterized by full valence electron shells, making them chemically inert. Hydrogen is a non-metal in Group 1, which is highly reactive and does not share the inert properties of the noble gases.

The fourth period of the periodic table begins with potassium (atomic number 19) and ends with krypton (atomic number 36). It includes the s-block elements, the first row of transition metals (d-block), and the p-block elements, totaling 18 elements in the complete period.

The fourth and fifth periods of the periodic table are long periods. The fourth period involves filling the 4s, 3d, and 4p orbitals, while the fifth period involves filling the 5s, 4d, and 5p orbitals. Each of these periods accommodates exactly 18 elements, reflecting the capacity of the subshells being filled.

The Modern Periodic Law, formulated by Henry Moseley, establishes that the properties of elements are a periodic function of their atomic numbers. This corrected the earlier Mendeleev's law, which was based on atomic mass. By arranging elements by atomic number, the periodic table accurately reflects the electronic configuration and chemical behavior of elements.

In Mendeleev's periodic table, the horizontal rows were designated as periods. These rows organized elements based on increasing atomic mass, while the vertical columns were referred to as groups, grouping elements with similar chemical properties together.

Newlands' Law of Octaves was based on the observation that every eighth element had properties similar to the first. However, this pattern only worked consistently for lighter elements up to calcium (atomic number 20). Beyond calcium, the introduction of transition metals disrupted the simple octet pattern, rendering the law less effective for heavier elements.

In the periodic table, horizontal rows are referred to as periods, representing the principal quantum number of the valence shell. Vertical columns are called groups, where elements share similar valence electron configurations and chemical properties.