The period number corresponds to the principal quantum number (n) of the outermost shell. Elements in the third period have their valence electrons in the third shell, which corresponds to the M shell (n=3), meaning K (n=1) and L (n=2) shells are also filled.

The first period of the periodic table contains only two elements: hydrogen and helium. Helium is a noble gas (inert gas) with a full valence shell (1s²), making it the only inert gas found in the first period.

John Newlands proposed the Law of Octaves in 1864. He noted that when elements are arranged by increasing atomic mass, every eighth element exhibits properties similar to the first, analogous to musical octaves. This was an early attempt to identify periodicity in chemical properties.

Alkali metals are found in Group 1 of the periodic table. The atomic numbers 3 (Lithium), 19 (Potassium), 37 (Rubidium), and 55 (Cesium) all belong to Group 1, characterized by having a single valence electron in their outermost s-orbital.

Henry Moseley (often misspelled as Mojle) used X-ray spectroscopy to show that the frequency of X-rays emitted by elements is related to their atomic number (Z) rather than their atomic mass. This discovery corrected the anomalies in Mendeleev's periodic table and established the modern periodic law, which states that properties are a periodic function of atomic number.

J.W. Döbereiner was a German chemist who, in 1817, identified the existence of triads of elements with similar properties. He observed that the atomic weight of the middle element was roughly the average of the other two. This work laid the groundwork for later developments in periodic classification, although it was limited to a small number of elements and did not account for all known elements at the time.

Neon, Krypton, and Argon are all members of Group 18 in the periodic table, known as noble gases, characterized by their full valence electron shells and chemical inertness. Platinum, however, is a transition metal located in Group 10, which exhibits different chemical and physical properties compared to the noble gases.

The period number of an element is determined by the number of occupied electron shells. Since the configuration 2, 8, 8, 2 shows four occupied shells (K, L, M, and N), the element is located in the fourth period.

The element with atomic number 16 is Sulfur (S). Its electron configuration is 1s2 2s2 2p6 3s2 3p4. The highest principal quantum number (n) in the ground state configuration is 3, which corresponds to the third period of the periodic table. Elements in the third period have their valence electrons in the third energy shell, and Sulfur is located in Group 16 of this period.

Henry Moseley conducted X-ray spectroscopy experiments that established a direct relationship between the frequency of X-rays emitted by elements and their atomic numbers. This discovery proved that atomic number is the true basis for periodic trends, correcting Mendeleev's mass-based arrangement.