The zigzag line, often called the staircase line, acts as a boundary in the periodic table. Elements to the left of this line are primarily metals, while elements to the right are non-metals. Elements touching the line are often classified as metalloids.

Hydrogen is the first element in the periodic table, characterized by the atomic number 1. It consists of a single proton and a single electron, making it the simplest and most abundant element in the universe. It is placed in Group 1 due to its electronic configuration, though it exhibits unique properties distinct from alkali metals.

The modern periodic table is organized into 18 vertical columns known as groups and 7 horizontal rows known as periods, based on the electronic configuration of the elements.

John Newlands proposed the Law of Octaves in 1864, arranging elements by atomic weight. His table ended with Thorium, as the law failed to account for elements beyond calcium effectively, and the noble gases were not yet discovered.

The elements sodium (Na) through argon (Ar) have atomic numbers ranging from 11 to 18. In the modern periodic table, these elements occupy the third period, where the third principal energy level (n=3) is being filled with electrons.

John Newlands, a British chemist, proposed the Law of Octaves in 1864. At that time, he observed that when elements were arranged by increasing atomic weight, every eighth element exhibited similar properties to the first. He based this classification on the 56 elements known to science at that period.

In the modern periodic table, the vertical columns are referred to as groups. Elements within the same group typically share similar valence electron configurations, which leads to similarities in their chemical properties. While 'groups' is the plural, 'group' is the standard singular term for a single column.

Mendeleev recognized that hydrogen and oxygen form stable compounds with almost all other elements. By examining the formulas of hydrides and oxides, he could determine the valency and chemical behavior of elements, which served as the foundation for his periodic classification system. This approach allowed him to organize elements systematically based on their combining capacities and chemical reactivity patterns.

The modern periodic table is based on the work of Henry Moseley (often misspelled as Mojle in some texts). He established that the properties of elements are a periodic function of their atomic numbers, which corrected the flaws in earlier mass-based tables.

Group 1 (the alkali metals) consists of Hydrogen, Lithium, Sodium, Potassium, Rubidium, Cesium, and Francium. Counting these elements, there are exactly 7 elements in this group, making it the correct answer based on the standard periodic table structure.