Question

In the periodic table, the period number indicates the number of:
A. neutrons
B. protons
C. electrons
D. none of the above

Answer 1

Hint: In the modern periodic table, there are seven horizontal periods and eighteen vertical groups. The number of the period in the periodic table is determined by the number of outermost shells. The elements are arranged in the periodic table based on their similarity in the chemical properties.

Complete step by step answer:
A period in the periodic table is a horizontal row of chemical elements. All elements in a horizontal row have the same number of outermost electron shells. Each next element in a period has one more proton and is less metallic than its predecessor. Arranged this way, groups of elements in the same column have similar chemical and physical properties, reflecting the periodic law. For example, the halogens lie in the second-last column (group 17) and share similar properties, such as high reactivity and the tendency to gain one electron to arrive at a noble-gas electronic configuration. As of 2020 today, a total of 118 elements have been discovered and confirmed. In the periodic table, the period number indicates the number of outermost shells and not the number of electrons, protons and neutrons.

So, the correct answer is Option D.

Note:
Modern quantum mechanics explains the periodic trends in properties in terms of electron shells. As atomic number increases, shells fill with electrons in approximately the order shown in the ordering rule diagram. The filling of each shell corresponds to a row in the table. In the s-block and p-block of the periodic table, elements within the same period generally do not exhibit trends and similarities in properties (vertical trends down groups are more significant). However, in the d-block, trends across periods become significant, and in the f-block elements show a high degree of similarity across periods.