Modern Periodic Table and Periodic Law

Correct answer: A. In increasing order of atomic number

In the modern periodic table, elements are placed in increasing order of atomic number. Atomic number is the number of protons in an atom’s nucleus, and the modern periodic law is based on it. Arrangement by atomic mass caused some anomalies, so option B is not correct. Exam tip: remember that the modern periodic law is based on atomic number, not atomic mass.

Correct answer: A. Group

A vertical column in the periodic table is called a group. Elements in the same group generally have the same number of valence electrons, so they show similar chemical properties. A period is a horizontal row, so option B is incorrect. Exam tip: remember vertical = group and horizontal = period.

Correct answer: A. They have the same number of valence electrons

Elements in the same group have the same number of valence electrons in their outermost shell. Since valence electrons mainly determine bonding and chemical reactions, these elements show similar chemical properties. The same number of electron shells indicates the same period, not the same group. Exam tip: remember that groups relate to valence electrons, while periods relate to electron shells.

Correct answer: A. Period

A horizontal row in the modern periodic table is called a period. The table contains seven periods, and elements in a period have the same number of occupied electron shells, although their valence electrons and properties change across the row. A vertical column is called a group. Therefore, group describes the vertical arrangement, while period describes the horizontal arrangement, making option A correct.

Correct answer: A. It decreases.

From left to right across a period, the number of protons increases while the added electrons enter the same main shell. The effective nuclear charge therefore increases and pulls the electrons closer to the nucleus, so atomic size decreases. Option B states the opposite trend. Exam tip: atomic radius decreases across a period but increases down a group.

Correct answer: A. Noble gases

The elements in Group 18 are called noble gases. This group includes helium, neon, argon, and other elements whose outermost electron shell is complete, giving them very low chemical reactivity under ordinary conditions. Halogens belong to Group 17, alkali metals are in Group 1, and transition metals occupy the central d-block. Hence, option A correctly identifies Group 18 elements.

Correct answer: A. 6

Carbon has 6 protons in its nucleus. The atomic number of an element equals the number of protons in its nucleus; therefore, carbon has atomic number 6. The number 12 is associated with the common mass number of carbon, not its atomic number. Exam tip: Remember that atomic number equals the number of protons.