Which of the following statement is incorrect?

Elements with similar chemical properties occur only within the same period.

The correct answer is option 4. Elements with similar chemical properties occur only within the same period.

Let us examine each statement about the periodic table to understand why they are correct or incorrect.

1. Mendeleev arranged the elements in a tabular form according to increasing atomic weight

Dmitri Mendeleev created one of the first periodic tables in 1869. He organized the elements by increasing atomic weight (now known as atomic mass) and grouped elements with similar chemical properties into columns. Mendeleev left gaps for elements that were not yet discovered, predicting their properties based on the trends he observed. Mendeleev indeed arranged elements by increasing atomic weight, and this method helped him to discover the periodic law. Thus, the statement is correct.

2. One of the defects in Mendeleev’s periodic table is the position of lanthanides

The lanthanides are a series of 15 elements from lanthanum (La) to lutetium (Lu). During Mendeleev's time, the chemistry and properties of the lanthanides were not well understood, and they were not distinctly separated from other elements in the periodic table. This lack of understanding and clear positioning was indeed a defect in Mendeleev’s table. The placement of lanthanides was a known issue in Mendeleev's periodic table. Thus, the statement is correct.

3. Maximum number of groups in Bohr’s periodic table is 18

Niels Bohr's model of the atom contributed to the modern understanding of the periodic table by explaining the electronic structure of atoms. The modern periodic table, influenced by Bohr’s atomic theory, consists of 18 groups. Each group represents elements with similar electron configurations and, consequently, similar chemical properties. The periodic table indeed has 18 groups, reflecting the maximum number of groups in the modern understanding. Thus, the statement is correct.

4. Elements with similar chemical properties occur only within the same period

A period is a horizontal row in the periodic table. Elements in the same period have the same number of electron shells but do not necessarily share chemical properties. A group is a vertical column in the periodic table. Elements in the same group have the same number of valence electrons, leading to similar chemical properties. Group 1 (alkali metals like lithium, sodium, and potassium) exhibit similar chemical properties such as high reactivity with water, which is due to each having one valence electron. However, these elements are in different periods. The chemical properties of elements are similar within the same group, not within the same period. Thus, elements with similar chemical properties are found in the same group, not restricted to the same period. Thus, the statement is incorrect.

The statement that is incorrect is 4. Elements with similar chemical properties occur only within the same period.

This is because elements with similar chemical properties are actually found within the same group, not the same period. Elements in the same group have the same number of valence electrons, which accounts for their similar chemical behavior.