How many groups are there in the modern periodic table? 

18

The correct answer is option (3) 18.

The modern periodic table is organized into 18 vertical columns called groups or families. Each group contains elements that share similar chemical properties because they have the same number of valence electrons. Here's a detailed explanation:

1. Organization by Similarity:  The periodic table is arranged such that elements with similar chemical properties are placed in the same vertical column or group. This arrangement allows for easy identification of trends and patterns in the properties of elements.

2. Valence Electrons: The number of valence electrons, which are the electrons in the outermost energy level of an atom, determines an element's chemical behavior. Elements in the same group have the same number of valence electrons, leading to similar chemical properties.

3. Numbering of Groups: The groups in the periodic table are numbered from 1 to 18. The numbers generally correspond to the number of valence electrons in the elements of that group. For example, Group 1 elements (e.g., lithium, sodium, potassium) all have one valence electron, while Group 18 elements (the noble gases) have eight valence electrons (except for helium, which has two).

4. Main Group and Transition Metals: The groups are divided into two main categories: the main group elements and the transition metals. The main group elements are found in Groups 1 and 2, and Groups 13 to 18, while the transition metals occupy the middle section of the periodic table.

5. Predictable Trends: Elements within the same group exhibit predictable trends in properties such as atomic radius, ionization energy, and electronegativity as one moves down the group. These trends are due to the increasing number of electron shells as one moves down a group, which affects the atomic structure and behavior of the elements.

In summary, the modern periodic table is organized into 18 groups, each containing elements with similar chemical properties due to their shared number of valence electrons. This organization facilitates the understanding of the periodic trends and behaviors of the elements.