The group number of an element is projected from the number of electrons in the valence shell of the element.

For At.No=21. Configuration - 2,8,8,3

It belongs to Group 3.
And the element is Scandium (Sc).

**Finding the Group Number of an Element with Atomic Number 21**

To determine the group number of an element based on its atomic number, we need to understand the periodic table and its organization. The periodic table is divided into periods (rows) and groups (columns). Each element is assigned a unique atomic number, which represents the number of protons in its nucleus. Here's how to find the group number of an element with atomic number 21:

**Step 1: Understand the Periodic Table**
The periodic table consists of seven periods numbered from 1 to 7 and multiple groups labeled from 1 to 18. The elements are arranged based on their increasing atomic number, from left to right and top to bottom.

**Step 2: Identify the Period**
Since the atomic number of the element in question is 21, we need to determine which period it belongs to. We can see that the first two periods (1 and 2) contain 2 and 8 elements respectively. Thus, elements with atomic numbers up to 10 belong to period 1, and those from 11 to 18 belong to period 2.

**Step 3: Determine the Group**
To find the group number, we need to locate the element within its respective period. Since the atomic number of the element is 21, it lies in period 3. Now, we count the elements in period 3 until we reach the desired atomic number.

**Step 4: Count the Elements**
Period 3 starts with sodium (atomic number 11) and ends with argon (atomic number 18). So, we count from sodium to argon to determine the position of the element with atomic number 21.

1. Sodium (Na) - atomic number 11
2. Magnesium (Mg) - atomic number 12
3. Aluminum (Al) - atomic number 13
4. Silicon (Si) - atomic number 14
5. Phosphorus (P) - atomic number 15
6. Sulfur (S) - atomic number 16
7. Chlorine (Cl) - atomic number 17
8. Argon (Ar) - atomic number 18

**Step 5: Determine the Group Number**
Since the element with atomic number 21 is located between sodium (group 1) and magnesium (group 2), it belongs to group 3.

Therefore, the group number of the element with atomic number 21 is 3.

By following these steps and understanding the periodic table's organization, we can determine the group number of any element based on its atomic number.