Problem 33 - A Complete Resource Book in Chemistry for JEE Main

Question

The atomic number of an element is 17 . The number of orbitals electron pairs in its valence shell is (a) 3 (b) 4 (c) 6 (d) 8

Step-by-Step Solution

Answer

The number of orbitals with electron pairs in the valence shell is 2.

Step 1: Identify the element

The atomic number is 17, which corresponds to the element Chlorine (Cl) on the periodic table.

Step 2: Determine the electron configuration

The electron configuration for chlorine with atomic number 17 is: 1s² 2s² 2p⁶ 3s² 3p⁵.

Step 3: Identify the valence shell

The valence shell is the outermost shell of an atom that contains electrons. For chlorine, the valence shell is the third shell (n=3), which contains the subshells 3s and 3p.

Step 4: Count the orbitals in the valence shell

In the 3rd shell of chlorine, the 3s subshell has 1 orbital and the 3p subshell has 3 orbitals.

Step 5: Calculate electron pairs

Each orbital can hold up to 2 electrons, making full pairings in 2 orbitals (one in 3s and one in 3p) with one unpaired in p. Hence, the number of orbitals that hold electron pairs is 2.

Key Concepts

Atomic Number

The atomic number is a unique identifier for each element found on the periodic table. It tells you the number of protons in the nucleus of an atom of that element. For example, Chlorine has an atomic number of 17, meaning it has 17 protons. This same number also indicates the number of electrons in a neutral atom of the element, helping determine its electron configuration. The atomic number not only helps identify elements but also reveals their placement and properties in the periodic table, such as their group and period.

Valence Shell

The valence shell is the outermost electron shell of an atom while performing chemical reactions. This shell contains the valence electrons, which are crucial in determining how an atom interacts with other atoms. For chlorine, with an electron configuration of 1s² 2s² 2p⁶ 3s² 3p⁵, the valence shell corresponds to the third shell. The valence shell plays a critical role in chemical bonding. It determines an element's reactivity and the type of bonds the atom can form. For chlorine, the 3p subshell lacks only one electron to be full, making it very reactive.

Orbitals

Orbitals are regions around an atom's nucleus where finding electrons is probable. They are divided into various types: s, p, d, and f. Each type has a different shape and energy level. In chlorine, the valence shell is the third shell, which includes one 3s orbital and three 3p orbitals.

• s-orbitals are spherical, holding up to two electrons.
• p-orbitals are dumbbell-shaped, and each p subshell has three orbitals, allowing a total of six electrons.

Knowing the number of orbitals is essential to understanding how atoms form bonds.

Electron Pairs

Electron pairs are formed when two electrons occupy the same orbital and spin in opposite directions. In the case of chlorine's valence shell, the 3s subshell can hold one electron pair in its single orbital. The 3p subshell, having three orbitals, can potentially hold three pairs of electrons, but in chlorine's case, it only has five electrons.

• The 3s orbital holds one pair of electrons (two electrons).
• Two of the 3p orbitals each hold one pair of electrons, while the third holds only one electron.

Thus, there are two full orbitals with electron pairs in chlorine’s valence shell, allowing it to easily engage in chemical reactions by pairing with electrons from other atoms.