Xth Std. CHEMISTRY BRIDGE COURSE

 Basics of chemistry

               Valency

🟣 Valency is the combining power or capacity of an element.

🟣 The valency of an element is related to how many electrons are in the outermost shell.

🟣 Elements in the same group of periodic table have the same valency.

Example:

Groups    :2  3  4  5  6  7  8

Valencies:2  3  4  3  2  1  0

🟣 Zero group elements are helium (He),neon (Ne), argon (Ar),krypton (Kr)xenon (Cr)and radon(Ra). These are also called noble gases because they do not usually react with other elements.

🟣 Valence electrons are responsible for forming the bonds.

🟣 Some elements have variable valency such as iron with valency 2 and 3, Cobalt with valency 3 and 2 and copper with valency 2 and 1.

Writing chemical formula of binary compound using valency

🔴 Find the valency  of the two elements first.

🔴 Write the chemical formula such that the sums of the valencies of the two elements are equal.

🔴 If a metal is present always place it first in the formula.

Examples:

WATER.          MAGNESIUM OXIDE

H       O                   Mg          O

↘️  ↙️                     ↘️        ↙️

 1     2                         2          2

  H2O                               MgO

Electronic configuration

💫 The distribution of electrons of an atom or molecule in atomic or molecular orbitals is called an electronic configuration.

Rules for writing electronic configuration:

🟣 Aufbau principle: it states that in the ground state of an atom or ion ,electrons fill atomic orbitals of the lowest energy levels first before occupying higher energy levels.

Example: 1s sub shell is filled before 2s subshell is occupied.

  

Pauli's Exclusion Principle: it states that no two electrons in the same atom can have identical values for all four Quantum numbers.

             OR

More than two electrons cannot occupy the same orbital.

Two electrons in the same orbital must have opposite spins.

Hund's rule: every orbital in a sublevel is a singly occupied before any orbital is doubly occupied.

🟣 All of the electrons in singly occupied orbitals have the same spin.

Example:

Atomic number of nitrogen is 7

Its electronic configuration is 1s² 2s² 2p³

It is placed in the periodic table in 2nd period and 15th group

Rough worksheet:

✨✨✨✨✨✨✨✨✨✨✨✨✨✨✨✨✨✨✨✨✨✨✨✨⚡⚡