Class X Chemistry Chapter 1: Periodic Table INTEXT Question 1

Periodic Table and Valency

• Henry Moseley stated that the elements should be arranged in a periodic table according to their atomic numbers.
• Definition of valency: The number of electrons an element can lose or gain to form a bond with another atom.
• The modern periodic table has 7 periods.

Valency and Groups

• Group 1 elements have valency 1 because they can lose one electron to form a bond with another atom.
• The valency of an element is determined by the number of electrons it can lose or gain to form a bond with another atom.
• Elements in the same group have the same valency because they have the same number of electrons in their outermost shell.

Periodic Table Structure

• A periodic table is a tabular arrangement of elements according to their atomic numbers, electron configurations, and recurring chemical properties.
• The horizontal rows in the periodic table are known as periods.
• The reason for periodicity in the periodic table is the recurring pattern of similar chemical properties in elements due to the same number of electrons in their outermost shell.

Electronic Configuration

• As you move from left to right across a period in the periodic table, the number of valence electrons increases by one in each element.
• Elements in the same group have the same number of electrons in their outermost shell.

Element Classification

• Halogens are elements classified as Group 17 in the periodic table, and two examples are Fluorine (F) and Chlorine (Cl).
• Elements placed at the extreme right of the periodic table are Noble Gases.
• Noble Gases have their outermost shell complete.
• Carbon is an element that has two outermost shells incomplete.
• The valency of elements in Group 1 is 1.