Topic:

Atoms and Molecules

Subtopic:Elements, Atoms, SymbolsL.O: To identify the first twenty elements by their name and symbols.

Atoms

Atoms are the building blocks of all matter. Atoms are very small beyond our

imagination. Atomic radius is measured in nanometres.

1 nm = 1 / 109 m (1 m = 109 nm)

Atoms of most elements are not able to exist independently.

Symbol Symbol represents an atom of an

element. H represents one atom of hydrogen. The first letter of a symbol is always

written as a capital letter and the second letter (if any) as a small letter.

First twenty elements and their symbols

Sl No: Name of the element Symbol

1 Hydrogen H

2 Helium He

3 Lithium Li

4 Beryllium Be

5 Boron B

6 Carbon C

7 Nitrogen N

Sl No: Name of the element Symbol

8 Oxygen O

9 Fluorine F

10 Neon Ne

11 Sodium (Natrium) Na

12 Magnesium Mg

13 Aluminium Al

14 Silicon Si

Sl No: Name of theelement Symbol

15 Phosphorous P

16 Sulphur S

17 Chlorine Cl

18 Argon Ar

19 Potassium (Kalium) K

20 Calcium Ca

HW Write the symbols of the following

elements:

Copper Silver Gold Zinc Mercury lead

Sub topic: Atomic massL.O: To illustrate the significance of atomic mass of elements and to identify the RAM of a few common elements.

Atomic Mass

According to Dalton, each element had a characteristic atomic mass.

Determining the mass of an individual atom being a difficult task, Relative Atomic Masses were determined using the laws of chemical combinations and the compounds formed.

Relative Atomic Mass (RAM) Carbon-12 isotope was chosen as the standard

reference for measuring atomic masses.

The Relative Atomic Mass (RAM) of all elements have been found with respect to an atom of carbon-12.

Definition: The RAM of an element is defined as the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom.

Atomic Mass Unit (amu)

One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12.

IUPAC (International Union of Pure and Applied Chemistry) recommended the use of u (unified mass)to amu.

RAM of a few elementsElement Symbol Atomic mass

(u)

Hydrogen H 1

Carbon C 12

Nitrogen N 14

Oxygen O 16

Sodium Na 23

Magnesium Mg 24

Sulphur S 32

Element Symbol Atomic mass (u)

Chlorine Cl 35.5

Calcium Ca 40

Copper Cu 63.5

Zinc Zn 65.4

Molecule

A molecule is the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.

Atoms of the same element or of different elements can join together to form molecules.