Introduction to Atomic Structure

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ATOMIC Structure and ATOM

Atom is the smallest particle of an element. It consists of a nucleus, which is composed of neutron and proton, whereas, electrons are found outside the nucleus.

Democritus, a Greek philosopher (460-370 BC) gave an idea that matter is composed of small non-dividing particle, which he named as ‘a-tomio’.

Later, atomic theory given by Dalton in 1808 made the concept more clearer.

DALTON’S ATOMIC THEORY

He stated that:

• Matter is composed of indivisible atoms.
• All the atoms of a given element have identical properties.
• Compounds are formed when atoms of different elements combine in a fixed ratio.
• No new product is formed in a chemical reaction, there is just reorganization.

ATOMIC MASS UNIT

Atomic mass unit can be defined as a mass exactly equal to one twelfth the mass of one carbon-12 atom. It is abbreviated as amu.

1 amu= 1.66056 *10^-24g

Mass of an hydrogen atom=1.6736 * 10 g

Mass of H atom in amu= 1.0078 amu

MOLECULAR MASS

Molecular mass is the sum of atomic masses of the elements present in a molecule.

e.g.

Molecular lass of carbon dioxide (CO₂)= 12.011u + 2*16.00u = 44.011u

MOLE CONCEPT

One mole is the amount of substance that contain as many particles as there are atoms in exactly 12g of C-12.

One mole of C-12 weighs = 12g/mol

Mass of a C-12 atom = 1.992648 * 10^-23 g

The no. of atom in one mole = 6.0221367*10²³

AVOGADRO CONSTANT

The number of entities in one mol is known as Avogadro constant. It is denoted by N_A.

N_A=6.022 * 10²³

1 mol of H atoms= 6.022 * 10²³ atoms

1 mol of water molecules= 6.022 * 10²³ water molecules

Q.1. Which is the smallest particle of an element?

1. Compound
2. Matter
3. Substance
4. Atom

Ans. d

Q.2. What are compounds?

1. When atoms of different elements combine in a fixed ratio
2. When atoms of different elements combine in a non fixed ratio
3. Same as atoms
4. Smallest particle of an element

Ans. a

Q.3. What is the molecular mass of CO₂?

33. 33.011u
34. 44.011u
35. 50u
36. 60u

Ans. b

Q.4. 1 mol of H atom is equal to

6. 6.022 * 10²³ atoms
7. 6.032 * 10²³ atoms
8. 6.022 * 10²² atoms
9. 6.022 * 10²⁴ atoms

Ans. a

Q.5. Dalton Atomic Theory states:

1. All the atoms of a given element have identical properties.
2. Compounds are formed when atoms of different elements combine in a fixed ratio.
3. No new product is formed in a chemical reaction, there is just reorganization.
4. All

Ans. d