Discovery of Structure of Atoms

Dalton’s Atomic Theory

The idea of atomic theory

was revived by John Dalton

(1766-1844) 2,000 years

after Democritus first

proposed about it.

In 1800, John Dalton performed a series of

experiments showing the matter consists of lumpy

particles called “atoms”.

His Findings led to the formulation of what is commonly

known as “Dalton’s Atomic Theory”.

Dalton formulated his theory based on Antoine

Lavoiser’s (1743-1794) law of conservation of mass and

Joseph Proust’s (1754-1826) law of definite proportion.

Law of Conservation of

Mass

Law of Definite Proportion

The total mass of substances

before and after a complete

reaction are equal.

Elements combine in a fixed

proportion to form a

compound.

The following are the major postulates of Dalton’s Atomic theory:

1.Matter is composed of small

indivisible particles called atoms.

2.Atoms of the same element are

identical.

3.Compounds are formed by a

combination of two or more different

kinds of atoms.

4.A chemical reaction is

a rearrangement of atoms.

The formulation of Dalton’s atomic

theory marked the start of the modern

era of chemistry.

However, during his time, (1800s), some

scientists still did not believe that there

was an indivisible atom found in all

matter.

In the 1900s, the theory became

widely accepted upon discovery of

subatomic particles.

Let us look at the Cathode Ray Tube experiment

The CRT by J.J Thomson

is a glass tube with two

electrodes: the positive

electrode called anode

and the negative

electrode called

cathode.

Once you apply a sufficiently high voltage, a stream

of ELECTRONS flows towards the "cathode"; this is

what you call a cathode ray. Inside the tube there

exists near perfect vacuum, so there are barely any

other gas molecules, ions etc.

Compare it to a lightning.

JJ Thomson decide to apply a magnetic field in his experiment.

J.J. Thomson was able to have a ratio of the mass of the bent particles

He found out that it was 2000x lighter

than a Hydrogen atom. And that was

the time the electron was discovered.

The Discovery of Proton, Electron and Neutrons

In 1886, Eugen Goldstein (1850-1930) discovered the

positively charged subatomic particle.

However, it was Ernest Rutherford (1871-1937) who coined

the term proton for the positively charged particle in an atom.

Then using CRT experiment, J.J. Thomson (1856-1940)

discovered that an atom is also composed of negatively

charged particles which he named electrons.

In 1932, James Chadwick (1891-1974)

discovered the neutral particles, which he

called neutrons, in the nucleus of an atom.

Atomic model proposed by

different scientists

J.J. Thomson’s Plum Pudding Model

In 1904, he proposed the “plum

pudding” model of divisible atom.

According to him, atoms consist

of a large sphere of uniform

positive charge embedded with

smaller negatively charged

particles called electrons

(corpuscles).

Ernest Rutherford’s Nuclear Model

Based on this model, the atom has a very small positively charged nucleus that contains most of the mass of the atom.

The electrons move around the nucleus

The protons found inside the nucleus are positively charged.

The number of protons is equal to the

number of electrons.

Later he postulated the existence of a

neutral particle in the nucleus to make up

for he calculated mass deficiency in the

atom studied.

Niels Bohr’s Planetary model

In his model (1.) the

electrons move around

the nucleus in fixed orbit.

An electron in a

particular orbit has

constant energy.

2. An electron can absorb energy and move to a

higher energy orbit of larger radius;

3. An excited electron can fall back to its original

orbit by emitting energy as radiation;

4. Electrons can only exist in certain discrete

energy levels.

James Chadwick Model

According to him, the nucleus

of an atom contains neutrons,

electrically neutral particles

with a mass similar to that of

proton.

Quantum Mechanical Model

In this model, the electrons are considered to be moving around the nucleus- but not in a fixed orbit.

Their exact location is not known, it

is only their probable location that is

being considered.

This location is around the

nucleus, in an arbitrary cloud.

This “electron cloud” is that

region around the nucleus

where electrons are most

likely to be found.