The atomic history

Chapter 4Atoms and Elements

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The Divisibility of Matter• Infinitely divisible

For any two points, there is always a point between.

• Ultimate particleUpon division, eventually a

particle is reached which can no longer be divided.

“Nothing exists except atoms and empty space; everything else is opinion.” - Democritus 460–370 B.C.

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Atomic Theory of Matter

The belief that atoms are the fundamental building blocks of matter was reemerged in the early 19th century, championed by John Dalton.

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Dalton’s Atomic Theory (1808)1. Each Element is composed of tiny, indestructible

particles called atoms. Tiny, hard, indivisible, spheres.

2. All atoms of an element are identical. They have the same mass, volume, and other physical and

chemical properties. So, atoms of different elements are different. Every carbon atom is identical to every other carbon atom.

They have the same chemical and physical properties. However, carbon atoms are different from sulfur atoms.

They have different chemical and physical properties.

3. Atoms combine in simple, whole-number ratios to form molecules of compounds.

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Modern structure of the atomSubatomic Particle Symbol

Electron e-

Proton p

Neutron n

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The Discovery of Electrons

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The Atom,1900:

• Thompson came up with a model called “Plum pudding” model. In this model, he presented atoms as positive spheres of matter with negative electrons imbedded in them.

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Rutherford’s model of the atom

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In 1932 Chadwick observed that beryllium, when bombarded by alpha particles, released an unknown radiation that was neutral. Chadwick interpreted this radiation as being composed of particles of mass approximately equal to that of the proton, but without electrical charge--neutrons.

The Neutron

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• The nucleus contains protons and neutrons.• The nucleus is only about 10-13 cm in diameter in comparison to 10-8

cm as the atomic diameter• Protons identify the element• Electrons are responsible for the chemical behavior of the element• Neutrons stabilize the element

Subatomic particle

Mass (in amu) Charge (Coulombs)

Number charge

Electron 0.000548 –1.602 x 10-19 -1

Proton 1.00073 +1.602 x 10-19 +1

Neutron 1.00087 0 0 10