Atomic Theory

Atomic Theory

Development of The Atomic TheoryThe Birth of Modern Atomic TheoryAtomic Theory PresentationAtomic Models

• Atomic theory is the idea that matter is made up of little units called atoms. When the ancient Greek philosopher Democritus came up with the idea in the 5th century BC, is was originally meant to refer to indivisible units. As of 1897, the British scientist J.J. Thomson discovered that atoms are in fact made up of smaller particles. Today atomic theory refers to matter being made up of units that are indivisible only some of the time. Exceptions include plasmas such as fire, other ionic arrangements such as those found in the body, radioactive materials, and many more.

• The word "atom" (from the ancient Greek adjective atomos, 'indivisible'[) was applied to the basic particle that constituted a chemical element, because the chemists of the era believed that these were the fundamental particles of matter.

Atomic Theory Definition

The idea that everything is made up of a few simpleparts originated during the 400's B.C. in the philosophy of atomism.

Atomism was founded by the Greek philosopher Leucippus, but his disciple, Democritus developed the philosophy more fully.

Democritus gave his basic particle the name atom, which means uncuttable. He imagined atoms as small, hard particles, all composed of the same substance but of different sizes and shapes.

During the 300's B.C., a Greek philosopher named Epicurus incorporated Democritus' ideas about atoms into his philosophy. About 50 B.C., the Roman philosopher and poet Lucretius presented the fundamental principles of atomism in his long poem, On the Nature of Things.

Development of the atomic theory

During the Middle Ages, the idea of atoms waslargely ignored. This neglect resulted partly fromthe fact that atomism had been rejected byAristotle, an ancient Greek philosopher whosetheories dominated medieval philosophy andscience.

The idea that atoms form the basic units of all matter did survive, however.

During the 1500's and 1600's, such founders of modern science as Francis Bacon and Isaac Newton of England and Galileo of Italy believed in atoms. But those scientists could add little more to the atomic theory than Democritus had described.

Development of the atomic theory

• Dunki’n-Democritus

• Donut-Dalton

• Tastes-Thomson J.J.

• Really-Rutherford

• Better-Bohr,Niels

• ‘Di ba-De Broglie

Mnemonic Device

Ancient Greek philosopher born in Abdera, Thrace, Greece. He was an influential pre-Socratic philosopher and pupil of Leucippus, who formulated an atomic theory for the cosmos. Democritus is recognized as the earliest proponent of the concept of atomism.

Leucippus, the founder of the atomism, was the greatest influence upon him.

The theory of Democritus and Leucippus held that everything is composed of "atoms", which are physically, but not geometrically, indivisible; that between atoms lies empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size.

Of the mass of atoms, Democritus said "The more any indivisible exceeds, the heavier it is." But their exact position on weight of atoms is disputed.

Democritus, along with Leucippus and Epicurus, proposed the earliest views on the shapes and connectivity of atoms. They reasoned that the solidness of the material corresponded to the shape of the atoms involved.

Democritus’ Life and Contribution

In 1750, Rudjer Boscovich, a scientist born in what is now Croatia, suggested that Democritus might have been wrong in believing that atoms are "uncuttable." Boscovich thought that atoms contain smaller parts, which in turn contain still smaller parts, and so forth down to the fundamental building blocks of matter. He felt that these building blocks must be geometric points with no size at all. Today, most atomic physicists accept a modern form of this idea.

The development of the atomic theory advanced greatly when chemistry became an exact science during the late 1700's. Chemists discovered that they could combine elements to form compounds only in certain fixed proportions according to mass. In 1803, a British chemist named John Dalton developed an atomic theory to explain this discovery.

The birth of the modern atomic theory

• Around 1803, Dalton developed an atomic theory to explain the ratios in which elements combine to form compounds. It was the cornerstone for modern atomic theory.

• 5 main points of Dalton’s atomic theory:• 1. Elements are made of extremely small particles called atoms.• 2. Atoms of a given element are identical in size, mass, and other

properties: atoms of different elements differ in size, mass, and other properties.

• 3. Atoms cannot be subdivided, created, or destroyed.• 4. Atoms of different elements combine in simple whole-number ratios to

form chemical compounds.• 5. In chemical reactions, atoms are combined, separated, or rearranged.• - He also developed an assumption based on the faith of nature’s simplicity

that when atoms combine in only one ratio, it must be presumed to be binary one, unless some cause appear to the contrary.

• - Limitations included that nowadays, Dalton’s second and third points to the atomic theory and proven wrong.

John Dalton’s Life And Contribution

There were three fundamental laws established

by Dalton and other scientists of his time to

support the atomic theory. These laws are the:• a) Law of Conservation of Mass

The law of conservation of mass states that in a chemical reaction, matter is neither created nor destroyed, or, more accurately, there is no detectable change in mass during an ordinary chemical reaction.

 • b) Law of Definite Proportions

The law of definite proportions states that different samples of any pure compound contain the same elements in the same proportions by mass.

 • c) The Law of Multiple Proportions

The law of multiple proportions states that the mass of one element that can combine with a fixed mass of another element are in a ratio of small whole numbers.

John Dalton’s Life And Contribution

3 Fundamental Laws

In 1896 , he took the cathode ray experiments a step further by firstly improving Perrin’s version to more clearly prove cathode rays do carry negative charges. With this , Thomson then went on to discover the electron through his demonstration of cathode rays responding to electrode fields just as negatively charged particles would.

Thomson had figured out a way to determine the charge of the mass by using both an electric and magnetic field.

Used mutually perpendicular electric and magnetic fields to determine the speed of cathode rays. Then with only one field turned on, he measured the deflection of rays. These deflections depended on magnitude of field, length of path in the field, and the speed, mass, and charge of cathode-ray particles.

With calculation, he found reasonably consistent values for the charge-mass ratio, which allowed him to conclude that all cathode rays consist of identical particles with exactly the same negative charge.

1897 - English chemist and physicist; discovered 1st subatomic particles

J.J Thomson’s Life And Contribution

• Atoms contain negatively charged particles called electrons and positively charged matter.

• Created a model to describe the atom as a sphere filled with positive matter with negative particles mixed in Referred to it as the plum pudding model

J.J Thomson’s Life And Contribution

Small, dense, positively charged particle present in nucleus called a proton Electrons travel around the nucleus, but their

exact places cannot be described. 1912 - New Zealand physicist discovered the nucleus By 1909, he had shown that some radioactive elements, such as radium

and thorium, emitted positively charged helium ions, which are also known as alpha particles and when passed through a thing sheet of mica, a beam of alpha particles will spread out.

He had a technique that allowed him and his assistants, Hans Geiger and Ernest Marsden, to measure the proportion of alpha particles scattered at different angles from various materials. They would produce a pencil-shaped beam of alpha particles and position a thin sheet of gold foil at a right angle to the beam. Then they would use a screen coated with zinc sulfide, which would detect the scattered particles by letting off faint flashes of light visible with a microscope. By moving the screen and microscope around the foil, they were able to measure the rates at which alpha particles appear at various angles. Eventually, they concluded the positive charge in a gold atom must be concentrated in an incredibly tiny volume, so most of gold was actually empty space.

Ernest Rutherford’s Life And Contribution

When using aluminum foil instead of gold, they proved that the positive charge and most of the mass of an atom are contained in a radius less than 10 -14

Discovered nucleus and disproved the raisin-bun model Lead to planetary model of atom consisting of electrons orbiting the

nucleus of anatom and there being an electrostatic attraction between positive nucleus and negative electrons, which provides the centripetal force to keep the electrons in orbit.

Limitations included the fact that Rutherford’s model was later adjusted by NielsBohr because in Rutherford’s model, the electrons should spiral into the nucleus in a few microseconds due to a constant acceleration, which would emit electromagnetic waves that would take energy from the orbiting electrons.

Ernest Rutherford’s Life And Contribution

Corrected the critical flaw in Rutherford’s model. Focuses on the quantization of energy of electrons - Basic principles of Bohr’s model: 1. Electrons can orbit the nucleus only at certain specific distances fromthe nucleus. These distances are particular multiples of the radius of thesmallest permitted orbit meaning the orbits in an atom are quantized. 2. The electron's distance from the nucleus determines both the kineticand electric potential energy of an electron in orbit. So forth the energy in anelectron is also quantized and each orbit corresponds to a specific energylevel for the electron. 3. Only by emitting or absorbing photons of equal energy to the differencebetween energy levels can an electron move from one energy level toanother. When an electron continues to orbit at particular energy level, noenergy is radiated. Also, since the size and shape of the orbit remains thesame and at a fixed energy level, these orbits are often referred to asstationary states.

Niel’s Bohr’s Life And Contribution

Limitations included him not explaining as to why energy is quantized, why orbiting electrons do not radiate electromagnetic energy, why a magnetic field splits the main spectral lines into multiple closely spaced lines, and the fact that it is not accurate for electrons to have two or more electrons.

Electrons travel around the nucleus in definite paths and fixed distances.

Electrons can jump from one level to a path in another level.

1913 - Danish physicist; discovered energy levels

Niel’s Bohr’s Life And Contribution

In 1924, Louis de Broglie developed his theory that particles have wave properties. He concluded this through diffraction experiments. So forth, the principles of interference and standing waves apply for electrons orbiting a nucleus.

For most sizes of orbit, successive cycles of the electron wave will be out of phase, and destructive interference will reduce the amplitude of the wave. For constructive interference to occur, the circumference of the orbit must be equal to a whole number of wavelengths.

The wave nature of matter provides a natural explanation for quantized energy levels.

In 1926, Erwin Schrödinger derived an equation for determining how electron waves behave in the electric field surrounding a nucleus. The solutions to his equation are functions that define the amplitude of the electron wave in the space around a nucleus.

In quantum model, electrons behave as waves, which do not have a precise location.

Quantum Model (De Broglie)

The atomic spectra is a range of characteristic frequencies of electromagnetic radiation that are readily absorbed and emitted by an atom.

An electron can jump from one fixed orbital to another. If theorbital it jumps to has a higher energy, the electron must absorb photon of a certain frequency. If it’s a lower energy, the electron must give off a photon of a certain frequency.

The frequency depends on the difference in energy between the orbitals. This relates to Bohr’s model because Bohr’s model describe show in order

for electrons to move from one orbital to another, the electron must release or absorb a photon of appropriate energy.

ATOMIC SPECTRA

Models Presented

Solid Sphere Model or Bowling Ball Model

Proposed by John DaltonPlum Pudding Model or

Raisin Bun ModelProposed by J.J. Thomson

Nuclear ModelProposed by Ernest

Rutherford

Bohr Model or Planetary Model

Proposed by Niels Bohr

Electron Cloud Model Proposed by Erwin

Schrodinger

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