The Atom: A Look Into the

Ultimate

ESSENTIAL QUESTIONS:1.Why was an image of

individual atoms considered such an important breakthrough?

2.How scientists discovered the component particles within atoms?

3.How can we possibly know anything about atomic structures?

Early Models of

the Atom

The Beginning...•Thales of Miletus -(640-546 BC) was the first important Greek scientist of record to speculate about the composition of matter.He assumed that water was the essence of all things, that it was the primary element.

•Anaximenes- (525 BC) chose air as his primary element.

Me Heraclitus- (535-475 BC) thought that it was fire.

Pythagoras -(582-507 BC) forsook the concept of a single, primary element for one involving all three- water, fire, air – plus a fourth, earth.

Empedocles- (ca. 450 BC) developed the “four elements” concept into a more definite form.

Aristotle- explained gravity by assuming that each of the four elements had a natural home to which it returned when free to do so.

The word “atom” comes from the Greek word a-tomos which means “indivisible”.

In 15th century, Greek philosophers Leuccippus of Miletus and Democritus of Abdera explained that matter was made up of

particles in continuous motion These particles have the following qualities:• indivisible because of their extremely small size•Indivisible as their name indicates solid• eternal because they are perfect •surrounded by an empty space•Having an infinite number of shapes

The idea of atomicity of matter did not gain much acceptance it was rejected by Plato and Aristotle.“they could keep on dividing matter in smaller and smaller pieces forever but at no point would you reach the indivisible particle”De Rerum Natura, a poem wrote by Lucretius that helped the atomic theory to survive.

Four elements VS. Atomistic theory of the Greeks Left

behind for 2000 years

16th and 17th Century PhilosophersGalileo and Newton accepted the idea of matter

Robert Boyle came up with the definition of elements – any substance that cannot be broken down into a simpler substance.

Anton Lavoisier wrote in his Elements of Chemistry ...if, by term elements we mean to express those simple and indivisible atoms of which matter is composed...”

Pierrre Gasendi-atoms form groups which he called molecule or corpuscular and holding together in groups are mechanical forces

John Dalton’s Atomic Theory•He began the modern atomic theory•He chose the word atom to express the idea indivisibility• He pictured an atom as a tiny indestructible sphere with mass.•He is known as one of the fathers of modern physical science•.

Postulates of Dalton’s Theory1. Matter is made up of extremely small indestructible particles called atoms.2. All atoms of a given element are alike.3. Atoms enter into combination with other atoms to form compounds, but remain unchanged during ordinary chemical reactions.4. Atoms can combine in simple numerical ratios such as 1:1, 1:2, 1:3, and so on.

Dalton’s Atomic Theory gives sufficient explanation to the three laws of chemical combination making his theory valid and acceptable.•Law of Conservation of Mass•Law of Definite Composition/Proportion•Law of Multiple Proportions

Atomic Weight/Mass•Equal to the proportionate average of the masses of the isotopes of a particular element

•Atom is mass unit (amu) unit for mass which is equivalent to 1/12th the mass of a carbon-12 atom

•This means that the mass of one C-12 is exactly 12 amu.

Inside the Atom•Electrical Nature of Matter-Coulomb’s Law formulated by Charles Coulomb stated that...“Bodies bearing the same charge repel one another, while bodies of opposite charges attract one another.”

-Luigi Galvani Was successful in passing a current by making use of the energy derived from a chemical change

Count Alessandro Volta-his discovery of battery sparked widespread investigation into the phenomenon of electrolysis

Sir Humphry Davy and Michael Faraday-studied the electrical composition of numerous compounds

Michael Faraday-proposed the quantitative law to express magnitude of electrolytic effects-George Stoney proposed the name electrine for the charge

Thompson’s Atomic ModelPlum Pudding/Raisin Bread Model“atom was composed of negatively charged sphere wherein the electrons were loosely embedded on the surface”

Natural Radioactivity-Henri Becquerel discovered radio activity-Father of Atomic and Nuclear Physics

Pierre and Marie Currie discovered radium and polonium.

Ernest Rutherforddiscovered the three rays- alpha, beta and gamma rays.

The Electron Sir Joseph John Thompson-he further concluded that cathode rays are negatively charged fundamental particles found in all atoms-cathode rays are now called electrons

Eugene Goldstein-discovered new particles which he called canal rays (now ions)

Wilhelm Roentgen -discovered X-rays

The Nucleus-According to Rutherford there must be a small and massive center inside each gold atom that was positively charged-he concluded that atom must be mostly empty space to explain why most alpha particles went straight through the gold foilRutherford’s Nuclear Model of an Atom-he was able to calculate the relative sizes of the nucleus and “electron space” based on the number of alpha particles deflected and their angles of deflection.

The Structure of the Nucleus and the Neutron

Rutherford’s found out that the number of positive charges in the nucleus increases from atom to atom by single unit when the elements are arranged consecutively. This means that the nuclear charge is the basis of the atom’s individuality

He also established that the positive charge is due to the presence of a fundamental particles called proton.

Atomic number (Z) – is the number of protons in the nucleusSince atom is electrically neutral, the number of protons = number of electronsJames ChadwickDiscovered neutronIt has mass roughly equal to that proton but has no electric chargeAtomic mass (A)- is equal to the sum of the masses of the protons and neutrons

IsotopesAtoms of the same element having different atomic masses

Thank You...God Bless Us All!