Early Theories of Matter

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Presentation transcript:

Early Theories of Matter You will learn: 1. The early ideas about the atom. 2. What led to the discovery of electrons, protons, and neutrons.

I. The Philosophers A. Democritus (460-370 BCE) – 1st proposed the idea that matter was composed of tiny indivisible particles he called atomos. B. Aristotle (384-322 BCE) – very influential, rejected Democritus’ idea.

1. Since Democritus could not prove his idea, his hypothesis was rejected. 2. Atomic theory wasn't rigorously tested until the 1700's. 3. The following laws apply to all compounds.

a. Law of Definite Proportions i. A given compound contains the same elements in exactly the same proportions regardless of the size of the sample or the source of the compound. Ex: NaCl is always 39.34% Na and 60.66% Cl

b. Law of Conservation of Mass i. Mass of the products must equal the mass of the reactants!

c. Law of multiple proportions i. The mass ratio of the elements in a compound that combines with a fixed mass of another element can be expressed in small whole numbers.

II. John Dalton (1776- 1844) A. Revived Democritus’ idea based on his research. 1. Used the Law of Conserv. of Mass, Law of Mult. Proportions and Law of Def. Proportions to support Democitus’s hypothesis of the atom.

B. Dalton’s Atomic Theory All matter is made of indivisible and indestructible atoms. All atoms of a given element are identical in their physical and chemical properties. Atoms of different elements differ in their physical and chemical properties. Atoms of different elements combine in simple whole-number ratios to form compounds. Chemical rxns consist of the combination, separation or rearrangement of atoms.

Dalton’s Atomic Theory cont. 6. Atoms cannot be created, divided into smaller particles, or destroyed 7. Different atoms combine in simple whole number ratios to form compounds 8. In a chemical reaction, atoms are separated, combined or rearranged.

C. Modifications to Dalton’s Atomic Theory 1. Atoms are divisible subatomic particles (Protons, Neutrons, and Electrons) 2. All atoms of a given element are not identical in regard to atomic mass. a. They vary in the # of sub atomic particles 3. Atoms can be created or destroyed through nuclear fusion and fission.

D. Defining the Atom 1. The smallest particle of an element that retains the properties of the element Ex: Imagine placing six billion Cu atoms (equal to the world’s population) side by side would result in a line of copper atoms less than one meter long.

E. Atomic Mass 1.The mass of a Cu atom is 1.0552 x 10- 25kg! -This is an average! Cu Isotope 69.2% = 1.04497 x 10-25kg Cu Isotope 30.8% = 1.0552 x 10-25kg

III. The mole! A. We don’t work on the microscopic level; we live in the visible world! B. The Mole is used to bridge the gap between the microscopic and macroscopic levels.

Mole continued... C. A mole is a collection of 6.022 x 1023 particles. 1. A mole of any atom that has a mass in grams numerically equal to it’s atomic mass in atomic mass units”

Mole con’t. D. Who came up with this idea? Amedeo Avogadro 1. This is why the mole is also called Avogadro’s constant or Avogadro’s # 2. “If each of the 6 billion people on Earth counted one mole of one element at a rate of 1 atom per second it would take over 3 million years to count all the atoms!”

E. Discovering the electron 1. Experiments in the mid-1800’s lead to the first modifications of Dalton’s Theory. 2. While looking at electricity J.J. Thomson (1856-1940) – used the CRT to find the ratio of the particles charge to its mass.

a. Cathode – an electrode through which electrons leave a metal b. Anode – an electrode through which electrons enter a metal

Discovering the electron (cont.) 3. Thomson concluded that the mass was extremely small and is given credit for identifying the 1st subatomic particle – the electron. a. Plum pudding model of the atom

Discovering the electron (cont.) 4. Robert Millikan (1868-1953) determined the charge of an electron using oil droplets. a. The charge has been established as –1. b. Knowing the charge and the charge to mass ratio, he calculated the mass of an electron to be 9.11 x 10-28 g

F. Discovering the nucleus 1. Ernest Rutherford was experimenting with gold foil and alpha particles. 2. He discovered that when he shot the alpha particles at the foil, they were deflected in different directions. 3. Atoms must have empty spaces inside them.