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The Building Blocks of Matter Chapter 3: The Atom.

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1 The Building Blocks of Matter Chapter 3: The Atom

2 From Philosophical Idea to Scientific Theory Democritus – Particle theory of matter in 400 BC Atomos - indivisible – atom kept as name even though not indivisible Aristotle followed Demcritus and did not believe in atoms. Aristotle thought all matter made of fire, wind, water and air. In 1700s all chemists thought of elements as the simplest form of matter Chemical reaction thought as transformation of a substance or substances into one or more new substances 1790s quantitative analysis study began and led to the following – Law of conservation of mass – mass is neither create or destroyed during ordinary chemical reactions or physical changes – Law of definite proportions – a chemical compound contains the same elements in exactly same proportions by mass regardless of the size of the sample or source of the compound – Law of multiple proportions – If two or more different compounds are composed of the same two elements, then the ratio of masses of the seond element combined with a certain mass of the first element is always a ratio of small whole numbers

3 Foundations of atomic theory 1790s quantitative analysis study began and led to the following – Law of conservation of mass – mass is neither create or destroyed during ordinary chemical reactions or physical changes – C + O = CO – Law of definite proportions – a chemical compound contains the same elements in exactly same proportions by mass regardless of the size of the sample or source of the compound – Law of multiple proportions – If two or more different compounds are composed of the same two elements, then the ratio of masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers – C O COC O O CO2 – + = ++ =

4 Dalton’s Atomic Theory All matter is composed of extremely small particles called atoms. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. Atoms cannot be subdivided, created or destroyed. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. In chemical reactions, atoms are combined, separated, or rearranged. Introduced in 1808 by English school teacher, John Dalton. He turned Democritus’ idea into a scientific theory that could be tested.

5 Modern Atomic Theory Altered Dalton’s theory in the following ways: – All matter is composed of atoms and atoms of any one element differ in properties from atoms of another element remain unchanged – Atoms are now known to be divisible – Atoms of the same element can have different masses

6 Discovery of the Electron Experiments with cathode ray tube – Cathode rays were deflected by a magnetic field in the same manner as a wire carrying electric current, which was known to have a negative charge. – The rays were deflected away from a negatively charged object. – – These observations led to the hypothesis that the particles that compose cathode rays are negatively charged. – J.J. Thomson then did experiments to measure the ratio of the charge of cathode rays to their mass. – They were named electrons..youtube.comCathode Ray Tube - YouTu#197476

7 Charge and Mass of the Electron Cathode rays have identical properties regardless of the element used to produce them Showed that atoms were divisible and electrons were part of the atom J.J. Thomson showed that electrons have large charge to mass ratio In 1909 Robert Millikan measured the charge on the electron This led to measuring the mass as 1/1837 the mass of the simplest Hydrogen atom Other inferences: – Because atoms are electrically neutral, they must contain a positive charge to balance the negative electrons – Because electrons have so much less mass than atoms, atoms must contain other particles that account for most of their mass.

8 Millikan Experiment Oil Drop Experi#197598

9 Discovery of the Atomic Nucleus In 1911, Ernest Rutherford, Hans Geiger an Ernest Marsdan did an experiment with gold foil to show that the atom contain a small dense area called the nucleus. Experiment-#19761F

10 Composition of the Atomic Nucleus ParticleSymbolsChargeMass #Relative MassActual Mass Electron e -, 0 e X Protonp +, 1 H X Neutronn o, 1 n X Nuclear Forces – Short- range proton-neutron, proton-proton, and neutron- neutron forces hold the nuclear particles together and are referred to as nuclear forces.

11 The sizes of Atoms Radius of an atom is the distance from the center of the nucleus to the outer portion of the electron cloud. Because they are so small they are measured in the picometer. 1 pm = m = cm Consider that 1 cm is the same fractional part of 1000 km(about 600 mi) as 100 pm is of 1 cm. Atomic radii range – 40 to 270 pm Nuclei of atoms have a smaller radii of about pm Nuclei of atoms have a high density of 2 X 10 8 metric tons/cm 3


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