Presentation on theme: "The Building Blocks of Matter"— Presentation transcript:
1 The Building Blocks of Matter Chapter 3: The AtomThe Building Blocks of Matter
2 From Philosophical Idea to Scientific Theory Democritus – Particle theory of matter in 400 BCAtomos - indivisible – atom kept as name even though not indivisibleAristotle 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 matterChemical reaction thought as transformation of a substance or substances into one or more new substances1790s quantitative analysis study began and led to the followingLaw of conservation of mass – mass is neither create or destroyed during ordinary chemical reactions or physical changesLaw 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 compoundLaw 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 followingLaw of conservation of mass – mass is neither create or destroyed during ordinary chemical reactions or physical changesC O = COLaw 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 compoundLaw 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 numbersC O CO C 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 unchangedAtoms are now known to be divisibleAtoms of the same element can have different masses
6 Discovery of the Electron Experiments with cathode ray tubeCathode 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 themShowed that atoms were divisible and electrons were part of the atomJ.J. Thomson showed that electrons have large charge to mass ratioIn 1909 Robert Millikan measured the charge on the electronThis led to measuring the mass as 1/1837 the mass of the simplest Hydrogen atomOther inferences:Because atoms are electrically neutral, they must contain a positive charge to balance the negative electronsBecause electrons have so much less mass than atoms, atoms must contain other particles that account for most of their mass.
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 Particle Symbols Charge Mass # Relative Mass Actual MassElectron e-, 0e X 10-31Proton p+, 1H XNeutron no, 1n X10-27Nuclear 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 AtomsRadius 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 = cmConsider 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 pmNuclei of atoms have a smaller radii of about pmNuclei of atoms have a high density of 2 X 108 metric tons/cm3