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Defining The Atom.  Atom: the smallest particle of an element  The Greek philosopher Democritus (460 B.C. – 370 B.C.) was the first to suggest the existence.

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Presentation on theme: "Defining The Atom.  Atom: the smallest particle of an element  The Greek philosopher Democritus (460 B.C. – 370 B.C.) was the first to suggest the existence."— Presentation transcript:

1 Defining The Atom

2  Atom: the smallest particle of an element  The Greek philosopher Democritus (460 B.C. – 370 B.C.) was the first to suggest the existence of atoms  He believed that atoms were indivisible and indestructible  His ideas were not based on experiments, just philosophy

3  Changed chemistry to a quantitative science  Measured the mass of a system before and after a reaction in a closed system ▪Stoichiometry  Isolated and named hydrogen and oxygen  Discovered how respiration and combustion are related  Major experiment involved cinnabar: red mercury oxide

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5  Copper carbonate from a variety of sources  Lab-made was 57.48% Cu, 5.43% C, 0.91% H, and 36.18% O  Natural was 57.48% Cu, 5.43% C, 0.91% H, and 36.18% O

6  A compound always contains the same elements in certain, definite proportions  Also known as the law of constant composition

7  ~2000 years pass before more was known about the atom  John Dalton; English Chemist & School teacher; 1803  Thought the atom looked like a marble.(Round, solid sphere)  His findings were based on scientific experiments, not philosophy

8  Dalton (1803)  Known as the FOUNDER of the atomic theory  Dalton invented the word ATOM as the basic unit of matter  Dalton also claimed that all atoms of a given element are IDENTICAL  He also discovered that atoms of different elements have different PROPERTIES AND MASSES  Found that combining atoms of different elements formed COMPOUNDS

9 1. Compounds are composed of elements 2. Atoms of the same elements are identical and differ from other elements. 3. Atoms chemically combine in whole- number ratios 4. Atoms cannot be created or destroyed

10  Atoms are extremely small  A penny containing pure copper contains 2,400,000,000,000,000,000,000,000 atoms  Population of our planet: ~6,000,000,000  If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long  Can only be observed using a scanning tunneling (electron) microscope

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13 Dmitri Mendeleev 1869 Arranged elements into 7 groups with similar properties. He discovered that the properties of elements "were periodic functions of their atomic weights". This became known as the Periodic Law.

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15  A magnet will deflect the cathode ray  Particles in the cathode ray are negatively charged

16 h?v=IdTxGJjA4Jw

17 J.J. Thomsom 1897 While using a CATHODE RAY TUBE he discovered that the ray was deflected (due to a magnetic/electrical field From this discovery he concluded that atoms contain small negatively charges particles called ELECTRONS Theory famously referred to as the PLUM PUDDING model because he visualized the ELECTRONS being EMBEDDED within the structure of the atom (just like raisin bread) The MASS of the rest of the atom (besides the electrons) was thought to be EVENLY DISTRIBUTED and POSITIVELY CHARGED

18 “Plum Pudding” model: the electrons were like plums embedded in a positively charged “pudding,” J. J. Thomson

19  The philosopher who first suggested the existence of atoms was ____________. A. John Dalton B. Democritus C. Ernest Rutherford

20  The three subatomic particles that make up an atom are. A. proton, neutron, electron B. nucleus, proton, electron C. cathode rays, canal rays, neutrons

21  True or False? A Scanning Tunneling Electron Microscope is the only device we can use to see atoms. A. True B. False

22  John Dalton’s model of the atom looked like a ___________ A. soccer ball B. marble C. football

23  Democritus believe that atoms were _____ A. Indivisible & Invisible B. Indestructive & Invisible C. Indivisible & Indestructive

24  True or False? There were only ~200 years between Democritus’ theory and John Dalton’s work with the atom. A. True B. False

25  The scientist who discovered the electron was _____________ A. JJ Thomson B. Robert Millikan C. Ernest Rutherford

26 1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a proton Mass of the electron is 9.11 x g The oil drop apparatus

27  Discovered the “Proton”  Goldstein proved that when negative particles are formed, so are positive  These positive particles were found to have a mass 1837x that of an electron

28 932 – James Chadwick (1932)  discovered the “neutron”  a particle with no charge, but a mass equal to a proton

29  In 1911, Designed the Gold Foil Experiment  Aimed a beam of alpha ( α) particles at a thin piece of gold foil (only a few atoms thick.)  Most α particles passed through the foil…  A small amount of the α particles were deflected  To their surprise, some α particles bounced straight back. 1 in ~8000 did not go through the foil

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31  Most went straight through  Mostly empty space  Some were deflected at huge angles  Positive charge in center of atom  Others bounced straight backwards!!  Small, dense solid area in center of atom  Nuclear Model

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34  Conclusions:  There is a nucleus in the center of the atom where most of its mass is.  The nucleus is positive  Atoms are mostly empty space  Disproved J.J. Thomson’s Plum Pudding Model  If an atom is the size of a football stadium, the nucleus is the size of a marble

35  Planetary Model 1913  Nucleus surrounded by orbiting electrons at different energy levels  Electrons have definite orbits  Utilized Planck ’ s Quantum Energy theory  Worked on the Manhattan Project (US atomic bomb)

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37  Change to Dalton’s atomic theory: atoms are divisible into subatomic particles:  Electrons, protons, and neutrons

38 ParticleChargeMassLocationElectron (e - ) virtually nothing Electroncloud Proton+11Nucleus Neutron01Nucleus

39  Elements are different because they contain different numbers of PROTONS  atomic number = the number of protons in the nucleus  # protons in an atom = # electrons

40 Mass number = number of protons and neutrons in the nucleus Mass # = p + nElementpne Mass # Oxygen Arsenic Phosphorus

41  Contain the symbol of the element, the mass number and the atomic number. X Mass number Atomic number Subscript → Superscript →

42 Symbols n Find each of these: a) number of protons b) number of neutrons c) number of electrons d) Atomic number e) Mass Number Br 80 35

43 Symbols n If an element has an atomic number of 34 and a mass number of 78, what is the: a) number of protons b) number of neutrons c) number of electrons d) complete symbol

44 Symbols n If an element has 91 protons and 140 neutrons what is the a) Atomic number b) Mass number c) number of electrons d) complete symbol

45 Symbols n If an element has 78 electrons and 117 neutrons what is the a) Atomic number b) Mass number c) number of protons d) complete symbol

46  Atoms of the same element can have different numbers of neutrons.  Thus, different mass numbers.  Isotopes change the number of neutrons & the mass number for an atom

47  Frederick Soddy proposed the idea of isotopes in 1912  Isotopes: atoms of the same element having different masses, due to varying numbers of neutrons.  Elements occur in nature as mixtures of isotopes.

48  Element name – mass number:  carbon-12  carbon-14  uranium-235

49 IsotopeProtonsElectronsNeutronsNucleus Hydrogen–1 (protium) (protium)110 Hydrogen-2(deuterium)111 Hydrogen-3(tritium)112

50  The Gold number on the P.T. is the Average Atomic Mass of the element  This is based on the abundance of each variety of that element in nature.  Instead of grams, the unit we use is the Atomic Mass Unit (amu)  It is defined as one-twelfth the mass of a carbon-12 atom.

51  Each horizontal row is called a period  Each vertical column is called a group, or family  Elements in a group have similar chemical and physical properties


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