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Atomic Structure The Idea of the Atom.

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Presentation on theme: "Atomic Structure The Idea of the Atom."— Presentation transcript:

1 Atomic Structure The Idea of the Atom

2 Early Models Greek philosophers – 450 B.C. what is the smallest particle? Democritus - Particles are atomos An atom is the smallest particle of an element that retains the chemical identity of that element.

3 Aristotle Wins Greek society - slave based
Beneath them to work with hands – no experiments Settled disagreements by arguments Aristotle more famous than Democritus Aristotle wins Didn’t believe or look into atoms until…

4 Late 1700s Chemists believed elements:
couldn’t be broken down by ordinary means Combine to form compounds Advancements in balances  better experiments

5 Laws Discovered Law of Conservation of mass: mass is neither created nor destroyed during chemical or physical changes Then discovered no matter where or how a compound is made it is made of fixed proportions of elements  Law of Definite Proportions Ex: Sodium Chloride, table salt, always has 39.34% Na and 60.66% Cl by mass

6 Laws Discovered Elements combine to form more than one compound Ex: carbon monoxide, CO, and carbon dioxide CO2 Law of Multiple Proportions

7 John Dalton 1808 –DALTON’S ATOMIC THEORY
All matter is composed of small particles called atoms. *All atoms of a given element are identical, but they differ from those of any other element.

8 Dalton’s Atomic Theory
*Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in whole-# ratios to form compounds In a chemical reaction, atoms are combined, separated, or rearranged.

9 1800s – The atom IS divisible
Atom has two regions: Nucleus: center of atom Protons: (+) charged Neutrons: no charge Electron cloud: region around nucleus containing electrons, (-) charged Subatomic particles: p+, no, and e-

10 - + J.J. Thomson’s Experiment Cathode - Ray Tube Voltage source
Moving electrical current (Cathode Ray) through glass tube with inert gas (Noble Gas)

11 J.J. Thomson’s Experiment
Voltage source + - By adding magnets, found the moving (-) particles

12 Thomson’s Model Same properties no matter what element used
All atoms had ELECTRONS!! Thomson’s Model Plum pudding: bunch of positive stuff, with the electrons able to be removed.

13 Ernest Rutherford - 1911 Atoms have e- = (-) charge
Atoms are neutral  need (+) to balance Must be other particles to account for mass of atom… Rutherford used alpha particles to test. Alpha particle: (+) charge, (2 p+ and 2 no) Experiment: beam of high speed alpha particles into thin gold foil

14 Lead block Uranium Gold Foil Florescent Screen
Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004

15 His prediction…. Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004

16 What he expected Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004

17 What he happened Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004

18 Results… Most particles passed through 1 in 8000 bounced back
Some particles completely reversed path!!!

19 Small dense, positive piece at center, nucleus
Plum-pudding is WRONG! Atom is mostly empty Small dense, positive piece at center, nucleus Alpha particles are deflected by nucleus if they get close. + Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004

20 + Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004

21 Electron Cloud Model Current model of atom
Nucleus surrounded by negative cloud “electron cloud”

22 Size of Atom Atom = size of football stadium
Nucleus = smaller than a dime in center Electron = each one smaller than Roosevelt’s eye on the dime Atom is mostly empty space!

23 Composition of Atom Proton has (+) charge equal in magnitude to (-) charge of electron Atoms neutral  # p+ = # e- p+ and no about same mass e- = 1836 times smaller than p+ Nucleus has majority of mass Nuclear Forces: short-range forces that hold nucleus together p+-p+, p+-no, no-no

24 Counting Atoms Atomic Number (Z): number of p+ of each atom of the element Identifies Elements!!

25 Atomic Number What is the atomic number for… Hg
What is the element with … 20 p+ Neutral atoms  # p+ = # e- In neutral atoms, atomic number = # e-

26 Isotopes Def: atoms of same element with different # of no (different masses) 3 H isotopes: Protium: 1 p % Deuterium: 1 p+ 1 no % Tritium: 1 p+ 2 no

27 Mass Number Use mass number to determine isotopes
Mass #: # of p+ and n0 Atom with 17 p+ and 20 no has mass # of… Chlorine-37 – Hyphen Notation

28 Cl Writing Isotopes 37 17 Nuclear symbol: Mass number Elemental symbol
Atomic number

29 Finding # of Neutrons 27Al # of neutrons = mass # - atomic #
How many p+, no and e- are in… Selenium – 79 27Al Oxygen - 18 13

30 Isotopes Write the complete chemical/nuclear symbol for the isotope with 21 protons, 24 neutrons, and 21 electrons.

31 Relative Mass of Atom Mass of Oxygen-16 = 2.656 x 10-23 g
Use relative atomic masses Pick standard  other masses are expressed in relation to standard Standard: Carbon –12 atom

32 Carbon - 12 Has mass of 12 atomic mass units (amu)
1 amu: is exactly 1/12 the mass of a carbon-12 atom Carbon-12: how many p+ and no? 1 p+  1 amu ( amu) 1 no  1 amu ( amu) 1 e-  0 amu ( amu)

33 Carbon - 12 What element is 4 x mass of Carbon –12?
What element is 1/3 mass of Carbon –12?

34 Average Atomic Masses Most elements are mixture of isotopes
Mass of element is average mass of isotopes We need the % of each isotope Average Atomic Mass: weighted average of the atomic masses of the naturally occurring isotopes of an element.

35 Weighted Average Box with 100 marbles of 2 sizes:
25% have mass of 2.00g 75% have mass of 3.00g What is the average mass of a marble? 2.75g (sig figs!!)

36 Average Atomic Mass Or (atomic weight) found on periodic table (ROUND TO 2 DECIMAL PLACES!!) Average Atomic Mass = (% Iso. #1)(Mass Iso.#1) + (%Iso.#2)(Mass Iso.#2) + … 100

37 Average Atomic Mass Find the atomic mass of Li if…
7.5% is Lithium – 6 = amu 92.5% is Lithium – 7 = amu

38 Example – AAM = 16.00 amu Isotope Fractional Abundance
Oxygen – 16 (15.99 amu) 99.757% Oxygen – 17 (17.00 amu) .038% Oxygen – x .235%

39 The Mole

40 Quantities 1 dozen = 1 gross = 1 ream of paper =
12 1 gross = 144 1 ream of paper = 500 In chemistry: 1 mole = x 1023

41 The Mole SI unit for amount of substance (mol)
Def: the number of particles in exactly 12 g of carbon – 12. 12 g of carbon –12 has x 1023 atoms Avogadro’s #, after Amadeo Avogadro The number is HUGE!!

42 Molar Mass Def: mass of 1 mole of a pure substance
1 mole Carbon –12 = 12 g 1 atom Carbon – 12 = 12 amu Mass of 1 mole of He atoms? 4.00g/mol Same as atomic mass from periodic table, different units

43 Molar Mass So: 4.00g He, 6.94 g Li and g Hg all have x 1023 atoms = 1 mole!!! Molar mass (g/mol)  conversion factor!!

44 Gram/Mole Conversions
Mass, in g, of 3.6 mol of C? 43g How many moles are in 23.5 g S? 0.733 mol p. 85 problems

45 Conversions with Avogadro’s #
Diatomic Molecules: elements that exist as 2-atom molecules in natural state 7 diatomic molecules H2, O2, N2, F2, Cl2, Br2, I2 Remember!!! HONClBrIF

46 Conversions with Avogadro’s #
How many moles of oxygen are in 2.00x1022 molecules of oxygen? How many atoms of sodium are in 3.80 mol of sodium?

47 Conversions with Avogadro’s #
How many molecules of hydrogen are in g of hydrogen? Mass of 5.0x109 atoms of neon? p.86 problems

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