atomic structure the idea of the atom early models greek philosophers – 450 b.c. what is the...

Atomic Structure

The Idea of the Atom

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.

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…

Late 1700s Chemists believed elements:

couldn’t be broken down by ordinary means Combine to form compounds

Advancements in balances better experiments

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

Laws Discovered Elements combine to form more than one

compound Ex: carbon monoxide, CO, and carbon dioxide CO2

Law of Multiple Proportions

John Dalton 1808 –DALTON’S ATOMIC THEORY

1. All matter is composed of small particles called atoms.

2. *All atoms of a given element are identical, but they differ from those of any other element.

Dalton’s Atomic Theory3. *Atoms cannot be subdivided, created, or

destroyed

4. Atoms of different elements combine in whole-# ratios to form compounds

5. In a chemical reaction, atoms are combined, separated, or rearranged.

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-

Voltage source

+-

Moving electrical current (Cathode Ray) through glass tube with inert gas (Noble Gas)

J.J. Thomson’s ExperimentCathode - Ray Tube

Voltage source

J.J. Thomson’s ExperimentJ.J. Thomson’s Experiment

• By adding magnets, found the moving (-) particles By adding magnets, found the moving (-) particles

+

-

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.

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

Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

Lead block

Uranium

Gold Foil

Florescent Screen

Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

His prediction….

Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

What he expected

Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

What he happened

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

Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

Plum-pudding is WRONG!

+

Atom is mostly empty Small dense,

positive pieceat center, nucleus

Alpha particlesare deflected by nucleus if they get close.

Mr. Green. Mr. Green’s Homepage. 10 Sept. 2003. http://www.tvgreen.com/index.htm. 21 Aug. 2004

+

Electron Cloud Model Current model of atom Nucleus surrounded by negative cloud

“electron cloud”

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!

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

Counting Atoms Atomic Number (Z): number of p+ of

each atom of the element

Identifies Elements!!

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-

Isotopes Def: atoms of same element with different

# of no (different masses) 3 H isotopes:

Protium: 1 p+ 99.9885% Deuterium: 1 p+ 1 no 0.0115% Tritium: 1 p+ 2 no

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

Writing Isotopes Nuclear symbol:

Cl37

17Elemental symbol

Mass number

Atomic number

Finding # of Neutrons # of neutrons = mass # - atomic # How many p+, no and e- are in… Selenium – 79

27Al

Oxygen - 18

13

Isotopes Write the complete chemical/nuclear

symbol for the isotope with 21 protons, 24 neutrons, and 21 electrons.

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

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 (1.007276 amu) 1 no 1 amu (1.008665 amu) 1 e- 0 amu (0.005486 amu)

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

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.

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!!)

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

Average Atomic Mass Find the atomic mass of Li if… 7.5% is Lithium – 6 = 6.015 amu 92.5% is Lithium – 7 = 7.016 amu

Example – AAM = 16.00 amuIsotope Fractional Abundance

Oxygen – 16 (15.99 amu) 99.757%

Oxygen – 17 (17.00 amu) .038%

Oxygen – x .235%

The Mole

Quantities 1 dozen =

12 1 gross =

144 1 ream of paper =

500 In chemistry: 1 mole = 6.022 x 1023

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 6.022 x 1023 atoms Avogadro’s #, after Amadeo Avogadro The number is HUGE!!

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

Molar MassSo: 4.00g He, 6.94 g Li and 200.59 g Hg all

have 6.022 x 1023 atoms = 1 mole!!! Molar mass (g/mol) conversion factor!!

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

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

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?

Conversions with Avogadro’s # How many molecules of hydrogen are in

0.020 g of hydrogen? Mass of 5.0x109 atoms of neon? p.86 problems