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Rhonda AlexanderRhonda Alexander

Atomic Structure & PeriodicityAtomic Structure & Periodicity

Chapter 3 - AtomsChapter 3 - Atoms

DemocritusDemocritus – a Greek philosopher 400 BC – a Greek philosopher 400 BC

* ‘Atomos’ – atoms are indivisible.* ‘Atomos’ – atoms are indivisible.

A – not Tomos - cutting

Dalton’s Atomic TheoryDalton’s Atomic Theory John DaltonJohn Dalton – English school teacher - 1808 – English school teacher - 1808 He proposed an explanation for several lawsHe proposed an explanation for several laws

•All matter is composed of small particles called Atoms•Atoms of a given element are identical in size, mass, &

other properties•Atoms cannot be subdivided, created, or destroyed•Atoms combine in simple whole-number ratios•In a rxn, atoms are combined, separated, or rearranged

Law of Conservation of Mass Law of Conservation of Mass (Energy)(Energy)

Matter is neither created nor destroyed.Matter is neither created nor destroyed. It only changes form.It only changes form.

If 5 g of element A combines with 10 g If 5 g of element A combines with 10 g of element B to form AB. How many of element B to form AB. How many grams of AB are formed?grams of AB are formed?

Law of Definite ProportionLaw of Definite Proportion

A chemical compound contains the same A chemical compound contains the same elements in exactly the same proportions elements in exactly the same proportions by mass regardless of the size of the by mass regardless of the size of the sample or the source of the compoundsample or the source of the compound

Table salt – NaCl 39.34 % Na & 60.66 % Cl

Law of Multiple ProportionsLaw of Multiple Proportions

Two or more different compounds are Two or more different compounds are composed of the same two elements, the composed of the same two elements, the masses of the 2masses of the 2ndnd element combine with a element combine with a certain mass of the 1certain mass of the 1stst element can be element can be expressed as ratios of small whole expressed as ratios of small whole numbers. numbers.

Water vs. Peroxide H2O H2O2

Water vs. Hydrogen PeroxideWater vs. Hydrogen Peroxide

Cathode Ray tubeCathode Ray tube

6.6. ***Sir Joseph John Thomson – 1897 ******Sir Joseph John Thomson – 1897 ***Electrons are composed of Electrons are composed of negatively negatively charged subatomic particlescharged subatomic particles..

Discovery of the Electron

1. Different gases glow with different colors if a current is passed through the tube.

2. Glass directly opposite the cathode glows.3. An object placed in between will cast a shadow.4. A paddle wheel will roll along on its rails from cathode

towardthe anode.

5. Rays are deflected away from a negative electrode.

Cathode Ray tubeCathode Ray tube

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Development of Atomic Models

Robert A. Millikan 1909Robert A. Millikan 1909

American physicist showed that the American physicist showed that the mass mass of the electron is 9.109 X 10 of the electron is 9.109 X 10 –31–31 kg kg

Confirmed the electrons carry a negative charge and its mass.

Brought about more questions about the atomic structure.

1. Atoms are neutral 2. Atoms have mass

Robert Millikan: Oil Droplet Experiment

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Rutherford Experiment: Nuclear AtomNot in handout

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Recording of Rutherford

Discovery of the Atomic Discovery of the Atomic NucleusNucleus

Ernest Rutherford – 1911 Gold Foil Ernest Rutherford – 1911 Gold Foil ExperimentExperiment

‘As if you had fired a 15 – inch shell at a piece of tissue paper And it came back to hit you.’

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IN NUCLEUS

Protons - + chargep+

Nuetrons – neutralno

Isotopes – atoms of the same element w/ different masses.

In The early 20th century, Rutherford showed that most of an atom’s mass is concentrated in a small positively charged region called the nucleus.

Electron cloud - 90% probability of finding the electron within this space

Electron: - charge ( e-)

Bohr Model

After Rutherford’s discovery, Bohr proposed that electrons travel in definite orbits around the nucleus

90 % probability of finding the electron within this space

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Structure of the AtomStructure of the Atom

AtomAtom – is the – is the smallest particlesmallest particle of an of an elementelement that retains the that retains the chemicalchemical propertiesproperties of the element of the element

Atom has 2 regions: Nucleus – protons & neutrons

Electron Cloud - Electrons

Weighing and Counting Atoms

Atomic number (Z) – number of protons in a nucleus

Mass Number – total number of protons & neutrons in a nucleus

Atomic Mass Units – amu 1/12 the mass of C-12

Charge = p+ - e-

Mass # = p+ + no

Atomic Mass – Weighted average mass of all the isotopes of the element

Examples of PEN

Atomic number = number of protons

If atom is neutral, then the number of protons must equal number of electrons.

Carbon Mass Number – Atomic Number = # of Neutrons

12 6

C 12 - 6 = 6 neutrons

1. Hydrogen 2. Sodium 3. Oxygen

4. Copper 5. Gold

IsotopesIsotopes

Isotopes of Hydrogen include Isotopes of Hydrogen include Hydrogen -1Hydrogen -1Hydrogen – 2, DuteriumHydrogen – 2, DuteriumHydrogen – 3, tritiumHydrogen – 3, tritium

They have the same number of They have the same number of protonsprotons but different numbers of but different numbers of neutronsneutrons and a and a different different mass numbermass number

isotopes

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Average Atomic MassAverage Atomic Mass – – is the is the weighted weighted average of the atomic average of the atomic

massesmasses of the naturally occurring of the naturally occurring isotopes of an element.isotopes of an element.

WeightedWeighted Average of Isotopes Average of Isotopes

Copper–63 69.17%, Copper-65 30.83%, Copper–63 69.17%, Copper-65 30.83%, Calculate the average atomic mass.Calculate the average atomic mass.

(63 x .6917) + ( 65 x .3083) =

43.5771 + 20.0395 = 63.6166

Carbon-12 98.90% , Carbon-13 1.1% 12.011