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UNIT 3 ATOMIC STRUCTURE

Mrs. Medina

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Early Models of the Atom

• All matter is composed of tiny fundamentals particles –ATOMS: smallest particles of an element

that retain the identity of the element in a chemical reaction

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• Greek philosopher (~400 BC) suggested existence of atoms – He claimed atoms were

tiny, indivisible, and indestructible

• His ideas agree with

modern theory but they were not based on scientific experiments.

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Democritus’s Atomic Philosophy Dalton’s Atomic Theory

• John Dalton (1766-1844), – English chemist and

schoolteacher.

• He used experimental methods and transformed Democritus’s ideas about atoms into scientific theory. – He studied the ratios in which

elements combine – He formulated an atomic

theory. 4

Dalton’s Atomic Theory

1. All elements are composed of tiny indivisible atoms

2. Atoms of the same element are identical and atoms of different elements are different.

3. Atoms of different elements can mix physically or chemically in whole-number ratios to form compounds.

4. Chemical reactions occur when atoms are joined, separated or rearranged.

5. Atom of one element can never became an atom of another element during a chemical reaction.

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Sizing up the Atom • Atoms are around

1 x 10-10 meters in size. – Line up 100,000,000

copper atoms side by side, they would only produce a 1 cm long line.

– A copper penny contains about 2.4x1022 atoms.

– Earth’s population = 6x109

• Atoms can be seen with a scanning tunneling microscopes

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Subatomic particles

• There are 3 kinds of subatomic particles –Electrons

–Protons

–Neutrons

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Electrons

• In 1897, JJ Thomson (English physicist) discovered the electron.

• Electrons are negatively charged subatomic particles.

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How were electrons discovered?

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ANODE

CATHODE

The green beam, or cathode ray, travels from cathode to the anode.

• A low pressure gas was placed in a tube with electrodes on the end

• Ran an electric current and saw a green beam.

How were electrons discovered? • A cathode ray can be

deflected by a magnet or electrically charged metal plates.

• The beam was attracted the positive and repelled from the negative. – Thus, Thomson hypothesized

the cathode ray was a stream of tiny negatively charged particles.

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• Originally called corpuscles; later renamed electrons.

How were electrons discovered?

• How did Thomson test his hypothesis? –He set up an experiment to measure the

“charge to mass” ratio of these electrons.

–He used different gases and different electrode metals

–=> The ratio was constant

• Conclusion: These electrons must be part of the atoms of all elements!

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More on Electrons

• In 1916, Robert Millikan (American physicist) reported the charge and mass of an electron.

• 1 unit of negative charge

•1

1840th the mass of a

hydrogen atom

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Protons and Neutrons • What is left after an atom loses a

negatively charged electron? – Something that has a positive

charge. • In 1886, Eugen Goldstein, observed

rays that travel in the opposite direction as the cathode rays (electrons). – He called them canal rays. – These are positively-charged particles

known as protons and have a mass 1840 times that of an electron.

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Protons and Neutrons

• In 1932, James Chadwick (English physicist) confirmed the existence of the neutron. –Neutrons are subatomic particles with

no charge but with a mass nearly equal to a proton.

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The Plum Pudding Model

• How were all these particles put together in the atom? – JJ Thomson thought the

electrons were evenly distributed in the atom

–Plum pudding model: electrons are stuck into a lump of positive charge like raisins stuck in dough.

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Rutherford and the nucleus

• In 1911, Ernest Rutherford proved this model was wrong: –electrons were floating around a tiny

positive core which we now know as the nucleus.

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Rutherford’s Gold Foil Experiment • Objective: To test the current atomic

theory, the plum pudding model. –Use positively charged alpha particles and

fire them at a very thin sheet of gold foil.

• If the plum pudding model was correct, all the alpha particles should have gone straight through with a slight measurable deflection due to the spread out positive charge.

• What happened???

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Rutherford’s Gold Foil Experiment

• The great majority of alpha particles passed straight through with NO deflection.

• But, a small fraction bounced back off the gold foil at very large angles.

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The atom is mostly empty space

A very small but massive center has a

positive charge

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The nuclear atom • The nucleus is the tiny central core of an

atom and is composed of protons and neutrons.

• In the nuclear atom,

–The protons and neutrons are located in the nucleus.

–The electrons are distributed around the nucleus and occupy almost all the volume of the atom.

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Rutherford’s problem:

In the following pictures, there is a target

hidden by a cloud. To figure out the shape of

the target, we shot some beams into the cloud

and recorded where the beams came out. Can

you figure out the shape of the target?

Target

#1

Target

#2

• Elements are different because they contain different numbers of protons – The atomic number = number of protons

– The atomic number (# protons) identifies an element.

• Atoms are electrically neutral

–Protons = Electrons

7 protons 1 proton 19 protons

ATOMIC NUMBER MASS NUMBER

• Mass Number = the total number of protons and neutrons

• Number of neutrons = mass number – atomic number

Neon Isotopes

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Isotopes

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• Are there 3 different kinds of neon atoms? YES, but they are all Neon atoms. –ISOTOPE: atoms that have the same

number of protons but different numbers of neutrons. • They are atoms of the same element. • They are chemically alike because they have

the same number of protons and electrons.

–Elements occur in nature as mixtures of isotopes.

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ATOMIC MASS • In nature, most elements occur as

mixture of two or more isotopes. – Each isotope has a fixed mass and

abundance • The ATOMIC MASS of an element is a

weighted average mass of the isotopes

• A weighted average reflects: – The mass of the isotopes – The relative abundance of the isotopes

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How to calculate the Atomic Mass

Name Percent abundance

Mass (amu) Average atomic mass

Hydrogen-1 99.985% 1.0078

1.00794 Hydrogen-2 0.014% 2.0141

Hydrogen-3 0.001% 3.0160

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Arithmetic Mean (1.0078 + 2.0141 + 3.0160) /3 = 2.0126 ???

Weight Average (1.0078*0.99985 + 2.0141*0.00014 + 3.0160*0.00001 = 1.00796 Yeah!!!!

To calculate Atomic Mass: Multiply the mass of each isotope by the natural

abundance in decimal form Add all the products

The Periodic Table: A Preview

• A “periodic table” is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties – The periodic table allows you to easily

compare the properties of one element to another

The Periodic Table: A Preview

• Each horizontal row (there are 7 of them) is called a period

• Each vertical column is called a group, or family

– Elements in a group have similar chemical and physical properties

– Identified with a number and either an “A” or “B”