“Atomic Structure”

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Essential Question: Describe how the model of the atom has changed since the Greek idea of “Atomos”

Defining the Atom

• Democritus

– He believed that atoms were indivisible and

indestructible “Atomos”

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Dalton’s Atomic Theory (experiment based!)

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3) Atoms combine in whole-number ratios

to form compounds

4) In chemical reactions, atoms are

combined, separated, or rearranged –

but never changed into atoms of

another element.

1) All elements are composed of tiny

indivisible particles called atoms

2) Atoms of the same element are

identical.

John Dalton

Discovery of the Electron

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In 1897, J.J. Thomson used a cathode ray tube to deduce

the presence of a negatively charged particle: the

electron

Thomson’s Atomic Model

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Thomson believed that the electrons were like plums

embedded in a positively charged “pudding,” thus it was

called the “plum pudding” model.

Ernest Rutherford’s

Gold Foil Experiment - 1911

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The Rutherford Atomic Model• Based on his experimental evidence:

– The atom is mostly empty space

– All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus”

– The neutron was discovered

by Chadwick in 1932

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Bohr Model

• Bohr refined Rutherford's model of the atom by finding that electrons were found in distinct energy levels around the nucleus. When electrons move to different energy levels energy is released.

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Quantum Atomic Model

• This is the current view of atoms, it is a mathematical model rather than a physical model. It describes the electrons as being found in regions of probability around the nucleus, an “electron cloud”

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Changes in Atomic model

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Democritus/Dalton Thompson(plum pudding) Rutherford

Bohr

Schrödinger (quantum Model)

Subatomic Particles

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Particle Charge Mass (g) Location

Electron

(e-) -1 negligible Electron

cloud

Proton

(p+) +1 1 Nucleus

Neutron

(no) 0 1 Nucleus

Atomic Number

The atomic number of

an element is the

number of protons in

the nucleus.

The protons determine

the identity of the

element

Mass Number

The mass number is the

number of protons and

neutrons in the nucleus of

an isotope.Carbon-14 indicates the mass is 14

Carbon-13 indicates the mass is 13

These are isotopes of carbon

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How to find number of protons

The number of protons is the same as the atomic number.

Atomic number = number of protons

How to find number of electrons

The number of electrons is the same as the number of protons.

Number of electrons = number of protons

How to find number of neutrons

1. Round mass number to the ones place. 35.453 = 35 24.805 = 25

2. Then subtract atomic number from mass number.

Mass # - Atomic # = number of neutrons

Mass number

Complete Symbols

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XMass

number

Atomic

numberSubscript →

Superscript →

Symbols

Find each of these:

a) number of protons

b) number of

neutrons

c) number of

electrons

d) Atomic number

e) Mass Number

Br80

35

18

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Nuclide p+ n0 e- Mass #

Oxygen - 10

- 33 42

- 31 15

8 8 1818

Arsenic 75 33 75

Phosphorus15 3116

Isotopes and Atomic Mass Notes

Essential Question: Why is the mass listed on the periodic table an average?

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Isotopes are….

• Atoms of the same element thathave a different numbers of neutrons.

• Thus, different mass numbers.

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• To show different isotopes the

mass # is written after element:

–carbon-12 carbon-14

Elements occur in nature as

mixtures of isotopes.

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Isotope Examples

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Examples of Isotopes:

Isotope Protons Electrons Neutrons Nucleus

Hydrogen–1

(protium) 1 1 0

Hydrogen-2

(deuterium) 1 1 1

Hydrogen-3

(tritium)

1 1 2

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Average Atomic Mass This is based on the abundance

(percentage) of each isotope of that

element in nature.

Instead of grams, the unit we use is the

Atomic Mass Unit (amu)

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• It is defined as one-twelfth the mass of a carbon-12 atom.

– Carbon-12 chosen because of its isotope purity.

• Each isotope has its own atomic mass, thus we determine the average from percent abundance.

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To calculate the average:

• Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results.

• If not told otherwise, the mass of the isotope is expressed in atomic mass

units (amu)27

Atomic Masses

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Isotope Mass % in nature (mass ) x (% as a decimal)

Carbon-12 12 98.89% 12 x .9889 =11.87

Carbon-13 13 1.11% 13 x .0111 = 0.14

Carbon-14 14 0.01% 14 x .0001 =

.0014

Atomic mass is the average of all the

naturally occurring isotopes of that element.

Carbon = 11.87 + 0.14 + .0014 =12.011

The Periodic Table:

A Preview

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Each horizontal row (there are 7 of

them) is called a period

Each vertical column is called a

group, or family