“atomic structure” 1 essential question: describe how the model of the atom has changed since...

“Atomic Structure”

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

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Defining the Atom• Democritus

– He believed that atoms were indivisible and indestructible “Atomos”

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Dalton’s Atomic Theory (experiment based!)

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

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Discovery of the ElectronIn 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively

charged particle: the electron

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Thomson’s Atomic Model

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

J. J. Thomson

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Ernest Rutherford’sGold 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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Subatomic Particles

Particle Charge Mass (g) Location

Electron (e-) -1 negligible Electron

cloud

Proton (p+) +1 1 Nucleus

Neutron (no) 0 1 Nucleus

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Atomic Number• The “atomic number” of an element is

the number of protons in the nucleus

• In a neutral atom the # protons is equal to the # electrons

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Mass Number

Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0

Nuclide p+ n0 e- Mass #

Oxygen - 10

- 33 42

- 31 15

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

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Complete Symbols

X Massnumber

Atomicnumber

Subscript →

Superscript →

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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

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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 that

have 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)

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Atomic Masses

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

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The Periodic Table:A Preview

Each horizontal row (there are 7 of them) is called a periodEach vertical column is called a group, or family