Models of The AtomModels of The AtomChapter 4Chapter 4

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Dalton Model of the AtomDalton Model of the Atom

Dalton proposes that Dalton proposes that ““all matter is made up of all matter is made up of tiny particles, which are molecules or atomstiny particles, which are molecules or atoms””..

Molecules can be broken down into atoms by Molecules can be broken down into atoms by chemical processes.chemical processes.

Atoms cannot be broken down by chemical or Atoms cannot be broken down by chemical or physical processes.physical processes.

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DaltonDalton’’s Model of Atom (1808)s Model of Atom (1808)

Based on Based on law of definite compositionlaw of definite composition, the mass ratio , the mass ratio of carbon to oxygen in carbon dioxide was always the of carbon to oxygen in carbon dioxide was always the same. Carbon dioxide was composed of 1 carbon atom same. Carbon dioxide was composed of 1 carbon atom and 2 oxygen atoms.and 2 oxygen atoms.

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DaltonDalton’’s Atomic Theorys Atomic Theory1)1) An element is composed of tiny, indivisible, An element is composed of tiny, indivisible,

indestructible particles called atoms.indestructible particles called atoms.2)2) All atoms of an element are identical and have the All atoms of an element are identical and have the

same properties.same properties.3)3) Atoms of different elements combine to form Atoms of different elements combine to form

compounds.compounds.4)4) Compounds contain atoms in small whole number Compounds contain atoms in small whole number

ratios.ratios.5)5) Atoms can combine in more than one ratio to form Atoms can combine in more than one ratio to form

different compounds.different compounds.

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It was found that the first two parts of DaltonIt was found that the first two parts of Dalton’’s atomic s atomic theory were incorrect.theory were incorrect.

Today proposals of 3, 4, and 5 are still accepted.Today proposals of 3, 4, and 5 are still accepted.

DaltonDalton’’s theory will stay as an important step in the s theory will stay as an important step in the further development of atomic theory.further development of atomic theory.

ContCont’’dd……DaltonDalton’’s Atomic Theorys Atomic Theory

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Subatomic ParticlesSubatomic Particles

50 years later after Dalton50 years later after Dalton’’s proposal, it was found s proposal, it was found that atoms that atoms were divisiblewere divisible..

Two subatomic particles were discovered:Two subatomic particles were discovered:

Negatively charged Negatively charged electronselectrons, e, e––..

Positively charge Positively charge protonsprotons, p, p++..

An electron has a relative charge of An electron has a relative charge of --11 and a proton and a proton has a relative charge of has a relative charge of +1+1..

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Thomson Model of the AtomThomson Model of the Atomsubatomic model of the atom (1903)subatomic model of the atom (1903)

““Plum PuddingPlum Pudding””Model of the atom:Model of the atom:

Electrons were Electrons were distributed evenly distributed evenly throughout a throughout a homogeneous sphere homogeneous sphere of positive charge.of positive charge.

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Mass of Subatomic ParticlesMass of Subatomic Particles

Thomson was able to calculate only the massThomson was able to calculate only the mass--toto--charge ratio of a proton and an electron.charge ratio of a proton and an electron.

Millikan determined the charge of an electron (1911).Millikan determined the charge of an electron (1911).

Using the charge of electron found by Millikan, Using the charge of electron found by Millikan, Thomson calculated the masses of a proton and Thomson calculated the masses of a proton and electron:electron:

An electron has a mass of 9.11 An electron has a mass of 9.11 ×× 1010--2828 gg

A proton has a mass of 1.67 A proton has a mass of 1.67 ×× 1010--2424 gg

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Types of RadiationTypes of Radiation

There are three types of radiation:There are three types of radiation:

Alpha (Alpha (αα), Beta (), Beta (ββ), & Gamma (), & Gamma (γγ))

Alpha rays are composed of helium atoms stripped of Alpha rays are composed of helium atoms stripped of their electrons (helium nuclei).their electrons (helium nuclei).

Beta rays are composed of electrons.Beta rays are composed of electrons.

Gamma rays are high energy electromagnetic Gamma rays are high energy electromagnetic radiation.radiation.

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RutherfordRutherford’’s Gold Foil Experiments Gold Foil Experiment

Rutherford fired alpha particles at thin gold foils. If Rutherford fired alpha particles at thin gold foils. If the the ““plum puddingplum pudding”” model of the atom was correct, model of the atom was correct, most most αα--particles should pass through undeflected.particles should pass through undeflected.

However, some of the alpha However, some of the alpha particles were deflected particles were deflected backwards.backwards.

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Explanation of ScatteringExplanation of ScatteringSince atom is largely empty space, most of the alpha Since atom is largely empty space, most of the alpha particles passed through the foil.particles passed through the foil.

Atomic nucleusAtomic nucleus contains the atomcontains the atom’’s protons and it is s protons and it is located at the center of an atom.located at the center of an atom.

The The αα--particles that particles that bounced backwards bounced backwards did so after striking did so after striking the dense nucleus.the dense nucleus.

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RutherfordRutherford’’s Model of the Atoms Model of the Atoma new model of the atoma new model of the atom

Negatively charged electrons are distributed around a positively charged nucleus.

An atom has a diameter of about 1 × 10-8 cm and the nucleus has a diameter of about1 × 10-13 cm.

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Subatomic Particles RevisitedSubatomic Particles RevisitedRutherford predicted that because of the heaviness of the nucleus, nucleus must contain neutral particles in addition to protons.Neutrons, n0, were discovered about 30 years later. A neutron has no charge and it is about the size of a proton.

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Atomic NotationAtomic NotationAtomic notation represents the number of protons and neutrons in the nucleus of an atom.

Atomic number, Z, represents the characteristic number of protons in the nucleus. Atomic number is element specific.

Mass number, A, represents the total number of protons and neutrons in the nucleus of an atom.

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Using Atomic NotationUsing Atomic NotationExample:

The element name: Sodium (symbol Na).

The atomic number is 11 – sodium has 11 protons.

The mass number is 23 = the atom of sodium has 23 protons + neutrons.

The number is neutrons is: A – Z = 23 – 11 = 12 neutrons.

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IsotopesIsotopes

Atoms of the same element that have a different number of neutrons in the nucleus are called isotopes.

Isotopes have the same atomic number but different mass numbers.

All atoms of the same element have the same number of protons.

Most elements occur naturally with varying numbers of neutrons.

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ContCont’’dd……IsotopesIsotopes

To refer to an isotope state the name of the element followed by the mass number.

Examples:

Cobalt-60, Carbon-14

Question:

How many protons and neutrons does an atom of mercury-202 have?

The atomic number of Hg is 80, so it has 80 protons

Hg-202 has 202 – 80 = 122 neutrons

Co6037 C14

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Average Atomic MassAverage Atomic Mass

Not all isotopes of an atom are present in equal proportions, to find the average atomic mass:

Use isotopes of Copper:63Cu with a mass of 62.930 amu and 69.09% abundance65Cu with a mass of 64.928 amu and 30.91% abundance

Average atomic mass of copper:(62.930 amu)(0.6909) + (64.928 amu)(0.3091)

= 63.55 amu

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ReviewReview

Atoms are composed of protons, neutrons, and electrons.

The protons and neutrons are located in the nucleus and the electrons are outside the nucleus.

Atoms are mostly empty space.

The number of protons is referred to as the atomic number for the atom.

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ContCont’’dd……ReviewReview

All atoms of the same element have the same number of protons.

Isotopes are atoms with the same number of protons but different numbers of neutrons.

The mass number for an isotope is the total number of protons plus neutrons.

The atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes.