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CHAPTER 5

The Structure of Atoms

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Fundamental Particles

Particle Mass (amu) Charge

Electron (e-) 0.00054858 - 1

Proton (p,p+) 1.0073 +1

Neutron(n,n0) 1.0087 0

Three fundamental particles make up atoms:

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The Discovery of Electrons Late 1800’s & early 1900’s

Cathode ray tube experiments showed that very small negatively charged particles are emitted by the cathode material.

1897 – J. J. ThomsonModified the cathode ray tube and measured the charge to mass ratio of these particles. He called them electrons.(Nobel prize in physics, 1906)

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The Discovery of Electrons 1909 – Robert A. Millikan

Determined the charge and the mass of the electron from the oil drop experiment.(The second American to win Nobel prize in physics in 1923)

1910 – Ernest RutherfordGave the first basically correct picture of the atom’s structure.(Nobel prize in chemistry in 1908)

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Rutherford’s Atom

The atom is mostly empty space

It contains a very small, dense center called the nucleus

Nearly all of the atom’s mass is in the nucleus

The nuclear diameter is 1/10,000 to 1/100,000 times less than atom’s radius

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The Discovery of Protons 1913 – H.G.J. Moseley

Realized that the atomic number defines the element: Each element differs from the

preceding element by having one more positive charge in its nucleus

Along with a number of observations made by Rutherford and some other physicists, this led to the discovery of the proton The elements differ from each other by

the number of protons in the nucleus

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The Discovery of Neutrons 1932 – James Chadwick

recognized existence of massive neutral particles which he called neutrons(Nobel prize in physics in 1935) The atomic mass of an element is

mainly determined by the total number of protons and neutrons in the nucleus

The atomic number of an element is determined by the total number of protons in the nucleus

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Mass Number and Atomic Number

Mass number – A Atomic number – Z

Z = # protons A = # protons + # neutrons # protons = # electrons

EAZ

The way we typically write this:

Cl3717

Cl37

full nuclide symbol short nuclide symbol

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Isotopes Atoms of the same element but with

different masses The same element means that the

number of protons is the same, then different masses mean that

the number of neutrons differs

H11 H21 H31

protium(or hydrogen)

deuterium tritium

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Isotopes: Example

O16

O17

O18

U235

U238

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Experimental Detection of Isotopes

1919-1920 – Francis AstonDesigned the first mass-spectrometer(Nobel prize in chemistry in 1922)

Factors which determine a particle’s path in the mass spectrometer: accelerating voltage, V magnetic field strength, H mass of the particle, m charge on the particle, q

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Mass Spectrometry & Isotopes

Mass spectrum of Ne+ ions This is how scientists determine the masses and

abundances of the isotopes of an element

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Mass Spectrometry & Isotopes

Let’s calculate the atomic mass of Ne using the mass-spectrometry data

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Atomic Weight Scale

A unit of atomic mass (atomic mass unit) was defined as exactly 1/12 of the mass of a 12C atom

Two important consequences of such scale choice: The atomic mass of 12C equals 12 a.m.u. 1 a.m.u. is approximately the mass of one

atom of 1H, the lightest isotope of the element with the lowest mass.

The atomic weight of an element is the weighted average of the masses of its isotopes

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Naturally occurring chromium consists of four isotopes. It is4.31% 50Cr, mass = 49.946 amu83.76% 52Cr, mass = 51.941 amu9.55% 53Cr, mass = 52.941 amu2.38% 54Cr, mass = 53.939 amuCalculate the atomic weight of chromium

Isotopes and Atomic Weight

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Isotopes and Atomic Weight

Naturally occurring Cu consists of 2 isotopes. It is 69.1% 63Cu with a mass of 62.9 amu, and 30.9% 65Cu, which has a mass of 64.9 amu. Calculate the atomic weight of Cu to one decimal place.

A.W.(Cu) = (62.9 amu 0.691) + ( 64.9 amu 0.309) =

= 63.5 amu

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Electromagnetic Radiation

Any wave is characterized by 2 parameters:

Wavelength () is the distance between two identical points of adjacent waves, for example between their crestsIt is measured in units of distance (m, cm, Å)

Frequency () is the number of wave crests passing a given point per unit time (for example, per second)It is measured in units of 1/time, usually s-

1

1 s-1 = 1 Hz (Hertz)

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Electromagnetic Radiation

The speed at which the wave propagates:

c = The speed of electromagnetic waves in

vacuum has a constant value:

c = 3.00108 m/s This is the speed of light

Given the frequency of the electromagnetic radiation, we can calculate its wavelength, and vice versa

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Electromagnetic Radiation

Electromagnetic radiation can also be described in terms of “particles” called photons

Each photon is a particular amount of energy carried by the wave

Planck’s equation relates the energy of the photon to the frequency of radiation:

E = h (h is a Planck’s constant, 6.626·10-34 J·s)

Max Planck(Nobel prize in physics in 1918)

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Electromagnetic Radiation

What is the energy of green light of wavelength 5200 Å?