Chapter 5 The Structure of Atoms

CHAPTER 5

• The Structure of Atoms

Fundamental Particles

• Three fundamental particles make up atoms:

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.

The Discovery of Electrons

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

The Discovery of Protons

The Discovery of Neutrons

Mass Number and Atomic Number

• Mass number – A

• Atomic number – Z

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

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

Isotopes: Example

Experimental Detection of Isotopes

• 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

Mass Spectrometry & Isotopes

• Mass spectrum of Ne+ ions

• This is how scientists determine the masses and abundances of the isotopes of an element

Mass Spectrometry & Isotopes

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

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

Isotopes and Atomic Weight

• Naturally occurring chromium consists of four isotopes. It is

• 4.31% 50Cr, mass = 49.946 amu

• 83.76% 52Cr, mass = 51.941 amu

• 9.55% 53Cr, mass = 52.941 amu

• 2.38% 54Cr, mass = 53.939 amu

• Calculate the atomic weight of chromium

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

Electromagnetic Radiation

• Any wave is characterized by 2 parameters:

• Wavelength () is the distance between two identical points of adjacent waves, for example between their crests
• It 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)

Electromagnetic Radiation

• The speed at which the wave propagates:

• c =   

• The speed of electromagnetic waves in vacuum has a constant value:

• c = 3.00108 m/s
• This is the speed of light
• Given the frequency of the electromagnetic radiation, we can calculate its wavelength, and vice versa

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)

Electromagnetic Radiation

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