Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107
Fundamentals of Chemistry
Fall 2008

Lecture Notes: 11 September
© R. Paselk 2005

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*Humboldt State University* ® *Department of Chemistry*

Richard A. Paselk
Chem 107	Fundamentals of Chemistry	Fall 2008
Lecture Notes: 11 September	© R. Paselk 2005

*PREVIOUS*		*NEXT*

Atoms and Atomic Structure

Elements

Elements are substances which cannot be broken down further into simpler substances.

Atoms

Atoms are the smallest constituents of elements. The first successful atomic theory was that of John Dalton (1803).

Dalton's atomic theory states:

All matter is composed of ultimately small particles, called atoms.
Atoms are permanent and indivisible - they can neither be created nor destroyed.
Elements are characterized by their atoms. All atoms of a given element are identical in all respects. Atoms of different elements have different properties.
Chemical change consists of a combination, separation, or rearrangement of atoms.
Chemical compounds are composed of atoms of two or more elements in fixed ratios.

All of these statements are close to reality, and nearly describe chemical behavior. But here are exceptions. Thus atoms can be created and destroyed via nuclear processes. They consist of different forms called isotopes. Atoms are not the smallest particles, etc.

Atoms are now known to consist of three different types of particles: electrons, protons and neutrons (the common form of one very important atom, hydrogen, has only two kinds: a proton and an electron). The protons and neutrons reside in a small inner portion called the nucleus while the electrons reside in a relatively large cloud centered on the nucleus. Important properties of these particles are listed in the table below:

Particle	Charge	Relative Mass	Mass
Electron (e^-)	-1	1/1840	9.11 x 10^-28g
Proton (p or H⁺)	+1	ª1	1.67 x 10^-24g
Neutron (n)	0	ª1	1.67 x 10^-24g

Some important terms which you must know are:

Atomic number (Z) - the number of protons in the nucleus. This number is characteristic of a given element.
Atomic mass number (A) - the sum of the protons and neutrons in a given atom (p + n).
Atomic mass - the actual mass of an average atom in a sample. The characteristic atomic masses for Earth are shown on periodic tables.
Atomic Mass Unit: the atomic mass unit = amu is a unit of mass for atoms. It is defined as 1/12 the mass of one atom of ¹²C, where the mass of ¹²C is defined as 12 exactly.

Isotopes

Isotopes are forms of elements which differ only in the number of neutrons. This means different isotopes of the same element have essentially the same chemical properties but slightly different physical properties. They can also different substantially in terms of their nuclear stability. Let's finish our table of examples of isotopes:

Symbol	Z	A	p	n	e^-
¹⁴C	6	14	6	8	6
²³⁸U⁶⁺	?	?	?	?	?
³⁵Cl^-	?	?	?	?	?
^?O^?	?	18	?	?	10

You should be able to fill in the blanks in a table like this with, the aid of a periodic table, on a quiz.

Determination of Atomic Mass:

We want to be able to figure out the atomic mass of a sample with a particular isotopic composition.

Example: Cu occurs as an isotopic mixture of 69.09% ⁶³Cu (mass = 62.93 amu) and 30.91% ⁶⁵Cu (64.93 amu). What is the atomic mass of copper in this sample.