CHEM 1411 Concept Reviews: Atoms and Elements

Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction.

Law of Definite Proportions: All samples of a given compound, regardless of their source or how they are prepared, have the same proportions of their constituent elements.

Law of Multiple Proportions: When two elements (call them A and B) form two different compounds, the masses of element B that combine with 1 g of element A can be expressed in as a ratio of small whole numbers.

Precepts of Dalton’s Atomic Theory

1. Each element is composed of extremely small particles called atoms.

2. All atoms of the same element are identical, but the atoms of one element are different from the atoms of all other elements.

3. Atoms of one element cannot be changed into atoms of a different element by chemical reactions. In addition, atoms are neither created nor destroyed in chemical reactions.

4. Compounds are formed when atoms of more than one element combine. A given compound will always have the same elements combined in the same exact ratio.

Modern Atomic Theory

The current atomic theory states that an atom is composed of three fundamental particles; protons, neutrons, and electrons. The protons and neutrons combine to form a small but very dense mass in the center of the atom known as the nucleus. The electrons spin around the nucleus at blindingly fast speeds (close to the speed of light). Since the electrons are moving so fast, we speak of them as having a “cloud of probability” where they are likely to be.

Particle	Charge	Mass (amu)	Scientist Who Discovered It	Experiment that Led to Its Discovery
Proton	+1	1.0073	Ernest Rutherford	The "Gold Foil" Experiment
Neutron	0	1.0087	James Chadwick	The Beryllium-Alpha Bombardment Experiment
Electron	-1	$LaTeX: 5.486\times10^{-4}$ $5.486\times10^{-4}$	J.J. Thompson	The "Cathode Ray" Experiment

Atomic Mass Unit (amu): A unit of mass equal to the weight of approximately one proton or neutron. (It is strictly defined as EXACTLY $LaTeX: \frac{1}{12}$ $\frac{1}{12}$ ^th the mass of a carbon-12 atom, which has exactly 6 protons and 6 neutrons.)

Isotopes

Isotopes: Atoms that have the same number of protons (i.e.-the same element) but different numbers of neutrons.

Isotope Notation	Useful Equations
	$LaTeX: Mass\:Number\:=Protons+Neutrons$ $Mass\:Number\:=Protons+Neutrons$
	$Charge=Protons-Electrons$

The Mole: A Chemist’s “Dozen”

The mole refers to a specific number. That number is $LaTeX: 6.022\times10^{23}$ $6.022\times10^{23}$ . Just like a dozen refers to 12 of something, 1 mole refers to $LaTeX: 6.022\times10^{23}$ $6.022\times10^{23}$ of something. Most importantly though for chemists, 1 mole of an element contains $LaTeX: 6.022\times10^{23}$ $6.022\times10^{23}$ atoms of that element. 1 mole of a compound contains $LaTeX: 6.022\times10^{23}$ $6.022\times10^{23}$ molecules of that compound. The number $LaTeX: 6.022\times10^{23}$ $6.022\times10^{23}$ is known as Avogadro’s number. 1 mole of an element also has a mass equal to the atomic mass number of grams. In addition, the coefficients of a balanced equation can tell you the ratio of how many moles of one substance are needed to react with or produce how many moles of another.

Average Atomic Mass

The masses on the periodic table represent the average atomic mass of an element. These are calculated by taking the average mass of all isotopes of a given element weighted according to fractional abundance. The equation for this calculation is as follows:

$LaTeX: Avg.\:Atomic\:Mass=\sum\left[\left(isotope\:mass\right)\times\left(fractional\:abundance\right)\right]$ $Avg.\:Atomic\:Mass=\sum\left[\left(isotope\:mass\right)\times\left(fractional\:abundance\right)\right]$ for all isotopes of an element.

The Periodic Table

Periodic Law: When the elements are arranged in order of increasing mass, certain sets of properties recur periodically.

Period: A row in the periodic table. Each successive row repeats the series of the special properties of different element types.

Group: A column in the periodic table. Elements in the same group tend to have similar properties. Certain groups have special names that are often derived from the specific chemical properties of elements in the group.

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Representative elements: Elements that are labeled with an “A” and are sometimes simply called “A-group elements”. These are called “representative” because they represent the broad spectrum of physical and chemical properties present among the many elements in the periodic table.

Metals: Elements that tend to be shiny, malleable (able to be hammered into sheets), ductile (able to be drawn or “pulled” into wires), and conduct heat and electricity well.

Nonmetals: Elements that tend to be dull, brittle, and do not conduct heat or electricity well as solids or liquids. Most non-metals are in the gas phase at normal temperatures.

Ions: Atoms that have become charged by either gaining electrons (anions) or losing electrons (cations).