CH 2 Atoms, Molecules and Ions

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Atoms,Molecules,

and Ions

Chapter 2Atoms, Molecules,

and Ions



Atoms,Molecules,

and Ions

Law of Constant CompositionJoseph Proust (1754–1826)

Also known as the law of definiteproportions.

The elemental composition of a puresubstance never varies.



Atoms,Molecules,

and Ions

Law of Conservation of Mass

The total mass of substances presentat the end of a chemical process is the

same as the mass of substances presentbefore the process took place.



Atoms,Molecules,

and Ions

The Nuclear Atom

Since some particleswere deflected at largeangles, Thompson’s

model could not becorrect.

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Atoms,Molecules,

and Ions

The Nuclear Atom

Rutherford postulated a very small, densenucleus with the electrons around theoutside of the atom.

Most of the volume of the atom is emptyspace.




Atoms,Molecules,

and Ions

Subatomic Particles

Protons and electrons are the only particles thathave a charge.

Protons and neutrons have essentially the same

mass.The mass of an electron is so small we ignore it.




Atoms,Molecules,

and Ions

Atomic Number

All atoms of the same element have the

same number of protons:The atomic number (Z)




Atoms,Molecules,

and Ions

Atomic Mass

The mass of an atom in atomic mass units

(amu) is the total number of protons andneutrons in the atom.




Atoms,Molecules,

and Ions

Isotopes:

Atoms of the same element with different masses.

Isotopes have different numbers of neutrons.

116C

126C

136C

146C



Atoms,Molecules,

and Ions

Average Mass

Because in the real world we use largeamounts of atoms and molecules, we use

average masses in calculations.Average mass is calculated from theisotopes of an element weighted by their

relative abundances.



Atoms,Molecules,

and Ions

Periodic Table:

A systematic catalogof elements.

Elements arearranged in order of atomic number.




Atoms,Molecules,

and Ions

Periodicity

When one looks at the chemical properties of

elements, one notices a repeating pattern of reactivities.

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Atoms,Molecules,

and Ions

Periodic Table

The rows on the periodicchart are periods.

Columns are groups.

Elements in the samegroup have similar chemical properties.




Atoms,Molecules,

and Ions

Groups

These five groups are known by their names.




Atoms,Molecules,

and Ions

Diatomic Molecules

These seven elements occur naturally asmolecules containing two atoms.




Atoms,Molecules,

and Ions

Types of Formulas

Empirical formulas give the lowest whole-number ratio of atoms of each element in

a compound.Molecular formulas give the exact number of atoms of each element in a compound.



Atoms,Molecules,

and Ions

Types of Formulas

Structural formulas show theorder in which atoms arebonded.

Perspective drawings alsoshow the three-dimensionalarray of atoms in a compound.

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Atoms,Molecules,

and Ions

Ions

When atoms lose or gain electrons, theybecome ions.

Cations are positive and are formed by elements on theleft side of the periodic chart.

Anions are negative and are formed by elements on theright side of the periodic chart.

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Atoms,Molecules,

and Ions

Writing Formulas

Because compounds are electrically neutral,one can determine the formula of a compound

this way:The charge on the cation becomes the subscript on theanion.

The charge on the anion becomes the subscript on thecation.

If these subscripts are not in the lowest whole-number ratio, divide them by the greatest common factor.




Atoms,Molecules,

and Ions

Common Cations




Atoms,Molecules,

and Ions

Common Anions

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Atoms,Molecules,

and Ions

Inorganic Nomenclature

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Atoms,Molecules,

and Ions

Patterns in Oxyanion Nomenclature

When there are two oxyanions involvingthe same element:

The one with fewer oxygens ends in -ite● NO2− : nitrite; SO32− : sulfite

The one with more oxygens ends in -ate

● NO3− : nitrate; SO42− : sulfate





Atoms,Molecules,

and Ions

Patterns in Oxyanion Nomenclature

The one with the fewest oxygens has the prefix hypo-and ends in -ite

● ClO− : hypochlorite

The one with the most oxygens has the prefix per - andends in -ate

● ClO4− : perchlorate

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Atoms,Molecules,

and Ions

Acid Nomenclature

If the anion in the acidends in -ide, change theending to -ic acid and

add the prefix hydro- :HCl: hydrochloric acid

HBr: hydrobromic acid

HI: hydroiodic acid




Atoms,Molecules,

and Ions

Acid Nomenclature

If the anion in the acidends in -ite, change theending to -ous acid :

HClO: hypochlorous acid

HClO2: chlorous acid




Atoms,Molecules,

and Ions

Acid Nomenclature

If the anion in the acidends in -ate, change theending to -ic acid:

HClO3: chloric acid

HClO4: perchloric acid




Atoms,Molecules,

and Ions

Nomenclature of BinaryCompounds

The less electronegativeatom is usually listed first.

A prefix is used to denotethe number of atoms of each element in thecompound (mono- is notused on the first element

listed, however.)

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Atoms,Molecules,

and Ions


The ending on the moreelectronegative element ischanged to -ide.

CO2: carbon dioxide

CCl4: carbon tetrachloride




Atoms,Molecules,

and Ions


If the prefix ends with a or o and the name of theelement begins with a

vowel, the two successivevowels are often elidedinto one:

N2O5: dinitrogenpentoxide


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CH 2 Atoms, Molecules and Ions