Matter in Terms of Matter in Terms of its Compositionits Composition

The basic knowledge The basic knowledge && proven conception behind proven conception behind all all && every explanations in chemistry is that of every explanations in chemistry is that of

atomic structure of matteratomic structure of matter..AtomsAtoms are smallest particles of matter that do not are smallest particles of matter that do not

disappear disappear && are not further divided in chemical are not further divided in chemical rxnsrxns. .

In models, they are presented as hard balls that In models, they are presented as hard balls that combine according to certain rules. combine according to certain rules.

Chemists study those rules Chemists study those rules && apply them to apply them to produce new combinations with desired properties.produce new combinations with desired properties.

ATOMSATOMSJohn Dalton (1766John Dalton (1766--1844)1844)

1. 1. Elements are formed of minute, Elements are formed of minute, indivisible particles indivisible particles -- atomsatoms..

2.2. Atoms of the same element have the same mass Atoms of the same element have the same mass &&other properties.other properties.

3.3. Atoms of different elements differ in their mass Atoms of different elements differ in their mass &&other properties.other properties.

4.4. Atoms combine to form a compound in Atoms combine to form a compound in definitedefinite, , simple numerical ratios simple numerical ratios ((1:1, 1:2, 1:3, 2:3, etc.1:1, 1:2, 1:3, 2:3, etc.))

5.5. Atoms of two elements may combine in two or Atoms of two elements may combine in two or more simple ratios to form more than one compound.more simple ratios to form more than one compound.

DaltonDaltons postulates explain:s postulates explain:

The The Law of Conservation of MassLaw of Conservation of Mass::Mass of products is equal to the mass of Mass of products is equal to the mass of

reactants, or, in other words:reactants, or, in other words:mass does not change in chemical reactions. mass does not change in chemical reactions. It must be true since atoms are not destroyed It must be true since atoms are not destroyed &&

each has a definite mass, both before each has a definite mass, both before && after after the reaction.the reaction.

ExampleExample:: When 64g Cu combine with 32g S, 96 g When 64g Cu combine with 32g S, 96 g of of CuSCuS always form:always form:

Cu + S Cu + S CuSCuS64g 32g 96g64g 32g 96g

The The Law of Definite CompositionLaw of Definite Composition: : In different samples of any pure compound, elements In different samples of any pure compound, elements

combine always in the same, fixed, proportions by combine always in the same, fixed, proportions by mass.mass.

[since atoms, in a compound, still have their definite [since atoms, in a compound, still have their definite masses, masses, && combine in fixed numbers, the combine in fixed numbers, the compound has some definite mass composition].compound has some definite mass composition].

ExampleExample:: When 18 g of water, HWhen 18 g of water, H22O, decompose by O, decompose by electrical current, always the same masses of electrical current, always the same masses of hydrogen, Hhydrogen, H22 [[2g2g] ] && oxygen, Ooxygen, O22 [[16 g16 g] are ] are produced:produced:

HH22O O HH22 + + 1/21/2OO2218g 2g 16g18g 2g 16g

The The Law ofLaw of Multiple ProportionsMultiple Proportions::If two different compounds are formed of the same If two different compounds are formed of the same

two elements, the masses of the 2nd element two elements, the masses of the 2nd element combined with a certain mass of the 1st element combined with a certain mass of the 1st element can be expressed as ratios of small integer (whole) can be expressed as ratios of small integer (whole) numbers. numbers.

[Straightforward consequence of the Dalton[Straightforward consequence of the Daltons s 6th6thpostulate]. postulate]. Compare:Compare:

HH22O O 2g2g H : H : 16g 16g O, or O:H = O, or O:H = 8:18:1 by massby mass& & Oxygen: Oxygen: 2:12:1HH22OO22 2g2g H : H : 32g32g O, or O:H = O, or O:H = 16:116:1

SOSO22 32g32g S : S : 32g32g O, or S:O =O, or S:O =1:11:1==2:22:2&& Oxygen: Oxygen: 3:23:2SOSO22 32g S : 48g O, or 32g S : 48g O, or S:O =S:O =2:32:3

Atomic structure is now not Atomic structure is now not a theory, but a matter of a theory, but a matter of

fact.fact.

Atoms & their Atoms & their arrangement can be arrangement can be directly visualizeddirectly visualized

Atomic force microscopy image of a silicon crystal face

Molecular models:Molecular models:

as simple as that as simple as that of water, Hof water, H22OO

or benzene, Cor benzene, C66HH6 6

or as or as comnplicatedcomnplicatedas a protein, as a protein, with thousands with thousands of atoms, as this of atoms, as this myoglobinmyoglobin

ELEMENTELEMENT vs.vs. COMPOUNDCOMPOUND..About About 100100 different kinds of atoms are known. different kinds of atoms are known.

They are They are ELEMENTSELEMENTS..

ELEMENTSELEMENTS are those substances that cannot be decomposed to are those substances that cannot be decomposed to simpler ones. simpler ones.

An An elementelement consists of consists of (nearly)(nearly) identical atomsidentical atoms, , thoughthough aa compoundcompound is formed by atoms of different elementsis formed by atoms of different elements. .

Stable, lasting combinations of elements of constant Stable, lasting combinations of elements of constant composition are composition are

CHEMICAL COMPOUNDSCHEMICAL COMPOUNDS. .

Both elements Both elements && compounds are compounds are chemical substanceschemical substances. . Out of Out of about 100 elements, billions of compounds are known, about 100 elements, billions of compounds are known,

& every week new compounds are discovered or synthesized& every week new compounds are discovered or synthesized..

Pure Substances Pure Substances vs.vs. MixturesMixturesPure substance is Pure substance is uniform in its compositionuniform in its composition, ,

i.e. it has a definite composition i.e. it has a definite composition (the ratio of (the ratio of combined elements)combined elements) in any smallest possible in any smallest possible

sample taken sample taken && that cannot be arbitrarily that cannot be arbitrarily changed. changed.

Constant composition means that ratio of Constant composition means that ratio of atoms combined in a compound is definite, atoms combined in a compound is definite,

cancant be changed arbitrarily. t be changed arbitrarily.

Chemical formula of a compound states this with Chemical formula of a compound states this with subscriptssubscripts

HH22OO11

is formula of water stating that in water 2 hydrogen (H) atoms is formula of water stating that in water 2 hydrogen (H) atoms are combined with 1 oxygen (O) atom.are combined with 1 oxygen (O) atom.

((11 is always omitted)is always omitted)NaClNaCl

is formula of cooking salt stating that in it is formula of cooking salt stating that in it sodium (Na) atoms combine with chlorine (sodium (Na) atoms combine with chlorine (ClCl) atoms in a ) atoms in a

1:1 ratio.1:1 ratio.

HH22SOSO44is formula of sulfuric acid showing that in it atoms of hydrogenis formula of sulfuric acid showing that in it atoms of hydrogen

(H), sulfur (S) & oxygen (O) always combine in a ratio of (H), sulfur (S) & oxygen (O) always combine in a ratio of H : S : O = 2 : 1 : 4H : S : O = 2 : 1 : 4

Those ratios cant be changed & do not depend of the source of these compounds, or the way it were obtained or treated.

More elaborate & informative way to present a chemical More elaborate & informative way to present a chemical compound uses compound uses structural formulastructural formula or or molecular modelmolecular model..

BallBall--&&--Stick spaceStick space--fillingfilling

For water,For water, HH22O: O: H H H H

OO

in carbon dioxide (dry ice),in carbon dioxide (dry ice), COCO22, , 1 carbon atom (C) combines with 2 oxygen atoms. 1 carbon atom (C) combines with 2 oxygen atoms.

Those ratios cannot be changed at will. Those ratios cannot be changed at will. O=C=O O=C=O

If composition of some compound changes, that If composition of some compound changes, that means a chemical transformation (means a chemical transformation (reactionreaction) occurred ) occurred

& a new substance has been produced.& a new substance has been produced.

ExampleExample: Carbon, C, forms with oxygen two compounds:: Carbon, C, forms with oxygen two compounds:carbon monoxide COcarbon monoxide CO

& & carbon dioxide, COcarbon dioxide, CO22-- two substances with completely different properties: the formertwo substances with completely different properties: the former

is a poison, the latter we breathe out. is a poison, the latter we breathe out. (There are no compounds with intermediate composition, such as (There are no compounds with intermediate composition, such as

CC11OO1.81.8))

Ability carbon Ability carbon && oxygen to combine their atoms oxygen to combine their atoms in 1:1 or 1:2 ratio, is in 1:1 or 1:2 ratio, is chemical propertychemical property

of these two elements.of these t