chem - atoms, molecules

8/13/2019 Chem - Atoms, Molecules...

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Lecture 2:

Atoms, Molecules, Ions and

their Role in Chemical

Reactions

Atomic Theory and Trends in the

Properties of the Elements


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Objectives Daltons atomic theory

Atomic structure

Elemental symbols

Isotopes

Classification of matter

Periodic table of the elements

Molecular and non-molecular compounds

Molecular and empirical formulae Ions

Naming compounds -- self study

3.6


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Daltons Atomic Theory of

Matter

Nothing Exists Per Se Except Atoms

and the Void

- Lucretius (ca. 94-51 BC)


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Postulate 1

All matter is composed of indivisible

units called atoms.

An atom retains its identity during

chemical reactions

Cl

H C

Br -

Cl-

H

Br

C


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Postulate 2

An elementis a type of matter composed of

only one kind of atom

In the original theory, each atom of an element

was thought to have the same physical

properties (e.g. mass) of all the other atoms in a

sample of that elementWe now know that an element may have several

isotopes (atoms of different mass)

What determines the identity of an element?


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Postulate 3

A compoundis a type of matter composed ofatoms of two or more elements chemically

combined in fixed proportions

Law of Constant Composition

Is ozone, O3, a compound? Explain.

Are N2O and NO2the same compound?

Is air (composed of 20% oxygen and 80%

nitrogen by weight) a compound?


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Postulate 4

A chem ical react ionconsists of the

rearrangement of the atoms present in

the reacting substances to give newchemical combinations present in the

substances formed by the reaction

Is the following a chemical reaction?

H + ClHHCl + H


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Natural Laws Arising from

Daltons Theory Law of conservation of matter

arises from postulates 2 and 4

atoms can be changed in high-energy collisions and

natural decay processes, but not in chemical reactions Law of constant composition

arises from postulate 3

Law of multiple proportions (postulate 3)

If elements A and B combine to form more than onecompound, the masses of B that can combine with a

given mass of A are in the ratio of small, whole

numbers.


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Law of Multiple ProportionsExample: 10.5 g of nitrogen (N2) combines with 12.0 g of

oxygen (O2) to form a gas, A. Under different

conditions, 17.5 g of N2combine with 10.1 g of O2

to form a gas, B. Show that compounds A and B

obey the law of multiple proportions.

ANS: show that the mass of N2per g of O2needed to

form B is a simple ratio of that of A.

B: (17.5 g) (10.1 g) = 1.73

A: (10.5 g) (12.0 g) = 0.875

B A = 1.98 2


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Implications:


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Subatomic Facts Sheet


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Elemental Symbols

35Cl17

Mass number (i.e. thenumber of protonsand neutrons--notexactly the same as

the atomic mass)goes here.

Atomic number(number of protons)goes here (although itis not often included)

Ionic charge (if any)goes here

The symbol is read chlorine-35.

It is one of two isotopes of chlorine,the other being 37Cl (chlorine-37).


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Isotopes

An elements identity is established by

its atomic number

Different isotopes of an element differ in

their atomic masses

This is due to varying numbers of

neutrons

e.g. 12C has 6 protons and 6 neutrons;13C has 6 protons and 7 neutrons.

An atom of a specific isotope is called a

nuclide


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Atomic and Molecular Masses

The atomic mass unit (amu)

An amu is defined as1

/12the mass of oneatom of carbon-12 (12C)

1 amu = 1.66054 x 10-27kg

1 kg = 6.02214 x 1026amu


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Average Atomic Masses

The atomic masses you see in the periodicchart are weighted averages of the masses

of the naturally occurring isotopes

e.g. In nature, bromine is found as 79Br

(78.918336 amu) and 81Br (80.916289 amu),

in 50.69% and 49.31% abundance,

respectively. What is the average atomic

weight (mass) of Br?AWBr=

(0.5069)(78.918336)+(0.4931)(80.916289)

= 79.90 amu (4 significant digits)


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Finding the % Relative Abundance

e.g. Cl has an average atomic mass of 35.453amu, and is comprised of 35Cl (m35 = 34.969amu) and 37Cl (m37 = 36.966 amu). What arethe relative abundances of the two

isotopes?A. There are 2 isotopes, and the abundances

(X35and X37) must add to 1; therefore, wehave two equations

(1) X37+ X35= 1 and(2) X37m37+ X35m35= 35.453

Solving we find X35 = 0.75764 and X37 = 0.24236


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The Periodic TableIUPAC

N.A. Standard

Atomic symbol


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Molecules

A moleculeis a neutral, discrete assembly oftwo or more atoms that are chemically bound

The properties of a molecule are very different

from those of the atoms from which it iscomprised

Most elementsdo not occur in nature in atomic

form; e.g. oxygen occurs at O2or O3(ozone)

ONLY the noble gases occur as atoms at room

temperature and pressure

This is not to say that most elements occur as

molecules. . .


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If not molecules then what?

Molecular elements include: H2, N2, O2,

S8, P4, C60, F2, Cl2, Br2, and I2

The metals and most metalloids occur as

extended structures, not as discrete units

IT IS INCORRECT TO TALK ABOUTMOLECULES OF ELEMENTAL

METALS

Partial structure of elemental metal


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Molecular Compounds Molecular compounds are substances that

consist of discrete units (molecules) that containmore than one type of chemically bound

element

e.g. Water, H2O; hydrogen peroxide, H2O2 Most molecular compounds are comprised of

non-metals, e.g. organic compounds

Molecules containing metals are possible; e.g.

Fe(CO)5

Most metal-containing compounds are extended

structures; e.g. TiO2


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Molecular vs. Empirical

FormulaeMolecu lar form ulagives the actual numbers

and types of atoms in a molecule

e.g. Ethene: C2H4

Empir ical formulagives the relative numbersand types of atoms in a molecule

e.g. Ethene: CH2

C=CH

H

H

H

Molecularstructureof ethene


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Ions and Ionic Compounds

Ions are charged atoms or molecules They can be positive (cations, e.g. Na+) or

negative (anions, e.g. Cl-)

Atoms form ions in order to complete the noblegas configuration

Metal atoms tend to lose electrons, non-metals

tend to gain electrons

e.g. Na([Ne]3s1) Na+([Ne]) + e-

e.g. Cl([Ne]3s23p5) + e- Cl-([Ar])

Note that ionic charges result from gain and

loss of electrons, not protons


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Main-Group Monatomic Ions


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Polyatomic Ions

Atoms form molecules in order to obtain

noble gas configuration for all atoms

If chemical bonding does not satisfy this

requirement, charging the molecule might

e.g. NO3vs NO3-

NOO

O+

_

Lewis structure of(neutral) NO3

Octet not satisfied

NOO

O+

_ _

Lewis structure of(anion) NO3

-

e-


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Naming Inorganic Compounds and

Acids & Bases

(Self Study 3.6)Responsible For:

Simple cations and anions (e.g. Cu+ = copper(I) =cuprous; O2- = oxide)

Polyatomic oxo-anions and acid forms(e.g. NO3

-= nitrate; HCO3- = hydrogen

carbonate)

Ionic compounds (e.g. Cu2O = copper(I) oxide =cuprous oxide)

Acids (e.g. HClO4 = perchloric acid)

Binary molecular compounds

(e.g. NO2 = nitrogen dioxide)


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Chemical Reactions


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Fractions or Whole Numbers?

Fractions are acceptable because the

coefficients refer to molesof substance

If you like whole numbers, simply

multiply by the appropriate constant

C8H18 () +25/2O2 (g)8 CO2 (g) + 9 H2O ()

x 2

2 C8H18 () + 25 O2 (g)16 CO2 (g) + 18 H2O () Note: Use lowest whole-number coeffs

(e.g. 4, 50, 32, 36 would not be

acceptable)


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Physical States

The physical states of the reactants and productsare often indicated parenthetically:

C8H18 ()+25/2O2 (g)8 CO2 (g)+ 9 H2O ()

Other conditions, such as the temperature, solvent,

and whether heat () has been added may beindicated above or below the arrow

298 K


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Alkali metals with water: alkali metalsreact with water to produce hydroxides

and hydrogen

K + H2O KOH + H2(l)(s) (g)(aq)1/2

BalancedBalancedNot balancedAll alkali metals behave similarly.

This is part of the utility of the periodic table.


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Decomposition reactions: One reactant

forms two or more products.

KClO3 KCl + O2(s) (s) (g)3/2


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Summary

Atoms are arranged on the periodic tableaccording to their chemical properties

The identity of an element is determined by the

number of protons in the nucleus

The masses of isotopes vary according to the

number of neutrons

Electrons are responsible for chemistry

Chemical compounds can be molecular, ionic,or covalent, extended structures

Driving force behind chemistry is formation of

t bl t t

chem - atoms, molecules

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