7/30/2019 1 Development of the Atomic Model

1/8

Atomic Theory of Matter

5th

Century B.C

Leucippus and Democritus: atomos (small, indivisible particles)Rejected by Plato/Aristotle: favoured the theory of 4 elements

1799

Antoine Lavoisier & Joseph Priestley, Law of Conservation of Masso The total mass of all products of a chemical reaction is equal to the to

mass of all reactants of that reaction

Joseph Proust, Law of Definite Proportionso

Different samples of the same compound always contains constituent elements in the same proportion by mass

Ex: CuCO3 always contains:

5.3 parts copper, 4 parts oxygen, 1 part carbon1808

John Dalton: Daltons Atomic Theoryo All elements are composed oftiny indivisible particles called atomso Atoms of the same element are identical(same size, mass and chemi

properties). Atoms of one element are different from the atoms of

other elements

o Atoms of different elements combine in simple whole number ratiosform compounds

o In a chemical reaction, atoms are combined, separated or rearrangedBUT NEVER changed or destroyed

Law of Multiple Proportionso If two elements can combine to form more than one compound, t

masses of one element that combine with a fixed mass of the oth

elements are in ratios of small whole numbers

7/30/2019 1 Development of the Atomic Model

2/8

Discovery of Atomic Structure

Atoms are NOT INDIVISIBLE!3 subatomic particles:

o Electronso Protonso Neutrons

Discovery of Electrons

Joseph John Thomsono Discovered corpuscles (ELECTRONS)

o Determined the charge to mass ratio for electrons (1.76 x 108 coulombsper gram)

7/30/2019 1 Development of the Atomic Model

3/8

Robert Millikano Oil-drop Experimento Determined the charge of an electron (1.60 x 10-19 C)

Electron mass = 9.10 x 10-28

g

ATOMIC MODEL # 1: PLUM-PUDDING MODEL

Proposed by J. J. Thomson Negative charges embedded on a uniform,

positive sphere of matter

Discovery of Protons and Neutrons

Ernest Rutherfordo Disproved Thomsons Modelo Gold-Foil Experiment

7/30/2019 1 Development of the Atomic Model

4/8

ATOMIC MODEL # 2: ATOMIC PLANETARY MODEL

Proposed by Ernest Rutherford Most of the atom must be empty space must have a very small, dense area of + charge

(nucleus)

Protons have same charge as e-, but almost 2000xmore mass!

Problems:

1.What prevents the negative electrons from being drawn into the positinucleus? And the positive charges from repelling the nucleus into oblivion?

2.H atom has 1 proton & He atom has 2 protons, mass ratio should be 2instead the ratio is 4:1

there must be another particle

Discovery of Neutrons

James Chadwicko Discovered a neutral (uncharged) particle in the nucleus (NEUTRONS)o Neutron is has slightly greater mass than a proton

Table 1. Properties of Subatomic Particles

Particle Charge Mass (kg)

Proton Positive (+1) 1.6726 x 10-27

Neutron None (neutral) 1.67510 x 10-27

Electron Negative (-1) 9.1096 x 10-31

7/30/2019 1 Development of the Atomic Model

5/8

Modern View of Atomic Structure

1.Atoms are NOT indivisible (subatomic particles)2.Atoms have an equal number of electrons and protons.3.Atoms are also extremely small.4.Atoms have generally small masses.

a.Grams not appropriate unitb.Use amu (atomic mass unit)

1 amu = 1.66054 x 10-24

g

Therefore,

Mass of proton = 1.0073 amu

Mass of neutron = 1.0087 amu

Mass of electron = 5.486 x 10

-4

amu

Chemical Symbol

Atomic Number (Z)

o Number of protons in the nucleuso Fixed number and is a characteristic of

the element

Mass Number (A)

o Total number of protons and neutronso Atomic number + # of neutrons

FOR NEUTRAL ATOMS, # of protons = # of electronsFOR IONS, number of protons is unchanged

Ex:

7/30/2019 1 Development of the Atomic Model

6/8

Isotopes

o Proposed by Frederick Soddyo Atoms of a given element that differ in the number of neutrons a

consequently in mass.

o An atom of a specific isotope is called a nuclide.

Elements occur in nature as mixtures of isotopes!

7/30/2019 1 Development of the Atomic Model

7/8

Measuring Atomic Mass

o Average Atomic Mass Based on the abundance (percentage) of each variety of th

element in nature

o Atomic Mass Unit Defined as one-twelfth the mass of a carbon-12 atom

To calculate Average Atomic Mass:

1. Multiply the atomic mass of each isotope by its abundance.2.Add results.

Example:

1.Calculate the atomic mass of Cl given the relative abundances of its isotopes35

Cl75.77%37

Cl24.23%

ave atomic mass = (atomic mass35

Cl)(fraction35

Cl) + (atomic mass37

Cl)(fraction37

C

= (34.968 amu) (0.7577) + (36.965 amu) (0.2423)

= 35.45 amu

2.Naturally occurring carbon is composed of 98.93% 12C and 1.07% 13C. Tmasses of these nuclides are 12 amu (exactly) and 13.00335 am

respectively. Calculate the average atomic mass of carbon.

(Ans: 12.01 am

3.Element X has two natural isotopes. The isotope with a mass of 10.012 amhas a relative abundance of 19.91%. The isotope with a mass of 11.009 am

has a relative abundance of 80.09%. Calculate the atomic mass of t

element.

(Ans: 10.810 am

7/30/2019 1 Development of the Atomic Model

8/8

QUIZ

1. If an element has 91 protons and 140 neutrons, what is the:a.Atomic numberb.Mass numberc. Number of electronsd.Chemical Symbol

2.Fill in the gaps in the following table assuming each column represents aneutral atom:

Symbol121

Sb

Protons 38 94

Neutrons 50 108

Electrons 74 57

Mass no. 139 239

3.Element Zy consists of four naturally occurring isotopes with masses 198.324199.2374, 196.3646 and 200.3472. The relative abundances of these foisotopes are 1.4, 24.1, 22.1 and 52.4%, respectively. From these data, calcula

the average atomic mass of Zy.

EXPRESS YOURSELF!

What do you want to tell yourself regarding Chem14?