CHAPTER 4Atomic Theory

DEMOCRITUS

First person to purpose that matter was not infinitely divisible“atomos”

ARISTOTLE•Rejected atomic theory• did not believe in “nothingness” of space

JOHN DALTON (1766-1844) Modern atomic theory –

19th century Conservation of mass –

separation, combination, or rearrangement of atoms

Was all of his theory accurate? NO!!

Dalton’s Atomic Theory (1808)

Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in simple whole-number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged

All matter is composed of extremely small particles called atoms Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties

DEFINING THE ATOM The smallest particle of an element that

still retains the properties of the element

How small is an atom?World population: 6300000000Atoms in a penny:

29000000000000000000000 Scanning tunneling microscope – allows

individual atoms to be seen

CATHODE RAY TUBE

William Crookes

Discovered Cathode Ray Tube

JJ Thomson

Used Cathode Ray tube to discover Electron

Discovery of the Electron1897: J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Some ModernCathode Ray Tubes

CATHODE RAY RESULTS Cathode ray deflected in a magnetic

field, indicated charged particles Deflected towards positively charged

plate, indicating particles must have negative charge

Altering gas, altering material had no effect on results, so particles must be in all matter

Called…. ELECTRONS!!!! First subatomic particles! Meant Dalton was… wrong!!!

MASS AND CHARGE OF THE ELECTRON 1909 – Robert Millikan determines the

mass of the electron. Mass of the electron was much smaller

than that of the hydrogen atom, the smallest known atom

Meant atoms were divisible into subatomic particles

Mass = 9.1 x 10-28 = 1/1840 mass of hydrogen

Charge = -1

Conclusions from the Study of the Electron

Electrons are negative. Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons Electrons have so little mass that atoms must contain other particles that account for most of the mass

THOMSON’S PLUM PUDDING MODEL Atom breakable!! Atom has structure Electrons suspended in a positively charged

electric field must have positive charge to balance negative

charge of electrons and make the atom neutral mass of atom due to electrons atom mostly “empty” space

compared size of electron to size of atom Cookie dough model

PLUM PUDDING MODEL

Rutherford’s Gold Foil Experiment

Alpha particles are positively charged Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

Rutherford’s Findings Most of the particles passed right through

A few particles were deflected

GREATLY Deflected particles were repulsed by positive charge of nucleus

Conclusions: Nuclear Model

RUTHERFORD’S NUCLEAR MODEL

The atom contains a tiny dense center called the nucleus the volume is about 1/10 trillionth the volume

of the atom The nucleus is essentially the entire

mass of the atom The nucleus is positively charged

the amount of positive charge of the nucleus balances the negative charge of the electrons

The electrons move around in the empty space of the atom surrounding the nucleus

PROTONS AND NEUTRONS Rutherford - Protons

Subatomic particle in nucleus protons :+1 charge (equal, opposite of

electrons) Chadwick – Neutrons

Subatomic particle in nucleusMass nearly equal to a proton, but carries

no electrical charge

Atoms are indivisible!

Actually, Mr. Dalton, we have proved that part of your theory

wrong.

What!?!Atoms can be divided into electrons,

protons, and neutrons.

Atom – electrically neutral particle composed of protons, neutrons, electronsSpherical shape

Atoms consist of two regionsNucleus – 99.7% of mass

Very small, dense region in the center. Contains protons & neutrons.

Electrons Cloud Mainly empty space surrounding nucleus Very large compared to the nucleus. Contains electrons.

Subatomic particlesProtons, neutrons, and electrons

THE STRUCTURE OF THE ATOM

Atomic Particles

Particle

Charge

Mass (kg) Location

Electron

-1 9.109 x 10-

31

Electron cloud

Proton +1 1.673 x 10-

27

Nucleus

Neutron

0 1.675 x 10-

27

Nucleus

Atomic NumberAtomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. Identifies the atom.Element # of

protonsAtomic #

(Z)

Carbon 6 6

Phosphorus

15 15

Gold 79 79

Isotopes

Elements occur in nature as mixtures of isotopes.

Isotopes are atoms of the same element that differ in the number of neutrons

Isotopes…Again (must be on the test)

Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.Isotope Proto

nsElectron

sNeutron

sNucleus

Hydrogen–1 (protium)

1 1 0

Hydrogen-2

(deuterium)

1 1 1

Hydrogen-3 (tritium)

1 1 2

Mass NumberMass number is the number of protons and neutrons in the nucleus of an isotope.

Mass # = p+ + n0

Nuclide p+ n0 e- Mass #

Oxygen - 10

- 33 42

- 31 15

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

Atomic Masses

Isotope Symbol

nucleus % in nature

Carbon-12

12C 6 protons 6 neutrons

98.89%

Carbon-13

13C 6 protons 7 neutrons

1.11%

Carbon-14

14C 6 protons 8 neutrons

<0.01%

Atomic mass is the average of all the naturally isotopes of that element. Carbon = 12.0125

amuOn Periodic Table

PRACTICE PROBLEMS P. 104 # 15-17

WRITING NUCLEAR SYMBOLS

He3

2

Mass #

Atomic #

Atomic Symbol

How many protons, electrons, and neutrons?

2 protons, 2 electrons, 1 neutron

Mass # - Atomic # = # Neutrons

WRITING ISOTOPES USING HYPHEN NOTATION

Uranium-235, Helium-3, or Carbon-14

Name of atom

Mass #

How many proton, electrons, neutrons?92 protons, 143

neutrons, 92 electrons

ISOTOPE PROBLEMS

Uranium-235, Helium-3, or Carbon-14

Convert these hyphen notation to nuclear symbols.

U235

92 He3

2 C14

6