A History of Atomic Theory & Basic Atomic Structure

Chapter 3: The Atom

Big Idea: Physical, chemical and nuclear changes are explained using the location and properties of subatomic particles.

Section 3.1

The Atom: From Philosophical Idea to Scientific Theory

All matter is made up of tiny, indivisible particles called atomos

Today, we define atom as the smallest particle

of an element that retains the chemical identity of that element

Aristotle asked:

Democritus – 450 BC

What holds the tiny particles

together?Democritus: ???

Aristotle rejected Democritus reasoning and proposed that matter was a continuum composed of mass and form

Marble (mass) Statue (form)

Later the simplest forms of matter were proposed to be:

Earth, Water, Fire, Wind

Aristotle – 384 BC

The transformation of a substance(s) into one or more new substances is known as a chemical reaction.

Law of Definite Composition: a chemical compound contains the same elements in exactly the same proportions by mass (regardless sample size or source)

Foundations of Atomic Theory

Sugar: 42.1 % Carbon 51.4 % Oxygen

6.5 % Hydrogen

Whether you have a teaspoon or a truckload!

Law of Conservation of Mass: mass is neither created nor destroyed during ordinary chemical reactions or physical changes

Foundations of Atomic Theory

HgO Hg + O 433.2 g 401.2g + 32g

Law of Multiple Proportions: if two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers

H2O H2O2

Water Peroxide 2g H 2g H16g O 1:2 Ratio 32g O

2

Dalton’s Atomic TheoryAll 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.

Atoms cannot be subdivided, created, or destroyed.satisfies Law of Conservation of Mass

Dalton - 1808

Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

satisfies Law of Multiple Proportions

In chemical reactions, atoms are combined, separated, or rearranged.

Not all aspects of Dalton’s atomic theory have proven to be correct.

Atoms can be split into even smaller particles. A given element can have atoms with different masses

(called isotopes)

Some important concepts remain unchanged All matter is composed of atoms Atoms are rearranged in chemical reactions Atoms of any one element are never identical to

atoms of another element

Modern Atomic Theory

Section 3.2

The Structure of the Atom

Atom is the smallest particle of an element that retains the chemical properties of that element.

1897 - Joseph John Thomson’s cathode-ray tube (CRT)

The Discovery of the Electron

The Discovery of the Electron

Negatively Charged Electrode

Positively Charged Electrode

Joseph John Thomson’s cathode-ray tube (CRT)

Cathode Ray Tube Scientists studied the flow of electric current in a glass

vacuum tube with electrodes at each end.

When connected to electric current the remaining gas glowed forming a BEAM OF LIGHT.

The beam always originated at the NEGATIVE electrode and toward the POSITIVE electrode.

The electrode is named by what type of particle it attracts Cathode: Negative (-) Anode: Positive (+)

JJ Thomson used magnets to deflect the beam proving that particles had a negative charge.

These negatively charges particles were called electrons.

Major contribution to the atom: Electrons are in all atoms!

Cathode Ray Tube Experiment

CRT Video

Robert A. Millikan - 1909 Continued Thomson’s work –

performed the Oil Drop Experiment

confirmed the negative charge of an electron and measured the mass of an electron

The electron has mass, though 1836 x less than that of soon to be discovered proton.

Thomson proposed that the electrons of an atom were spread evenly throughout a positively charged ball of matter. Known as Plum-pudding model

J.J. Thomson’s Plum Pudding Model

Plum Pudding Video

The Discovery of the Atomic Nucleus Earnest Rutherford’s Gold Foil Experiment - 1909

Gold Foil Experiment Set up Gold Foil with a detection sheet around it. Set up radioactive source- emitted alpha particles.

ALPHA PARTICLES shot at gold foil.

MOST particles went through the gold foil

But SOME particles BOUNCED back

Gold Foil Conclusions1. The atom is made up of mostly EMPTY SPACE2. The center of the atom contains a POSITIVE

CHARGE3. Rutherford called this positive bundle of matter

the NUCLEUS

Rutherford’s major contribution to the atom was the discovery of the nucleus. The volume of this is very small compared with the total volume of an atom.

Rutherford’s Model of the Atom

atomic radius ~ 100 pm = 1 x 10-10 m

nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

When two protons are extremely close to each other, there is a strong attraction between them.

A similar attraction exists when neutrons are very close to each other or protons and neutrons.

The short-range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together are referred to as nuclear forces.

Composition of the Nucleus

Structure of the Atom The nucleus is a very small region located at the

center of an atom.

The nucleus is made up of at least one positively charged particle called a proton and usually one or more neutral particles called neutrons.

Surrounding the nucleus is a region occupied by negatively charged particles called electrons.

P, N, E are often referred to as subatomic particles.

Particle Symbol Charge Mass Number

Actual Mass (kg)

Electron e- -1 0 9.109 x 10-31

Proton p+ +1 1 1.673 x 10-27

Neutron no 0 1 1.675 x 10-27

Section 3.3

Ions and Isotopes

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei

XAZ

H11 H (D)2

1 H (T)31

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

An ion is an atom, or group of atoms, that has a net positive or negative charge.

cation – ion with a positive chargeIf a neutral atom loses one or more electronsit becomes a cation.

anion – ion with a negative chargeIf a neutral atom gains one or more electronsit becomes an anion.

Na 11 protons11 electrons

Na+ 11 protons10 electrons

Cl 17 protons17 electrons

Cl-17 protons18 electrons

Atomic Mass Unit

One atom is the standard – Carbon Mass of other elements are based off of the standard Carbon: 6 p and 6 n = 12 amu

1/12 mass of Carbon atom Periodic table lists weighted average atomic masses of elements

(like a GPA calculation)

Relative Atomic Mass