electron

neutron

proton

AtomsHistory, Theory,

Structure

What’s In the Box?

How can we describe something we can’t even see?

Compare to Black Box Experiment Can’t see internal structure Discovery by experimenting with how

matter behaves Constantly changing and/or refining

definition

How Do We Know Atomic Structure?

Our current understanding of atomic structure is based on:

What is Inside an Atom?

NucleusContains Protons (+) and Neutrons (N)

+N+

Electron Shells (Not exact locations) e-

Drawn as an outer “wall” only for reference; Energy

Historical Perspective

“Ancient” “Modern”

Aristotle: matter may be continuously divided without changing its intrinsic properties

Democritus:matter is composed of tiny, indivisible particles; atoms may not be divided

Dalton: Atoms may not be further divided.

Thomson: pudding theory; electrons

Rutherford: gold foil Bohr: quantum theory Schrodinger:

charge-cloud

1808 English Two very important tenets

◦ Law of Conservation of Mass◦ Law of Definite Proportions

Dalton

All matter is made up of atoms. All atoms of an element are identical in size,

mass, and chemical properties. Atoms of an element are different from atoms

of all other elements. Atoms may neither be divided nor destroyed. Atoms may be combined, separated, and

rearranged to form new compounds. Atoms of different elements combine in

whole-number ratios to form compounds.

Dalton’s Main Points

Thompson

1911 Rutherford’s Gold Foil Experiment Actually conducted by Rutherford’s

assistant Hugely advanced for time period

The Nucleus

Bohr’s model related structure and position of electrons to energy levels

Ground state Excited state Line-emission spectrum Continuous spectrum Only worked consistently for hydrogen

Bohr and Quantum Theory

Based on Bohr’s ideas with one main difference Electrons are not traveling in fixed positions Use of orbitals rather than orbits Areas where electrons are most likely to be

found For example, an electron will never be expected to be found in the atom’s nucleus

Charge-Cloud

Protons have a positive charge.◦ Mass of 1 unit◦ Located in nucleus◦ Identity of an atom

Neutrons have no charge◦ Mass of 1 unit◦ Located in nucleus◦ Help to stabilize nucleus

What’s Inside an Atom?

Located outside of the nucleus◦ Negative charge◦ Mass is insignificant to mass of whole atom◦ Responsible for chemical reactivity of an atom

Though mass is tiny, equal but opposite charge◦ Can change position◦ Relate to energy of an atom

Electrons

Protons Neutrons

Positive charge

Located in nucleus

Determine properties of an atom

Unique to each element

No charge (neutral)

Located in nucleus

Thought to provide stability to nucleus

Numbers may vary (isotopes)

•Negative charge•Located in energy levels around nucleus•Responsible for chemical reactivity of an atom

Electrons

Sub-Atomic Particles

All mass measurements are comparisons based on standards

All atoms based on mass of carbon-12 atom Weighted average of all known isotopes

based on measured frequency Combination of protons and neutrons

Atomic Mass

12.011

C6

Quantifying Atoms

Atomic Mass• represents

combined masses of protons and neutrons

Atomic Number• Represents

number of protons; in a neutral atom represents number of electrons

While all atoms of an element have the same number of protons, numbers of neutrons may differ

These atoms still retain the basic properties of the element

Small differences in behavior Atoms of the same element with differing

numbers of neutrons Indicated by symbol with number to

indicate number of neutrons or mass

Isotopes

Mendeleev (1834-1907); Russian; 1880’s◦ listed approximately 73 known elements in

columns in order of increasing atomic mass◦ left blank spaces for unknown elements

Moseley (1887-1915); British; worked w/ Rutherford; 1913◦ determined atomic number of atoms of known

elements◦ arranged elements by atomic number

The Periodic Table of the Elements: History

Periods◦ Horizontal

◦ 7

◦ Periodic Law

◦ Properties change as you go from left to right on the Periodic Table. Atomic mass increases Atomic number increases

Modern Periodic Table

Group or Family

Vertical

Similar physical and chemical properties

Significance of Group Number◦ For main group elements represents number of

valence electrons

Modern PT, cont.

Approximately 80% of known elements

Alkali: Group I◦ Highly reactive

Alkaline Earth: Group 2◦ Highly reactive

Transition, inner transition

Properties◦ malleable, ductile, lustrous◦ solids at room temp except Hg◦ good conductors of heat and electricity

Metals

Not lustrous

Poor conductors of electricity and heat

Halogens◦ Group 7

Noble gases◦ Group 8

Right side of Periodic Table

Nonmetals

Zig zag line

Properties of both metals and nonmetals

Used in computer chips and solar cells; semiconductors

Metalliods

Periodic Table of the Elements

Source: Jefferson Labs

Historical perspective◦ Models of atoms based on behavior rather than a

visual analysis All atoms of an element have same basic

properties◦ Isotopes

Atoms of same element with differing # of neutrons Identity of an atom based on protons Chemical reactivity of an atom based on

electrons Periodic Table of the Elements

◦ Organization and Patterns

Summary