The Atom

Law of Conservation of Mass/Matter

Matter cannot be created or destroyed

Total mass is constant in chemical reactions.

Originated with Antoine Lavoister (1700s)

Quantitative mass data of reactants and products in mercury oxide decomposition.

Law of Definite Proportions

Proposed by Joseph Proust (late 1700s)

Decompositions and research with copper carbonate

Compound composition and properties are fixed

All compound samples have the same composition

Same % of elements in the compound

Ex. H2O

Law of Multiple Proportions

2+ compounds with same 2 elements

Compositions of these compounds are related

Masses of elements related to each other in whole number ratios

Proposed by John Dalton in addition to his atomic theory.

Ex. CO2 (2:1), CO (1:1)

Terminology

Element– basic unit of a substance, contain only ONE type of atom, represented by symbol.

Example: Ag, only contains Ag atoms.

Atom—smallest particle of an element that still contains element properties. Example: One atom of Au, cannot have a smaller particle of

gold and still be gold.

Compound vs. Molecule

Compounds:

more than one element

elements combined in definite proportions

Molecule:

Smallest unit of a compound that still retains the properties of the compound.

How far back does the “atom” go?

Democritus

400 B.C.

Called the basic unit of matter an “atom”

The Atom and its Structure

Dalton Atomic Theory

1800s

Atoms make up elements.

Atoms form compounds as a whole and cannot be divided. Compounds formed from atoms joining in FIXED proportions

Dalton Atomic Theory (cont.)

All matter made of atoms

Atoms of an element have the same size, mass, etc.

Different atoms have various sizes, mass, etc.

Atoms cannot be divided, destroyed, or created.

Atoms rearrange in chemical reactions.

John Thomson

1897

Cathode-Ray experiments.

Discovered the electron particle and its possible charge (-).

Determined ratio between mass and charge of an electron

Robert Millikan

1909, American

Found the mass of an electron (VERY small) with Thompson’s data

Currently, mass of electron = 9.109 x 10-

31kg

Discovered electron charge

e = -1.602 x 10-19 C

Oil drop experiments.

Early Models of the AtomThompson

Must be a balance between negative and positive charges

“Raisin-Pudding” model

Uniform distribution of positive charge

Positive cloud with stationary electrons

Early Models of the AtomRutherford

How are electrons distributed in an atom?

Discovered alpha particles as 42He

Experiments with Au, Ag, and Pt foils bombarded with alpha particles

Early Models of the Atom Rutherford

Mostly empty space

Small, positive nucleus

Contained protons

Negative electrons scattered around the outside

James Chadwick 1932 discovered neutrons

contained in atom’s nucleus

No charge

Mass approximately same as proton mass

Early Models of the Atom Bohr

1913—hydrogen atom structure

Physics + quantum theory

Electrons move in definite orbits around the positively charged nucleus—planetary model

Does not apply as atoms increase in electron number

Erwin Schrödinger Quantum mechanics

1926---wave equation

Electrons behave more like waves than particles

Heisenberg’s Uncertainty Principle

Electron’s location and direction cannot be known simultaneously

Electron as cloud of negative charge

Modern Model of the AtomThe electron cloud

Sometimes called the wave model

Electron as cloud of negative charge

Spherical cloud of varying density

Varying density shows where an electron is more or less likely to be

How did we discover electron arrangement

in an atom?

ELECTROMAGNETIC

RADIATION ! ! !

Waves Repeated disturbance through a medium

(air, liquid) from origin to distant points.

Medium does not move

Ex. Ocean waves, sound waves

Characteristics of Waves

Wavelength

Distance between 2 points within a wave cycle

2 peaks

Frequency

# of wave cycles passing a point for a particular time unit

Usually seconds.

Wavelength and frequency are inversely proportional.

Electromagnetic Waves

Produced from electric charge movement

Changes within electric and magnetic fields carried over a distance

No medium needed

Electromagnetic Spectrum

Contains full range of wavelengths and frequencies found with electromagnetic radiation

Mostly invisible, visible range (390 nnm -760 nm)

Different materials absorb/transmit the spectrum differently.

Types of Spectra What is a spectra?

Spectrum– white light/radiation split into different wavelengths and frequencies by a prism

Continuous spectrum

No breaks in spectrum

Colors together

Line spectrum

Line pattern emitted by light from excited atoms of a particular element

Aided in determining atomic structure

Line Spectrum Pattern emitted by light from excited atoms of an

element

Specific for each element

Used for element identification

Flame Tests Some atoms of elements produce visible

light if heated

Each element has a specific flame color

Examples: Li, Na, Cs, Ca

A Bit of Quantum Theory……

Max Planck 1900

Related energy and radiation

Quantum---smallest amount of energy

Atoms can only absorb/emit specific quanta

Albert Einstein 1905

Added to Planck’s concept

Photons—

Bundles of light energy

Same energy as quantum

Photons release energy and electrons gain energy

Threshold frequency– minimum amount of energy needed by photon to extract electron

THEREFORE ……… Light is in the form of electromagnetic

waves

Photons can resemble particles

Gave raise to the possibility of thinking about wave AND particle qualities of subatomic particles (electron)

Atomic Structure Nucleus

Protons

Neutrons

Electrons

Atomic Structure Electrons

Tiny, very light particles

Have a negative electrical charge (-)

Move around the outside of the nucleus

Atomic Structure Protons

Much larger and heavier than electrons

Protons have a positive charge (+)

Located in the nucleus of the atom 

Atomic Structure Neutrons

Large and heavy like protons

Neutrons have no electrical charge

Located in the nucleus of the atom 

Atomic Structure

Describing Atoms

Atomic Number = number of protons

In a neutral atom, the # of protons = the # of electrons

Atomic Mass= the number of protons + the number of neutrons

Isotopes The number of protons for a given atom never changes.

The number of neutrons can change. 

Two atoms with different numbers of neutrons are called isotopes

Isotopes have the same atomic #

Isotopes have different atomic Mass #’s

Isotopes

Ions

An atom that carries an electrical charge is called an ion

If the atom loses electrons, the atom becomes positively charged.

If the atom gains electrons, the atom becomes negatively charged

Ions The number of protons does not change in

an ion.

The number of neutrons does not change in an ion.

So, both the atomic number and the atomic mass remain the same.

PEN Method for--- H O P

He F S

Li +1 Ne Cl -1

Be Na Ar

U-238 Mg +2 K