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. H 2 O

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. CO 2 (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 = x kg  Discovered electron charge  e = x C  Oil drop experiments.

Early Models of the Atom Thompson  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 Atom Rutherford  How are electrons distributed in an atom?  Discovered alpha particles as 4 2 He  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  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 Atom The 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

c = νλ c = speed of light, 3.0 x 10 8 m/s  Constant ν= frequency (s -1 or Hz) λ= wavelength (m)

Example 1:  Find the frequency of a green light that has a wavelength of 545 nm.

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  E = hν  h= x J  s (Planck’s constant)  E = energy per photon (J)  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

Example 1 Calculate the energy found in a photon of red light with a wavelength of nm

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 OP  HeFS  Li +1 NeCl -1  BeNaAr  U-238Mg +2 K