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 English schoolteacher. We now know this theory is not all the way true due to being able to divide an atom—nuclear chemistry.

Dalton Atomic Theory (cont.) Atoms cannot be divided, destroyed, or created. Atoms rearrange in chemical reactions. All matter made of atoms Atoms of an element have the same size, mass, etc. Different atoms have various sizes, mass, etc. Rearrange, combine, separate. True: all matter made of atoms, different atoms differ in properties, atoms do rearrange

John Thomson 1897 Cathode-Ray experiments. Discovered the electron particle and its possible charge (-). Determined ratio between mass and charge of an electron Rays move away from wire containing electric charge—most be negative charge. -tub opposite cathode glows—due to stream of particles/cathode ray. **Discovered electrons**--they were moving to opposite side and causing deflection., negative charge! Cathode = positive.

Robert Millikan Oil drop experiments. 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 Knew oil density, so watch the oil drop and measure mass/watch and note the particles radius. Electric field helped find the charge of electron. -Spray oil drops, electric field set up

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 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 Putting everything together. Gold foil experiment. –bombard gold foil with alpha particles/positive charge, most particles passed through foil -others deflected back!!! ---hit something in the atom/come in contact with something in atom, very small because only happened some of the time. -also positive center due to being repelled. Discovery of Nucleus

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 We will look more at this model later. -Based on the hydrogen atom.

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 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 ν= frequency (s-1 or Hz) 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. Frequency = 5.50x1014 Hz

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? Continuous spectrum Line spectrum 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 E = hν h= 6.626 x 10 -34 Js (Planck’s constant) 1900 Related energy and radiation E = hν h= 6.626 x 10 -34 Js (Planck’s constant) E = energy per photon (J) Quantum---smallest amount of energy Atoms can only absorb/emit specific quanta Energy increases with frequency

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 700.0 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 Electrons Protons Neutrons Atoms are made of subatomic particles.

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 Positive charge—more protons than electrons. Negative charge—more electrons so the negative charge is more than positive charge **Think like a balance** Balance of charges = neutral atom.

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