CHAPTER 4 – THE ATOM

John Dalton’s Atomic Theory (19 th Century) 1) All matter is composed of small particles called atoms 2) All atoms of an element are identical in size, mass, and chemical properties 3) Atoms cannot be created, divided into smaller particles or destroyed 4) Different atoms combine in simple whole number ratios to form compounds 5) In chemical reactions, atoms are separated, combined, or rearranged FALSE

Rule #4 + ELEMENT A ELEMENT B COMPOUND of A and B

Rule #5 +

What is an atom? Defn – smallest particle of an element that retains properties of the element How many copper atoms can fit on a penny? 29,000,000,000,000,000,000,000 atoms (29 x )

Electron Discovered by J.J. Thomson (1909) using a cathode ray tube (CRT) Electron – negative charged particle of matter (-1) - has no relative mass

Nucleus Discovered by Ernest Rutherford (1911) using Gold Foil Experiment Nucleus – dense, positively charged region in center of atom entialchemistry/flash/ruther14.swf

Composition of Nucleus a)Proton – positive (+1) charged particle of nucleus b)Neutron – neutral (0) charged particle of nucleus

Particle Symbol Location Charge Relative Mass Electron e - around nucleus -1 0 Neutron n 0 nucleus 0 1 Proton p + nucleus +1 1

What makes an atom of one element different from an atom of another element ?

Atomic Number Defn – number of protons in an atom How is periodic table arranged? by increasing atomic number How many electrons do atoms have? # electrons = # protons atomic number identifies atoms

reading the periodic table H Hydrogen Element Atomic number Symbol Atomic mass

Mass Number Defn – total number of protons and neutrons NOT SAME AS ATOMIC MASS!!!!!! H Hydrogen Cannot be found on periodic table NO NO NO NO NO!!!!

Mass Number Remember this formula to find # neutrons: MAN M – A = N (Mass #) – (Atomic #) = Neutrons

Refresh!!!! I’ve given you 3 numbers: 1)Atomic number 2)Atomic mass 3)Mass number The ones with the word “ATOMIC” is on the periodic table Not on periodic table

What’s in a cheeseburger?

Isotope Defn – atoms with same number of protons, but different number of neutrons HYDROGEN - 1 HYDROGEN - 2HYDROGEN - 3 = neutrons = protons = electrons

3 Different Hydrogen Isotopes HYDROGEN – 1 (protium) 1 proton 0 neutrons 1 electron HYDROGEN – 2 (deuterium) HYDROGEN – 3 (tritium) 1 proton 1 neutron 1 electron 1 proton 2 neutrons 1 electron

Ways to write isotopes 1) Hyphen notation (2 parts) name of element – # ORsymbol – # ex: carbon-14ORC-14 chlorine-35ORCl-35 hydrogen-3ORH-3 MASS NUMBER

Ways to write isotopes 2) Nuclear symbol (3 parts) C 14 6 element symbol mass number atomic number

Ex problem K Potassium # protons # neutrons # electrons Nuclear symbol Potassium-39 Potassium-40 Potassium K KK

Mass of individual atoms Atoms have very very tiny masses to work with(~ kg) Instead we use a relative mass – atomic mass unit (amu) Atomic Mass Unit (amu) – 1 amu ≈ mass of 1 proton or 1 neutron

amu’s and subatomic particles particleMass (amu) e-p+n0e-p+n

Atomic Mass Defn – weighted average mass of all isotopes of an element Where can it be found? found on periodic table K Potassium

Atomic Mass Recall all the hydrogen atoms in the world Hydrogen-1Hydrogen-2Hydrogen-3 Each has their own percent abundance in the world 99%0.2%0.8% Take averages of all the weights of each to find atomic mass atomic mass

Ex problem: Calculate atomic mass of chlorine. Isotope mass (amu) percent abundance 35 Cl % 37 Cl % What do you expect the average to be? About 35 amu Actual = amu

Ex problem: Calculate atomic mass of chlorine. Isotope mass (amu) percent abundance 35 Cl % 37 Cl % Mass contribution = (mass)(abundance)

Which isotope is most abundant? Ar-38, Ar-40, or Ar-41? Ar Argon Ar-40

Ex problem: Calculate atomic mass of chlorine. Mass contribution = (mass)(abundance) For 35 Cl= (35)( )= amu For 37 Cl= (37)( )= amu amu Matches periodic table for Cl