Download Lap 4: Atomic Structure Mead Chemistry Chapter 4 4.1 Defining the

Lap 4: Atomic Structure
Mead Chemistry
Chapter 4
4.1 Defining the Atom
A. Early Models of the Atom
 Def: Smallest particle of an element that still retains its identity in a chemical reaction
 Democritus’s Atomic Philosophy
 Greek philosopher
 460 BC -370 BC
 First to suggest existence of atoms
 Believed atoms were indivisible and indestructable
 Ideas proved to be true, but not based on scientific method
B. Dalton’s Atomic Theory
 English chemist 1766-1844
 Using experimental methods, he transformed Democritus’ ideas into a scientific theory
 5 parts to his atomic theory
1. All elements are composed of tiny, indivisible particles called atoms.
2. Atoms of one element are identical.
3. Atoms of any one element are different from any other element.
4. Atoms of different elements can join together to make compounds.
5. Chemical reactions occur when atoms are separated, joined or rearranged.
C. Sizing up the Atom
 Atoms are very small
 A penny contains 2.4 x 1022 atoms of Copper (Cu)
 24000000000000000000000
 Only 6 x 109 people on Earth
 Radii between 5 x 10-11m and 2 x 10-10 m
 0.00000000005m to 0.0000000002m
 Possible to see with a Scanning Tunneling Microscope
 Arranged iron (Fe) on copper plate (Cu)
4.2 Structure of the Nuclear Atom
A. Subatomic Particles
1. Atom can be divided into 3 subatomic particles
Proton, neutron and electron
2. Electron
a. Discovered in 1897 by J.J.Thomson
b. Negatively charged particle
c. Mass found by Millikan to be 1/1840 the mass of a hydrogen atom
3. Protons
a. Positively charged particle
b. Mass of 1840 times larger than electron
4. Neutrons
a. Particles with no charge
b. Mass same as proton
B. Atomic Nucleus
1. Early theory suggested that the electrons were spread out in the atom like “raisins in
plum pudding”
2. Actual structure was discovered by Rutherford in 1911
a. Gold Foil experiment
b. Shot a beam of positive particles at a piece of gold foil
c. If plum pudding theory is true, particles should pass easily through with only
slight deflection when positive particles hit the spread out positive protons
3. Did not happen as expected
a. Instead most particles passed easily through
b. Small number bounced back at very large angle
4. Rutherford’s new theory of atomic structure
a. Atom is mostly empty space
This is why most particles passed easily through
b. Positive charge and almost all mass is located in central area of atom
This is why some particles bounced off at huge angles
c. Called the center the nucleus
Def: tiny, central core of the atom that is composed of protons and
neutrons
5. Nuclear Atom
a. Protons and neutrons are in the nucleus
b. Electrons are distributed around the nucleus and occupy almost all of the
volume of the atom
c. If the atom was the size of a football field, the nucleus would be a marble
4.3 Distinguishing Among Atoms
A. Atomic number
 Elements are defined by the number of protons in the nucleus = Atomic number
 Examples:
 Hydrogen: atomic number 1= 1 proton
 Helium: atomic number 2 = 2 protons
 Copper: atomic number 29 = 29 protons
 Silver: atomic number 47 = 47 protons
 Arranged by atomic number on the periodic table
 Number of protons = number of electrons
 Atoms are neutral (net charge of 0)
 Example: Carbon
 Atomic number = 6
 6 protons (+6)
 Must have 6 electrons (-6)
 +6 and -6 = 0 charge
 Example: Sulfur
 Atomic number = 16
 16 protons, 16 electrons
B. Mass number
 Most of the mass of an atom is in the nucleus
 Nucleus contains protons and neutrons
 Mass of protons and neutrons are 1840 times larger than the mass of an electron
 Mass number = # protons + # neutrons
 Mass # = Atomic # + # neutrons
 Example: Gold-197 or Au
 Mass number = 197
 Atomic number = 79
 # protons:
 79
 # electrons:
 79 (same as protons)
 # neutrons:
 Mass # - atomic #
 197-79
 118 neutrons
C. Isotopes
 Def: Different varieties of atoms of one element
 Examples: different isotopes of helium
 Helium-5, Helium-6, Helium-7
 Must have same atomic number (# of protons) b/c that defines the element
 2 protons makes helium helium
 Mass number is different and so isotope is identified by mass number
 Example of Neon isotopes
 All neon varieties have atomic number of 10
 Neon-20
 Mass number 20
 Protons =10, Electrons =10, Neutrons =10
 Neon-21
 Mass number 21
 Protons =10, Electrons =10, Neutrons =11
 Neon-22
 Mass number 22
 Protons =10, Electrons =10, Neutrons =12
D. Actual masses of atoms
1. Too small to be useful and practical for calculations
2. Example: mass of atom of Flourine atom 3.2x 10-23 grams
3. Use a standard to compare relative masses
4. Use isotope of carbon, Carbon-12, to be the standard for measuring atomic mass
E. Atomic mass unit (AMU)
1. Carbon-12 has a mass of 12 amu
2. Compare all other elements to carbon
3. Different isotopes of an element will have different atomic masses b/c of the
different numbers of neutron
4. Atomic mass of an element = weighted average of the masses of all isotopes
5. Includes the mass of the different isotopes and their relative abundance
6. Relative abundance= how common a particular isotope is in a naturally occurring
sample of the element, given as a percent
7. Whichever isotope the average atomic mass is closest to has greatest abundance
a. Example: Atomic mass of Cu = 63.546 amu
Which isotope is more abundant, Cu-63 or Cu-65?
Atomic mass is closer to Cu-63
Cu-63 is more abundant or more common
b. Example: Chlorine’s atomic mass = 35.4 amu
Which isotope is more abundant, Cl-35 or Cl-37?
Cl-35
E. Calculating Atomic Mass
Formula: (isotope mass X abundance) + (isotope mass X abundance) /100
Example: Find the atomic mass of carbon if Carbon-12 has an abundance of 98.89% and
Carbon-13 has an abundance of 1.11%.
(12 X 98.89) + (13 X 1.11) /100 = 12.011 amu
4.4 Periodic Table Preview
1. Arrangement of elements placed into groups according to shared properties
2. Allows you to compare properties of elements
3. Horizontal row = period
7 periods on modern periodic table
Properties vary as you move across period and repeat in next period
4. Vertical column = group or family
Elements within group have similar chemical and physical properties
Identified by number and letter