Chapter 4 Atomic Structure
I. History of the Atom
A. Democritus (400 AD)
1. All matter consisted of extremely small particles that could not be divided
2. Atoms (uncut, indivisible)
B. Aristotle (400 AD)
1. No limit to number of times matter could be divided
C. Dalton (1700s)
1. Discovered that compounds have a fixed composition
2. Ratio remains constant
3. Developed the Atomic Theory
a. All elements are composed of atoms
b. All atoms of the same element have the same mass, and atoms of
different elements have different masses
c. Compounds contain atoms of more than one element
d. In a particular compound, atoms of different elements always
combine in the same way
4. Model of atom; solid sphere
D. J. J. Thomson (Joseph John 1856-1940) (electron)
1. Cathode Ray tube
2. Hypothesized that the beam was a stream of charged particles
3. Attracted to positive plate concluded that they were negative
4. 1/2000 the mass of a H atom
5. Proved that atoms were made up of smaller particles
6. Plum pudding model
7. Later named electron
E. Rutherford (Earnest 1899) (Proton)
1. Hypothesized that particles would travel in a straight path through gold and hit
screen
2. Hypothesis was not supported
a. 1/20,000 were deflected
3. Concluded that positive charge was not evenly distributed
a. Nucleus: A dense, positively charged mass located in the center of the atom
b. Total volume of atom is about a trillion times the volume of the nucleus
II. The Structure of an Atom
A. Proton: Positively charged subatomic particle that is found in the nucleus of an atom
B. Electron: A negatively charged subatomic particle that is found in the space outside the
nucleus
C. Neutron: A neutral subatomic particle that is found in the nucleus of an atom
1. James Chadwick
2. Mass is almost the same as a proton
D. Electron cloud: A visual model of the most likely locations for electrons in an atom
III. Structure of an Atom (continued)
A. Atomic Number and Mass number
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1. Atomic number: Represents the number of protons in an atom of an element
a. Proton number of an element never changes
1.
2.
b. Atoms of different elements have different atomic numbers
c. Electron number is always the same as the atomic number
1. Each positive charge is balanced by a negative charge for a
neutral atom
2. Mass number: The sum of the protons and the neutrons in the nucleus of an
atom
a. Number of neutrons =
mass number – atomic number
D. Isotopes: Atoms of the same element that have different numbers of neutrons and
different mass numbers
1. Same atomic number but different mass numbers
2. Same number of protons and electrons
3. Different number of neutrons
III. Modern Atomic Theory
A. Bohr’s model of the atom
1. Energy levels: The possible energies that electrons in an atom can have
a. Fixed orbits around nucleus
b. Each step up represents a higher energy level
c. Electron can move from one energy level to another when atom gains
or loses energy
B. Electron Cloud Model
1. Used to describe possible locations of electrons around nucleus
C. Atomic Orbitals
1. Orbital: A region of space around the nucleus where an electron is likely to
be found
D. Electron configuration: The arrangement of electrons in the orbitals of an atom
1. Ground state: All the electrons in an atom have the lowest possible energy
a. Stable
2. Excited state: An electron moves to an orbital of higher energy
a. Less stable
Chapter 5 Periodic Table
I. Organizing the elements
A. Mendeleev’s Periodic Table
1. Arranged in rows by increasing atomic mass
2. Elements with similar properties arranged in columns
a. Periodic table: An arrangement of elements in columns, based on a
set of properties that repeat from row to row
3. Prediction
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a. Predicted that eka-aluminum would be a soft metal with a low mp and
a density of 5.9 g/cmo
b. 1875 Gallium was discovered
1. Low mp (29.7o), density of 5.91 g/cm3
c. 1879 Scandium (Sc)
d. 1886 Germanium (Ge)
II. The Modern Periodic Table
A. Periodic Law: In the modern table, elements are arranged by increasing atomic
number
1. Periods: Horizontal rows of elements on the periodic table
2. Group (Family): Vertical columns of elements on the periodic table
1. Elements in groups have similar electron configurations and chemical
properties
2. Periodic law: Properties of elements repeat in a predictable way when
elements are arranged by increasing atomic number
B. Atomic Mass: Average atomic mass of isotopes found in nature, noted on periodic
table
a. Carbon-12 = 12 atomic mass units
b. Atomic mass unit (amu): 1/12 the mass of a carbon-12 atom
c. Average atomic mass:
C. Classes of Elements
1. Solids, liquids, gases at room temp
2. Whether they occur naturally or not (1-92)
3. Metals, nonmetals, and metalloids
a. Metals: Elements that are good conductors of electric current and heat
1. Ductile: Can be drawn into wires
2. Malleable: Can be pounded into sheets
3. Lustrous: Shiny
4. Transition metals: Metals found in the center of the table (groups 312)
5. Lanthanide and actinide series; properties are similar to one another
b. Metalloids: Elements with properties that fall between those of metals and
nonmetals
1. Ability to conduct electric current varies with temperature
c. Nonmetals: Elements that are poor conductors of heat and electric
current
1. Low boiling points; gases at room temperature
2. Solids are brittle
III. Representative groups (Main Groups): Groups 1 and 2 and 13-18
A. Valence electrons: An electron that is in the highest occupied energy level of an
atom
1. Same amount within a group; reason for similar properties
2. Play key role in chemical reactions
B. Alkali metals: Elements of group 1
1. One valence electron
2. Very reactive
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C. Alkaline Earth metals: Elements of group 2
1. 2 valence electrons
2. Harder and higher mp than those in group 1
D. Halogens: Elements in group 17
1. 7 valence electrons
2. Highly reactive non-metals
3. Fluorine; most reactive element on the table
E. Noble gases: Elements in group 18
1. He has 2 valence electrons, the rest have 8
2. Unreactive; stable
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