Download Chemistry--Chapter 5: Atomic Structure and the Periodic Table

Chemistry--Unit 1: Atomic Structure and the Periodic Table
Lecture Notes
I.
II.
Atoms
A. Early Models of the Atom
1. Democritus
a. 400 BC, first suggested the existence of indivisible atoms
b. No research, no experimental support
2. John Dalton
a. late 1700’s conducted research and experiments
b. result was Dalton’s atomic theory:
1) All elements are composed of tiny indivisible particles called
atoms (we know now atoms are divisible!).
2) Atoms of the same element are identical. The atoms of any one
element are different from those of any other element.
3) Atoms of different elements can physically mix together or can
chemically combine with one another in simple whole-number
ratios to form compounds.
4) Chemical reactions occur when atoms are separated, joined, or
rearranged. Atoms of one element, however, are never changed
into atoms of another element as a result of a chemical reaction.
B. Just How Small Is an Atom?
1. Unimaginably small!
2. A pure copper penny would contain 2.4 x 1022 atoms, while there are only
6 x 109 people on earth; a 1-cm line of copper atoms would contain 1 x 108
atoms.
3. 1981, scanning tunneling microscope, can see outside of individual atoms
Structure of the Nuclear Atom
A. Electrons
1. Negative charge, discovered by J.J. Thomson in 1897 using cathode ray
tube
2. An electron carries one unit of negative charge and its mass is about
1/1840 the mass of a hydrogen atom or 9.11 x 10-28g (more precisely,
9.10939 × 10–28 g); charge and mass of electron determined by Robert
Millikan in 1916
B. Protons and Neutrons
1. Protons have a positive charge, cathode ray tube again used in discovery,
have a mass about 1840 times that of an electron or 1.67 x 10-24g (more
precisely, 1.67262 × 10–24 g)
2. Neutrons have no charge but a mass nearly equal to that of a proton (more
precisely, 1.67493 × 10–24 g), found in 1932 by James Chadwick
C. The Atomic Nucleus
1. Thomson developed the “plum-pudding model” of atomic structure, in
which the atom was a large positively charged ball with small negatively
charged electrons embedded inside of it
2. The Gold Foil experiment conducted by Ernest Rutherford, attempted to
prove this but instead lead to the discovery of the nucleus, the small, dense
(2.3 × 1017 kg/m3), central region of the atom where the protons and
neutrons are located.
Chemistry--Unit 1: Atomic Structure and the Periodic Table
Lecture Notes
III.
IV.
3. Most of the atom is empty space and electrons, nucleus in the center
Distinguishing Between Atoms
A. Atomic Number--the atomic number is the number of protons; it distinguishes
one elements’ atoms from another; small whole numbers on periodic table
B. Mass Number--the total number of protons and neutrons (not found on p.t.),
the “14” in carbon-14
C. Isotopes--elements of the same atom that have a different number of neutrons,
so a different mass number
D. Atomic Mass
1. Numbers given on periodic table are in amu’s (atomic mass units),
obviously not grams
2. One amu is defined to be the mass of 1/12th of the mass of the carbon atom
3. The atomic mass of an element is a weighted average mass of the atoms in
a naturally occurring sample of the element
4. The atomic masses for elements are found on periodic table, larger
decimal numbers
The Periodic Table: Organizing the Elements
A. Development of the Periodic Table
1. Antoine Lavoisier-33 known elements at the time, classified as earths,
gases, nonmetals, and metals (1789), Dobereiner-the law of triads,
examples Li, Na, K, and Cl, Br, I (1829), then Newlands-the law of
octaves (1865)
2. Dmitri Mendeleev
a. 1869, arranged the 70 known elements in order according to atomic
mass and in groups according to similar properties
b. left blanks for elements not discovered yet
3. Henry Moseley, 1913, discovered the importance of the atomic number
and arranged the periodic table in order according to atomic number
B. The Modern Periodic Table
1. The periodic law states that when the elements are arranged in order of
increasing atomic number, there is a periodic repetition of their physical
and chemical properties.
2. Elements in the same column, called groups, have similar chemical and
physical properties, while elements across a period do not
3. Three general classifications, metals and nonmetals, separated by
metalloids
4. Groups 1 & 2 and 13-18 are the representative elements because they
exhibit a wide range of physical and chemical properties
5. Other areas are alkali metals, alkaline earth metals, transition metals,
inner transition metals (also called rare earth elements), halogens, and
noble gases