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Atomic Structure and the
Periodic Table
Ch. 5
Atoms
5-1
Early Models of the Atom
• Democritus – 4th
century BC – world
made up of empty
space and tiny
particles called atoms
(atomos)
‘indivisible’
– Hypothesized without
using experiments
Dalton’s Atomic Theory
1. All matter is made of atoms
2. Atoms are indestructible and cannot be divided
into smaller particles (later proved wrong)
3. All atoms of one element are exactly alike
(later proved wrong), but they are different
from atoms of other elements
4. Atoms combine in simple whole number ratios
to form compounds
5. In chemical reactions, atoms combine,
separate, or are rearranged.
Done in the 1800’s through experiments!
Structure of the Nuclear Atom
5-2
Atomic Structure
1. Electron: (e-)
– Discovered by Thompson’s cathode-ray tube
(1897)
• Cathode Ray = positive and negative electrodes
connected, beam travels from cathode (-) to
anode (+)
– Found electrons have negative charge
• Beam in cathode ray attracted to anode (+), must
be negatively charged.
– Relatively no mass (experiment by Millikan
1916)
Thompson’s Cathode-Ray
Experiment
2. Proton – has positive charge (p+), greater
mass than an electron
- Discovered by Goldstein in 1886
3. Neutron – has neutral charge (no), equal
in mass to proton
-Discovered by Chadwick in 1932
4. Nucleus
= small, positively charged central core,
made up of protons and neutrons
-Discovered by Rutherford’s gold foil experiment
in 1911
-Fired stream of positive particles at gold foil,
most passed right through (atom mostly empty
space) while a few bounced off (very small
positive nucleus at center)
Play movie
Development of Modern Atomic
Theory
• Law of conservation of matter (mass) = in
a chemical reaction matter is neither
created nor destroyed.
– All elements are recycled/rearranged!!
• Law of definite proportions = elements
within a compound are always in certain
proportions by mass
– Ex: NaCl (table salt) is always
39% Na and 61% Cl.
Distinguishing between Atoms
5-3
Atomic Number
• Atomic # = Number of protons in the nucleus of an
atom
atomic # = p+
• Determines the identity of the element
• Periodic table is organized by atomic number
• Ex: What is the atomic number of…
He ______ C ______
N ______ O ______
• Also! # of protons = # of electrons
p+ = e– Ex: How many electrons does Cl have? _______
Mass Number
• Mass # = Sum of the protons and neutrons
mass # = p+ + no
Why do we not include electrons???
neutron = mass # - atomic #
• Periodic Table symbols:
12
C
6
Mass # (rounded)
Element
Atomic #
Let’s Practice!
• How many?
Protons ____
Neutrons ____
Electrons ____
• Write it:
?
C
?
• How many?
Protons ______
Neutrons ______
Electrons ______
• Write it:
?
S
?
Remember!!!
•
•
•
•
Mass # = p+ + n
Atomic # = p+
e- = p+
n = mass # - atomic #
Atomic Mass + Isotopes
• Atomic Mass =
weighted average
mass of isotopes
• Isotopes = one
element with same
proton # but different
neutron #
– Carbon has 3
possible isotopes:
14 13 12
C
C
C
6
6
6
Carbon-14, Carbon-13,
Carbon-12
Try it!
• Ex: Isotope of Chlorine:
37
Cl
17
How many?
Protons
______
Neutrons
______
Electrons ______
Atomic Mass Unit
• Atomic Mass Unit (amu) = 1/12 the mass
of a carbon-12 atom.
• Used to compare element’s masses to a
standard.
• Units for measuring atom’s masses
The Periodic Table:
Organizing the Elements
5-4
Development of the Periodic Table
• In 1869, Mendeleev created Periodic
Table of Elements
– Organized it by increasing atomic mass
and elements with similar properties in
same columns.
– Able to predict future elements
properties
• In 1913, Moseley organized it by atomic
number.
Modern Periodic Table
• Organized by atomic number
• Periodic Law: grouped according to physical
and chemical properties
• Period = horizontal row of periodic table;
numbered down the side, 7 periods
• Group = vertical column, also called a
family; 18 groups
Group Designations
• Representative elements = Group 1,2,1318
– Group 1 – Alkali metals
– Group 2 – Alkaline earth metals
– Group 17 – Halogens
– Group 18 – Noble gases
• Transition metals = groups 3-12
Physical States of Elements
• Periodic Table shows the states of the
elements at room temperature and normal
pressure
– Most are solids
– Only two are liquids
– Gaseous elements are in the top right hand
corner except for Hydrogen
– The rest are made synthetically (manmade)through nuclear reactions (43, 61, 85,
87, and > 93)
Classifying Elements
• Metals - majority of elements, occupy left
side and center
• Nonmetals - occupy top right hand corner,
(includes H)
• Metalloids - along the boundary
Metals
• Have luster, many colors, conduct heat and
electricity, usually bend w/o breaking
(malleable)
• Almost all are solids at room temp. and have
extremely high melting points
Nonmetals
• Are abundant in nature
– Oxygen and Nitrogen: 99%
atmosphere
– Carbon in more compounds than all
others combined!
• Poor conductor of heat and electricity,
brittle when solid
• Many gases at room temp, low melting
point
Metalloids
• Have properties of both metals and
nonmetals
• 7 Metalloids: B, Si, Ge, As, Sb, Te, Po
(touch staircase, no Al + At)
• Some are semiconductors = element that
does not conduct electricity as well as
metal, but does conduct slightly better
than nonmetal
– Ex: Silicon – used in computer industry