Chapter 4
 Smallest part of an element
that retains the properties of
that element
 all elements are made of
indivisible atoms
 Two main regions of an atom:
I. Nucleus
II. Outer Cloud
I. Nucleus
 Most of the atom’s mass
 Very little space
~1/10,000th the volume.
 Contains two subatomic particles
 Protons (+)
 Neutrons (0)
 A. Protons (p+)
 Found in the Nucleus
 Positive Charge
 Mass is 1 amu
 Determines the element
B. Neutrons (n0 )
 Found in the Nucleus
 Neutral Charge
 Mass is the same as a proton
Determines nuclear
stability
II. Outer Cloud
 Most of the atom’s volume
 Very little mass
 Contains Electrons(-)
A. Electrons (e-)
 Move around the nucleus in
concentric shells
 Negative Charge
 Very Very Light!
 Determines Basic Chemical
Reactivity
 Atomic Number
 The # of Protons
 Found on the Periodic Table
 The “Z” Number
 Atomic Number (must be whole #)
 Give the atomic number for the
following elements:
C
 Be
 Ca
 Ar
 He
 Cu
 Fe
 Mass Number
 # of particles in the nucleus
 The “A” number
 # of Protons + # of Neutrons
 This is not on the periodic table!
 Example:
A carbon atom has 6 p+ and 7 n0.
What is its mass number?
 Mass Number
 Give the mass number for the
following elements:
 C: 6 p+ and 6 n0
 Be: 4 p+ and 3 n0
 Ca: 20 p+ and 18 n0
 Ar: 18 p+ and 21 n0
 He: 2 p+ and 1 n0
 Cu: 29 p+ and 34 n0
 Fe: 26 p+ and 34 n0
 Charge
 Nucleus = + charge
 Electrons = - charge
(opposites attract)
 Usually the + and – charges
exist in equal number
The atom is therefore
“neutral”
 Charge
 Ion = Charged Atom
 + charge = cation
 - charge = anion
 A Negative ION = ANION
 Charge
 Ion’s charge is determined by:
 p+ − e  Example:
 Nitrogen atom has 7 p+ and 10 e Charge is:
 -3
 Charge
 Find the charge for the following elements:
 Na: 11 p+ and 10 e O: 8 p+ and 10 e Al: 13 p+ and 10 e Be: 4 p+ and 2 e H: 1 p+ and 0 e Cl: 17 p+ and 18 e Ge: 32 p+ and 28 e-
 Atoms with the same number of protons
but different number of neutrons are
isotopes.
 Varied mass due to different numbers of
n0
 Isotopes
 Can’t change # of protons
 That would change the element
 Can change # of neutrons
 Compare
 Carbon sample 1: 6 p+ and 6 n0
 Carbon sample 2: 6 p+ and 7 n0
 Carbon sample 3: 6 p+ and 8 n0
 Which one is the isotope?
 Answer: They all are!
 Atomic Mass
 Weighted Average
 Based on ALL isotopes of an element
 This is why it is not a whole number
 NOT the same thing as mass number
 How do we indicate what isotope we are
talking about?
 Top Left is for mass number
 Bottom Left is for atomic number
Mass Number
Atomic Number
Cl
 What if we had a chlorine atom with 17
protons and 18 neutrons?
 Mass number = _______
 Atomic number = ____________
35
17
Cl
 What if we had a magnesium atom with
12 protons and 13 neutrons?
 Mass number = ____________
 Atomic number = _____________
25
12
Mg
 Practice: Given the following elements, write
the symbol with the mass number and
atomic number in the proper locations.
 C: 6 p+ and 6 n0
 Be: 4 p+ and 3 n0
 Ca: 20 p+ and 18 n0
 Ar: 18 p+ and 21 n0
 He: 2 p+ and 1 n0
 Cu: 29 p+ and 34 n0
 Fe: 26 p+ and 34 n0
 How else do we indicate isotope?
 Symbol
 Dash
 Mass Number
Cl
35
Cl-35
 An atom of Carbon has 6 protons and 8
neutrons.
C-14
 How else do we indicate isotope?
 Name
 Dash
 Mass Number
Chlorine
35
Chlorine-35
 An atom of Hydrogen has 1 proton and 2
neutrons.
Hydrogen-3
 On your worksheet, fill in the appropriate
boxes as demonstrated by your
instructor.
 Atomic number = # of protons
 Mass number = protons + neutrons
 Not the same thing as atomic mass!
 Charge = protons - electrons
 If the # of electrons are changed
 It becomes an ion
 If the # of protons are changed
 It becomes a different element
 If the # of neutrons are changed
 It is a different isotope