Chem 20 – Unit 2
Notes Section 1:
- Parts of the atom
- Ions
Atoms:
Particle
Symbol
Location
Electric
Charge
Proton
Neutron
Electrons
p+
no
e-
Nucleus
Nucleus
Orbitals
(clouds)
+
Neutral
-
-
Relative Mass
(atomic mass
unit: amu)
1
1
1/1837
Actual Mass
1.67 x 10-24g
1.67 x 10-24g
9.11 x 10-28g
The diameter of a typical atom is approximately 10-8cm
The diameter of a typical nucleus is approximately 10-12cm
o Therefore almost all of the space in an atom is empty
Terminology
a. All the elements have a name and a symbol
Ex: H – Hydrogen
He – Helium
b. Atomic Number
- the atomic number is defined by the number of protons in the nucleus of the atom
c. Mass Number
- the mass number is defined by the number of protons and neutrons in the nucleus
of the atom
How can we write this easily?
a
zX
X: symbol of the element
A: Mass #
Z: Atomic Number
Ex:
1
4
7
11
12
1H 2He 3Li 5B 6
C
19
23
24
9F
11Na
12Mg
To draw the characteristical atom for a certain element you need to know the atomic
number and the mass number of the element (which are found on the Periodic Table of
Elements)
Hand out Periodic Tables
In a neutral atom:
Atomic Number = #of Protons = # of electrons
Mass Number = # of protons + # of neutrons = Approximate Atomic Mass
# Neutrons = Mass Number – Atomic Number
Example:
How man protons, electrons and neutrons are in the following atoms?
Atom
Beryllium (Be)
Neon (Ne)
Sodium (Na)
Atomic Number
4
10
11
Mass Number
9
20
23
Solution:
a. From an atom, the number of electrons equals the number of protons and
is given by the atomic number. Be has 4 protons and 4 electrons. We get
the number of neutrons by subtracting the atomic number from the mass
number. Be has 9-4 = 5 neutrons
b. Ne 10 p+, 10 e-, and 20-10 = 10 n0
c. Na 11 p+, 11 e-, and 23-11 = 12 n0
Ions:
-
Atoms or molecules that have lost or gained electrons
Note: an electron has a negative charge
The charge on an ion must be symbolized i.e. Cu+2
o Ions are positive or negative atoms
o Positive Ions are called Cations, occurs when a neutral atom looses one or
more of its’ electrons
 H  H+ + eo Negative Ions are called Anions, they occur when a neutral atom gains one
or more electron
 S + 2e-  S-2
- An ion is produced by a chemical reaction and so it has totally different
characteristics than the atom
- Ex. 63Cu, Cu+2, Cu+1, 31P, P-3