
The basic unit of an element that can retain the
properties of that element in a chemical
reaction
1.
Protons (P+)

2.
Neutrons (N0)
3.
Electrons (e - )
Aka:

Subatomic Particles
Subatomic
Particle
Symbol
Location
Relative
Mass
Charge
Proton
P+
Nucleus
1 amu
+1
Neutron
N0
Nucleus
1 amu
0
Electron
e-
Electron
cloud
0
-1



Changing the # of protons changes the identity
of the element
Changing the # of neutrons change the mass of
the atom
Changing the number of electrons changes the
charge of the atom
Since like charges repel, how does the
nucleus stay together?
Strong forces in the nucleus

Atomic Number

Tells us how many protons are in the nucleus of each
atom of an element

Represented by the letter Z


What is the atomic
number of carbon?
How many protons
are in the nucleus of a
carbon atom?

What is the atomic
number of carbon?


6
How many protons
are in the nucleus of a
carbon atom?

6

Mass Number

Tells us the number of protons and neutrons in the
nucleus of each atom of an element

Represented by the letter A

Shorthand Notation:

Element name – mass number
 Ex: carbon–12, carbon-14, chlorine-35
 12, 14, 35 = mass # (protons + neutrons)

Shorthand Notation:
 AZX notation:
 X =Element Symbol
 A =Mass Number
 Z =Atomic Number
 Ex: Chlorine-35
 Cl always has 17 p+


What is the mass number of carbon-12?
How many neutrons are in the nucleus of a
carbon-12 atom?

What is the mass number of carbon-12?


12
How many neutrons are in the nucleus of a
carbon-12 atom? 12 - 6 = 6
Mass #



# P+
Mass number = # of protons + # of neutrons
Mass # = Atomic # + # of neutrons
Number of neutrons = Mass # - Atomic #

In a neutral atom, the number of electrons
equal the number of protons

Neutral:
 0 charge (so # of negative charges (e-) = # of positive
charges (P+)


Isotopes are atoms of the same element that
have a different number of neutrons.
Changing the number of neutrons will change
the mass number

Ex: Carbon-11
11
6C
Carbon-12
12
6C
Carbon-13 Carbon-14
13
6C
14

Ex:
6C

*Notice that all carbon isotopes have an atomic number (#of
protons) of 6. The number of neutrons change so the mass
will change*

Single atom extremely small


Ex: Fluorine atom mass = 3.155x10-23
More convenient to compare relative masses of
atoms using a reference isotope as a standard
 Chosen reference isotope is carbon-12
Carbon-12 has atomic mass of exactly 12 atomic
mass units
 What is the atomic mass of carbon?
 What is the atomic mass of chlorine?


Single atom extremely small


Ex: Fluorine atom mass = 3.155*10-23
More convenient to compare relative masses of
atoms using a reference isotope as a standard
 Chosen reference isotope is carbon-12




Carbon-12 has atomic mass of exactly 12 atomic
mass units
1 atomic mass unit (amu) = 1/12 mass of carbon-12
What is the atomic mass of carbon? 12.011
What is the atomic mass of chlorine? 35.45

Bottom Line: The atomic mass is a weighted
average of the masses of an element’s isotope

Use a weighted average

Formula:
(mass 1)(%abundance 1)+(mass2)(%abundance 2) +…
**% abundances should be in decimal form**
(take your percentage & divide by 100)

Formula:
(mass 1)(%abundance 1)+(mass2)(%abundance 2) +…
**% abundances should be in decimal form**

(take your percentage & divide by 100)
Ex: Boron forms 2 stable isotopes, boron-10 and boron-11. The
relative abundance of the isotope are 19.8% and 80.2%
respectively. What is the atomic mass?

Formula:
(mass 1)(%abundance 1)+(mass2)(%abundance 2) +…
**% abundances should be in decimal form**

(take your percentage & divide by 100)
Ex: Boron forms 2 stable isotopes, boron-10 and boron-11. The
relative abundance of the isotope are 19.8% and 80.2%
respectively. What is the atomic mass?
(10amu)(.198) + (11amu)(.802) = 10.80 amu
Check yourself: Atomic # should match periodic table



An ion is an atom or group of atoms that has a
charge
A neutral atom has equal numbers of protons
and electrons
Ions form as atoms gain or lose electrons

Don’t mess with the protons!!

Examples
7
1+
3Li
16
8
O2-
24
2+
Mg
12
Protons-
12
Neutrons-
12
Electrons-
10
*If the ion is positive, subtract e*If the ion is negative, add e-

Examples
7
1+
3Li
16
8
O2-
24
2+
Mg
12
Protons-
3
8
12
Neutrons-
7-3=4
16-8=8
12
Electrons-
2
10
10
*If the ion is positive, subtract e*If the ion is negative, add e-