Atomic Structure
proton (+)
nucleus
Neutron (o)
Electron (-)
Nucleus: at the center of
the atom
Protons:
- are positively charged
- are the same in number as the atomic
number
- equal the number of electrons in a
neutral atom
 Neutrons:
- are neutral
- number varies
(# neutrons = mass # - atomic #)

Electrons
are negatively charged
 move at high speed in “shells”
around the center of the atom
 Electrons are not confined to a
certain path, but will be found
within a “cloud”, or orbital area

Determining the numbers of
particles in each element
How many protons, electrons and
neutrons are in each of these
atoms?
Be
Ne
Na
Ag
Pb

4 +
10 +
11 +
47 +
82 +
4 -
10 -
11 -
47 -
82 -
5 o
10 o
12 o
61 o
125 o
Bell work 12-7

How many protons neutrons and
electrons does Sulfur have?
Isotopes
Atoms that have the same number
of protons, but a different number
of neutrons
 Example: Neon: 10 electrons

10 protons
10 neutrons
Neon-21:
Neon-22
10 protons
10 protons
10 electrons
10 electrons
11 neutrons
12 neutrons
Atomic mass
The atomic mass is actually the average
of the isotopes of each element
 Example:

Element X has two natural isotopes.
The isotope with a mass of 10.012 amu (10X) occurs 19.91% of the time in the
earth’s crust.
The isotope with a mass of 11.009 amu (11X) occurs 80.09% of the time in the
earth’s crust.
Calculate the atomic mass of element X.
Here’s how to solve it:
10X:
10.012 amu X 0.1991 = 1.993 amu
11X:
11.009 amu X 0.8009 = 8.817 amu
Atomic mass of element X = 10.810 amu
Try this one:
Calculate the atomic mass of bromine.
The two isotopes of bromine have atomic
masses and relative abundances of 78.92
amu (50.69%) and 80.92 amu (49.31%).
Answer: 79.90 amu
Here’s how they’re written:
Atomic mass
14
6
Atomic number
(# of protons)
So – the number of
neutrons is…
8
C
Element symbol