1.1 The Nature of Atoms
Learning Goals ...
... determine the number of protons, electrons and
neutrons in an atom or ion
... distinguish between a cation and an anion
... calculate the isotopic abundance of an isotope
Meet the Elements
The Modern Atomic Theory
If an atom loses or gains electrons it is no longer called an atom.
It is called an ion. An ion has a full outer shell of electrons. An ion
has an electric charge.
All chemical reactions involve atoms trying to get a full outer shell of
electrons. (the OCTET rule)
When an atom reacts with another atom, it will either
1. Lose electrons to form a stable positive ion
(called a CATION).
2. Gain electrons to form a stable negative ion
(called an ANION).
or 3. Share electrons to form a stable molecule.
Indicate the number of protons, neutrons, and electrons for
each atom or ion
_____ protons
14
N
_____ neutrons
7
Recall:
_____ electrons
80
35
_____ protons
Br
_____ neutrons
Atomic number = # of protons
Atomic Mass = protons + neutrons
_____ electrons electrons = protons (in a neutral atom)
40
20
2+
Ca
_____ protons
positive ion has lost electrons
_____ neutrons negative ion has gained electrons
_____ electrons
Isotopes
• Isotopes are atoms with the same number of protons and
electrons, but have a different number of neutrons.
• They are still the same element.
Ex) The most abundant Lithium atom is lithium-7. It has 4 neutrons.
There are a some Lithium atoms that may have 3 or 5 neutrons.
Li-6, Li-7 and Li-8 are considered isotopes of the element
lithium.
Indicate the number of protons, neutrons, and electrons for each
isotope.
35
17
_________ protons
Cl
_________ neutrons
_________ electrons
37
17
_________ protons
Cl
_________ neutrons
_________ electrons
ISOTOPIC ABUNDANCE
- the relative amount in which each isotope is present in an element
AVERAGE ATOMIC MASS
- the average of the masses of all the element’s isotopes
- this is the mass found on the periodic table
Calculating Average Atomic Mass
amu = atomic mass units
= 1.660538921(73)×10−31 g
Ex) Naturally occurring silver has two isotopes. Calculate the average
atomic mass of silver if one isotope has a mass of 106.9 amu (u)
and a relative abundance of 51.8% and the other has a mass of
108.9 amu and a relative abundance of 48.2%.
Average atomic mass = (106.9u)(0.518) + (108.9u)(0.482)
= 107.87u
Calculating Isotopic Abundance
Ex) Boron exists as two naturally occurring isotopes: Boron -10
(10.01u) and Boron-11 (11.01u). Calculate the relative
abundance of each isotope.
1- x
Let x represent the abundance of boron-10 and let y
represent the abundance of boron-11.
Note: you know your %’s need to add up to 100%.
As a decimal they need to add up to 1)
x+y = 1
y = 1- x
(1-x) = 10.81
(10.01)(x) + (11.01) (y)
10.01x + 11.01- 11.01x = 10.81
10.01x -11.01x = 10.81 - 11.01
-x = - 0.20
x = 0.20
We can get the average
atomic mass from the
periodic table M = 10.81u
20% boron-10
80% boron-11
Can I ...
... determine the number of protons, electrons and neutrons in an
atom or ion
... distinguish between a cation and an anion
... calculate the isotopic abundance of an isotope
HOMEWORK
WS "Atomic Structure"
WS "Calculating Relative Abundance of Isotopes"