1. Distinguishing among Atoms
IaN pg. 7
Key Concepts
What makes one element different from another?
How do you find the number of neutrons in an atom?
How do isotopes of an element differ?
How do you calculate the atomic mass of an element?
Why is a Periodic Table useful?
Main Ideas
Atomic Number
Mass Number
Isotopes
Atomic Mass
Periodic Table
Success is dependent upon effort Sophocles

2. Distinguishing Among Atoms

3. Elements are arranged in order of increasing atomic number
Elements are arranged in order of increasing atomic number. This is the number of protons in the atom.

4. Atoms are Neutral
This means that the number of protons (which each have a charge of positive 1) and the number of electrons (which each have a charge of negative 1) must be the same.

5. The Periodic Table gives us information about atomic structure…

6. Atomic Number
The number of protons is what distinguishes one element from another.
The periodic table is arranged in order of atomic number.

8. Sometimes you find elements represented like this…OR
Element Symbol
He - 4
Mass number
The atomic number is the number of protons… the mass number is the number of protons + neutrons.
Mass Number = total # of protons & neutrons in the nucleus.
# of Neutrons = Mass # - Atomic #

9. Number of neutrons = mass number – atomic number
Example:
How many neutrons are there in one atom of ?
Solution:
Mass number = 24
Atomic number = 12
Number of neutrons = mass number – atomic number
= 24 – 12 = 12

10. Fill in the rest of the chart…
Symbol of the Element
Atomic Number
Mass Number
Number of Electrons
Number of Protons
Number of Neutrons Ba 82 8 10 33 42 Ca 40

11. Now for some perspective…

12. How Small is an Atom?

13. Isotopes
Isotopes are elements that have the same number of protons, but different numbers of neutrons.
Isotopes and their percent abundance are used to determine the atomic mass of an element.

14. Atomic Mass Mass Number = Mass of the nucleus
Atomic Mass = Weighted average mass of the atoms in a naturally occurring sample.
Mass
Abundance of Isotopes
AMU – Atomic Mass Unit standard 1/12 the mass of Carbon – 12.

15. Calculating Atomic Mass
Required Variables: 1. # of Stable Isotopes of the element 2. Mass of each Isotope 3. Percent abundance of each Isotope (amu X %) + (amu X %) = atomic mass
For each Isotope of the element.

16. For example: Carbon-12 makes up about 98.89% of all carbon on Earth.
Carbon-14 is only about 1x10-12%
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Carbon’s mass on the periodic table is 12.01amu.
This is found by: (12 x .9889) + (13 x .0111) + (0)*
= 12.01amu
*Carbon-14’s contribution is too small to matter.
mass numbers
Percents written as decimals