1. Chapter 3
Section 3 Counting Atoms
Isotopes and Ions

2. Chapter 3 Atomic Number, review….
Section 3 Counting Atoms
Atomic Number, review….
Atoms of different elements have different numbers of protons.
Atoms of the same element all have the same number of protons.
The atomic number (Z) of an element is the number of protons of each atom of that element.

3. Chapter 3
Visual Concepts
Atomic Number

4. Chapter 3
Section 3 Counting Atoms
Isotopes
Isotopes are atoms of the same element that have different masses.
The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons.
Most of the elements consist of mixtures of isotopes.

5. Chapter 3
Section 3 Counting Atoms
Mass Number
The mass number is the total number of protons and neutrons that make up the nucleus of an isotope.
Round the mass from the periodic table to the nearest whole number.
The periodic table lists a mass that is an average of ALL isotopes that occur.

6. Chapter 3
Visual Concepts
Mass Number

7. Chapter 3 Designating Isotopes
Section 3 Counting Atoms
Designating Isotopes
Hyphen notation: The mass number is written with a hyphen after the name of the element.
uranium-235
Nuclear symbol: The superscript indicates the mass number and the subscript indicates the atomic number.
U-235 is an isotope

8. Chapter 3 Designating Isotopes
Section 3 Counting Atoms
Designating Isotopes
The number of neutrons is found by subtracting the atomic number from the mass number.
mass number − atomic number = number of neutrons
235 (protons + neutrons) − 92 protons = 143 neutrons
Nuclide is a general term for a specific isotope of an element.

9. Chapter 3 Designating Isotopes Sample Problem
Section 3 Counting Atoms
Designating Isotopes
Sample Problem
How many protons, electrons, and neutrons are there in an atom of chlorine-37?
Cl – has ? P+…..
Cl – 37 means P+ + n….
P+ = 17
37 – 17 = 20 neutrons
p+ = e-
E- = 17

10. S -2 or S 2- Al +3 or Al 3+ Chapter 3 Ions
Section 3 Counting Atoms
Ions
The atomic number tells you the element and the number of protons
When you gain or lose electrons, you form an ion.
Ions are designated with a superscript charge.
S -2 or S 2-
Al +3 or Al 3+
The -2 indicates that there are 2 MORE electrons than protons.
The +3 indicates that there are 3 LESS electrons than protons.

11. Ions – atoms with a charge
Chapter 3
Section 3 Counting Atoms
Ions – atoms with a charge
Cation
Positive charge
Loses electrons
Anion
Negative charge
Gains electrons
Gained or lost electrons form IONS. Electrons have negative charges.
Fe+2 
N-3 
was an atom that lost 2 electrons.
was an atom that gained 3 electrons.