1. Section 4.3 How Atoms Differ
Explain the role of atomic number in determining the identity of an atom.
Define an isotope.
Explain why atomic masses are not whole numbers.
Calculate the number of electrons, protons, and neutrons in an atom given its mass number and atomic number.
Section 4-3

2. Section 4.3 How Atoms Differ (cont.)
periodic table: a chart that organizes all known elements into a grid of horizontal rows (periods) and vertical columns (groups or families) arranged by increasing atomic number
atomic number
isotopes
mass number
atomic mass unit (amu)
atomic mass
The number of protons and the mass number define the type of atom.
Section 4-3

3. Section 4.3 Subatomic Particles
NUCLEUS
ELECTRONS
equal in a neutral atom
PROTONS
NEUTRONS
NEGATIVE CHARGE
QUARKS
Atomic Number
equals the # of...
POSITIVE CHARGE
NEUTRAL CHARGE
Most of the atom’s mass.
Section 4.3 Subatomic Particles

4. Each element contains a unique positive charge in their nucleus.
Atomic Number
Each element contains a unique positive charge in their nucleus.
The number of protons in the nucleus of an atom identifies the element and is known as the element’s atomic number.
Section 4-3

5. Isotopes and Mass Number
All atoms of a particular element have the same number of protons and electrons but the number of neutrons in the nucleus can differ.
Atoms with the same number of protons but different numbers of neutrons are called isotopes.
Section 4-3

6. Isotopes and Mass Number (cont.)
The relative abundance of each isotope is usually constant.
Isotopes containing more neutrons have a greater mass.
Isotopes have the same chemical behavior.
The mass number is the sum of the protons and neutrons in the nucleus.
Section 4-3

7. Mass Number = protons + neutrons Atomic Number = protons only
Atomic & Mass Numbers
Mass Number = protons + neutrons
Atomic Number = protons only
# of Neutrons = mass # - atomic #
Mass Number
always a whole number
NOT on the Periodic Table!
© Addison-Wesley Publishing Company, Inc.

8. Mass # Atomic # Nuclear symbol: Hyphen notation: carbon-12
Section 4.3 Isotopes

9. Nuclide p+ n0 e- Mass # Oxygen - 8 - 33 42 - 31 15
4.3 Mass Number
Mass number is the number of protons and neutrons in the nucleus of an isotope.
Mass # = p+ + n0
Nuclide p+ n0 e-
Mass #
Oxygen - 8 - 33 42
- 31 15
Atomic # 16 8 8 16
Arsenic 75 33 75
Phosphorus 15 31 16

10. Chlorine-37 atomic #: mass #: # of protons: # of electrons:
# of neutrons: 17 37 20
Section 4.3 Isotopes

11. 12C atom = 1.992 × 10-23 g Mass of Atoms
One atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom.
One amu is nearly, but not exactly, equal to one proton and one neutron.
12C atom = × g
Section 4-3

12. Mass of Atoms (cont.)
The atomic mass of an element is the weighted average mass of the isotopes of that element.
Section 4-3

13. EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.
Avg.
Atomic
Mass
16.00
amu

14. EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.
Avg.
Atomic
Mass
35.40 amu

15. A B C D Section 4.3 Assessment
An unknown element has 19 protons, 19 electrons, and 3 isotopes with 20, 21 and 22 neutrons. What is the element’s atomic number?
A. 38
B. 40
C. 19
D. unable to determine A B C D
Section 4-3

16. A B C D Section 4.3 Assessment
Elements with the same number of protons and differing numbers of neutrons are known as what?
A. isotopes
B. radioactive
C. abundant
D. ions A B C D
Section 4-3