1. Atomic Structure

2. What Is an Atom?
Atom: the smallest unit of matter that retains the identity of the substance.

3. Atomic Structure Atoms have two regions:
Nucleus: the center of the atom that contains protons and neutrons which is most of the mass of the atom.
Electron Cloud: surrounds the nucleus and takes up most of the space of the atom.

4. Inside the Atom’s Nucleus
Protons: positively charged subatomic particles.
Mass of 1 amu
Neutrons: neutrally charged subatomic particles.
Mass of 1 amu

5. Inside the Electron Cloud
Inside the electron cloud are electrons which are subatomic particles with a negative charge and relatively no mass  Mass of ~ 1/1836 amu

6. Subatomic Particles

7. Protons
Atomic Number = Number of Protons in an Atom Ex: Hydrogen’s atomic number = 1 Hydrogen has one proton Ex: Carbon’s atomic number = 6 Carbon has six protons The number of protons identifies the atom. It is the atom’s fingerprint.

8. Learning Check
State the number of protons for atoms of each of the following:
A. Nitrogen
1) 5 protons 2) 7 protons 3) 14 protons
B. Sulfur
1) 32 protons 2) 16 protons 3) 6 protons
C. Barium
1) 137 protons 2) 81 protons 3) 56 protons

9. Solutions
State the number of protons for atoms of each of the following:
A. Nitrogen
2) 7 protons
B. Sulfur
2) 16 protons
C. Barium
3) 56 protons

10. Neutrons Atomic Mass = Number of Protons and Neutrons in an Atom
# of Neutrons = Atomic Mass - Atomic Number
Ex: Lithium’s atomic mass = 7
Lithium’s atomic number = 3
# Neutrons = 7 – 3 = 4
Lithium has 4 neutrons

11. Electrons Most atoms are neutral--having no overall charge.
Because the only charged subatomic particles are the protons and electrons… they must balance each other out in an electrically neutral atom.
Therefore..
# p+ = # e-
…in a neutral atom.

12. Calculating p+, n, and e-
Beryllium Protons = Neutrons = Electrons = Magnesium

13. Calculating p+, n, and e-
Beryllium Protons = 4 Neutrons = (9 – 4) = 5 Electrons = 5 Magnesium Protons = 12 Neutrons = (24 – 12) = 12 Electrons = 12

14. Subatomic Particle Arrangement
All of the protons and the neutrons
Orbit 3: holds up to 8 e-
Orbit 1: holds up to 2 e-
Orbit 2: holds up to 8 e-

15. Carbon’s Arrangement Atomic Number = 6 Atomic Mass = 12 e- = 6
6 p and 6 n live in the nucleus

16. Atomic Mass
Atomic mass is the weighted average mass of all the atomic masses of the isotopes of that atom.
Atomic Mass Unit = amu
Your remember this.
1 amu the mass of 1 p+ or 1 n
You do NOT have to remember this.
Atomic Mass Unit (amu) is defined as one-twelfth the mass of a carbon-12 atom.
Atomic Number 11 Na
22.99
Symbol
Atomic
Mass

17. Why “Average” Atomic Mass?
Q: How heavy is an oxygen atom? A: It depends because there are different oxygen isotopes. We are more concerned with the average atomic mass. This is determined based on the abundance of each isotope. The standard unit for mass = grams. But, for atomic mass amu (atomic mass unit) is used. It is too small to use grams.

18. Calculating Average Atomic Mass
Cl-35 is about 75.5% and Cl-37 about 24.5% of natural chlorine.
35 x =
100
35.50
37 x =

19. Calculating Average Atomic Mass for Carbon
Isotope
Symbol
Composition of the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
7 neutrons
1.11%
Carbon-14
14C
8 neutrons
<0.01%
Atomic mass is the average of all the naturally occurring isotopes of that element.
Carbon =

20. Dalton’s Atomic Theory
Dalton’s first postulate was wrong. Atoms of the same element can be different.
They can have different # of neutrons. Thus, different mass numbers.
These are called isotopes.

21. Isotopes Isotopes of Chlorine 17 17 chlorine-35 chlorine-37
Atoms with the same number of protons, but different numbers of neutrons.
Atoms of the same element (same atomic number) with different atomic masses (aka mass numbers).
Isotopes of Chlorine
35Cl 37Cl
chlorine chlorine-37

22. Isotopes
Elements occur in nature as mixtures of isotopes.

23. Hydrogen Isotopes Hydrogen–1 (protium) 1 Hydrogen-2 (deuterium)
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) 2

24. LecturePLUS Timberlake
Learning Check
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.
12C C 14C
#p _______ _______ _______
#n _______ _______ _______
#e _______ _______ _______
LecturePLUS Timberlake

25. LecturePLUS Timberlake
Solutions
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.
12C C 14C
#p ___6___ ___6___ ___6___
#n ___6___ ___7___ ___8___
#e ___6___ ___6___ ___6___
LecturePLUS Timberlake

26. LecturePLUS Timberlake
Learning Check
An atom of zinc has an atomic mass of 65 amu. Find the atomic number then answer the questions.
A. Number of protons in the zinc atom
1) ) ) 65
B. Number of neutrons in the zinc atom
C. What is the mass number of a zinc isotope
with 37 neutrons?
1) ) ) 67
LecturePLUS Timberlake

27. LecturePLUS Timberlake
Solutions
An atom of zinc has an atomic mass of 65 amu. Find the atomic number then answer the questions.
A. Number of protons in the zinc atom
1) 30
B. Number of neutrons in the zinc atom
2) 35
C. What is the mass number of a zinc isotope
with 37 neutrons?
3) 67
LecturePLUS Timberlake

28. Learning Check Write the atomic symbols for atoms with the following:
A. 8 p+, 8 n, 8 e- ___________
B. 17 p+, 20 n, 17 e- ___________
C. 47 p+, 60 n, 47 e- ___________

29. Solutions Write the atomic symbols for atoms with the following:
A. 8 p+, 8 n, 8 e- ___________
B. 17 p+, 20 n, 17 e- ___________
C. 47 p+, 60 n, 47 e- ___________
16O
8 p+, 8 n, 8 e- 8
17p+, 20n, 17e- 37Cl 17
47p+, 60 n, 47 e-107Ag 47

30. Isotope Nomenclature We name the isotope based on its mass number.
Carbon-12 C C
Carbon-14 C C
Uranium-235 U U