1. The Building Blocks of Matter
Chapter 3 - Atoms:
The Building Blocks of Matter

2. From Philosophy to Science
There were two schools of thought of the composition of the cosmos…
is everything in the universe continuous and infinitely divisible
Or, is there a limit to how small you can get?
Particle theory was not the most popular early opinion, but was supported as early as Democritus in ancient Greece.
From Philosophy to Science

3. From Philosophy to Science
Democritus proposed that all the matter is composed of tiny particles called “Atomos”
These “particles” were thought to be indivisible
Aristotle did not accept Democritus’ atom, he was of the “matter is continuous” philosophy
Because of Aristotle’s popularity his theory was adopted as the standard
From Philosophy to Science

4. From Philosophy to Science
By the 1700’s nearly all chemists had accepted the modern definition of an element as a particle that is indivisible
It was also understood at that time that elements combine to form compounds that are different in their properties than the elements that composed them
However, these understandings were based on observations not empirical evidence
From Philosophy to Science

5. From Philosophy to Science
There was controversy as to whether elements always combine in the same proportion when forming a particular compound.
In the 1790’s, chemistry was revolutionized by a new emphasis on quantitative analysis because of new and improved balances
This new technology led to the discovery of some new scientific understandings
From Philosophy to Science

6. From Philosophy to Science
The Law of Conservation of Mass:
States that mass is neither created nor destroyed during ordinary chemical reactions or physical changes.
Which means the total mass of the reactants must equal the total mass of the products.
From Philosophy to Science

7. Law of Conservation of Mass+ 
Carbon, C
Oxygen, O
Carbon Monoxide, CO
Mass x
Mass y
Mass x + Mass y
Carbon Monoxide, CO
Mass x + Mass y +
Carbon, C
Oxygen, O
Mass x
Mass y 

8. From Philosophy to Science
The Law of Definite Proportions:
The fact that a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or the source of the compound
NaCl is NaCl no matter if it is table salt (small crystals) or rock salt (large crystals)
From Philosophy to Science

9. From Philosophy to Science
The Law of Multiple Proportions:
If 2 or more different compounds are composed of the same 2 elements, then the ratio of the masses of the 2nd element combined with a certain mass of the 1st element is always a ratio of small whole numbers
From Philosophy to Science

10. Law of Multiple Proportions+ =
Carbon
Oxygen
Carbon Monoxide, 1:1 1 1 +
Carbon
Oxygen =
Carbon Dioxide, 1:2 1 2

11. In 1808, John Dalton proposed an explanation for each of the proposed laws
He reasoned that elements were composed of atoms & that only whole #’s of atoms can combine to form compounds
His ideas are now called the Atomic Theory of Matter
Atomic Theory

12. All matter is composed of extremely small particles called atoms.
Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, & other properties.
ELEMENT 1
ELEMENT 2
ELEMENT 3
ELEMENT 4
Atomic Theory

13. Atomic Theory Atoms cannot be subdivided, created, or destroyed
atoms of different elements combine in simple whole # ratios to form chem compds
in chemical rxns, atoms are combined, separated, or rearranged + +
Atomic Theory

14. Through these statements, evidence could be gathered to confirm or discount its claims
Not all of Dalton’s claims held up to the scrutiny of experimentation
Atoms CAN be divided into even smaller particles
Not every atom of an element has an identical mass
Atomic Theory

15. Atomic Theory Dalton’s Atomic Theory of Matter has been modified.
What remains unchanged is…
All matter is composed of atoms
Atoms of any one element differ in properties from atoms of another element
Atomic Theory

16. One of the disputed statements of Dalton was that atoms are indivisible.
In the 1800’s it was determined that atoms are actually composed of several basic types of smaller particles
it’s the number and arrangement of these particles that determine the atom’s chemical properties.
Atomic Theory

17. The definition of an atom that emerged was: the smallest particle of an element that retains the chemical properties of that original element.
All atoms consist of 2 regions that contain the subatomic particles
The nucleus
The electron cloud around the nucleus
Atomic Theory

18. The nucleus is a very small region located near the center of the atom
In every atom the nucleus contains at least 1 proton, which is positively charged particle and usually contains 1 or more neutral particles called neutrons
(Hydrogen does not contain a neutron.)
Atomic Structure

19. The electron cloud is the region that surrounds the nucleus
This region contains 1 or more electrons, which are negatively charged subatomic particles
The volume of the electron cloud is much larger than the nucleus
Atomic Structure

20. With the exception of Hydrogen, every nucleus contains 2 kinds of particles protons and neutrons
they make up the mass of the atom (Mass Number = Protons + Neutrons)
Proton has a charge equal to but opposite of the charge of an electron.
Atoms are neutral because they contain equal #’s of protons & electrons
Atomic Structure

21. The atoms of different elements differ in the # of protons in their nuclei and therefore in their positive charge
The # of protons the atom contains determines the atom’s identity, also known as atomic number.
Only Oxygen contains 8 protons
Only Fluorine contains 9 protons
Only Neon contains 10 protons
Structure of the Atom

22. Particle
Symbol
Charge
Mass
Number
Relative Mass (amu)
Actual Mass (kg)
Electron e- -1
9.109 x 10-31
Proton p+ +1 1
1.673 x 10-27
Neutron n0
1.675 x 10-27

23. Ch 3.3: Atomic Number

24. # protons in an atom = # electrons
Elements are identified by the number of PROTONS they contain.
The “atomic number” of an element is the number of protons in the nucleus
PROTONS IDENTIFIES AN ELEMENT!!!
# protons in an atom = # electrons
Why? Because atoms are neutral!

25. Complete Symbol X
Mass
number
Atomic
Subscript →
Superscript →

26. Cl 35 17 # OF PROTONS + # OF NEUTRONS MASS NUMBER ATOMIC NUMBER
NUMBER OF PROTONS
# OF PROTONS +
# OF NEUTRONS

27. Mass Number
Mass number is the number of protons and neutrons in the nucleus of an isotope:
Mass # = p+ + n0
Element p+ n0 e-
Mass #
Oxygen - 10 - 33 42
- 31 15
Mass number 18 8 8 18
Arsenic 75 33 75
Phosphorus 16 15 31

28. Find the # of e-, p+ and n0 for sodium. (mass # = 23)
Practice Problems
Find the # of e-, p+ and n0 for sodium.
(mass # = 23)
Find the # of e-, p+ and n0 for uranium.
(mass # = 238)
Atomic # = 11 = # e- = # p+
# neutrons = = 12
Atomic # = 92 = # e- = # p+
# neutrons = = 146

29. Check for understanding:
If an element has 91 protons and 140 neutrons find the:
Atomic number
Mass number
number of electrons
element name 91
231 91
protactinium

30. (Isotopes: The # after the name is the mass #.)
An isotope refers to atoms that have the same # of protons, but a different number of neutrons.
Because of this, they have different mass #’s.
Ex---> (1) Carbon-12 & Carbon-13
(2) Chlorine-35 & Chlorine-37
(Isotopes: The # after the name is the mass #.)

31. Cl Cl 18 20 35 37 17 17 EXAMPLE OF AN ISOTOPE ATOMIC MASS
ATOMIC NUMBER 18
NEUTRONS 20
NEUTRONS

32. Br Question #1 80 35 Find each of these: Atomic number Mass Number
number of protons
number of neutrons
number of electrons Br 35

33. Question #2
If an element has an atomic number of 34 and a mass number of 78, what is the:
number of protons
number of neutrons
number of electrons
complete symbol

34. Units = atomic mass unit (amu) 12
Units = atomic mass unit (amu)
The atomic masses listed in the Periodic Table are a “weighted average” of all the isotopes of the element.

35. In chemistry, chlorine has 2 isotopes:
Weighted Average
Practice Problems:
In chemistry, chlorine has 2 isotopes:
Cl-35 (75.8% abundance) Cl-37 (24.23 % abundance)
What is the weighted average atomic mass of chlorine?
35 x = amu
37 x = amu
Add them up!!!
This rounds to 35.5 amu +
amu

36. Relating Mass Numbers to Atoms
The Mole: the amount of a substance that contains as many particles as there are atoms in exactly 12 grams of carbon-12.
Avogadro’s Number: the number of particles in exactly one mole of a pure substance = x 1023.
Molar Mass: the mass of one mole of a pure substance. Units = g/mol

37. This is when we get to use dimensional analysis!
The conversion factors we need are:
and of course…molar mass

38. Gram to Mole Conversions
Mass of Element in Grams
Number of Moles of Element
Number of Atoms of Element

39. Practice Problem
ALWAYS USE PARANTHESES AROUND YOUR CONVERSION FACTORS!!
You have 3.50 mol of Copper. What is it mass in grams?
You have grams of Carbon. How many atoms is this?
You have 4.85 x 1030 atoms of Magnesium. How many moles is this?