1. Physical Science Chapter 4 – Section 2
Structure of Atoms
Physical Science
Chapter 4 – Section 2

2. State Standards
CLE – Describe the structure and arrangement of atomic particles

3. What is an Atom?
The three main components of an atom are distinquished by mass, charge, and location
The chemistry of each element depends directly on these three components
Nucleus:
Protons ( positively charged )
Neutrons ( neutral – NO CHARGE )
Electrons ( negatively charged )

4. Electric Forces in Atoms
Protons and electrons attract each other
Electric force holds components together; the same forces hold solids and liquids together
Liquid water is held together by electric forces

5. Atomic Components Protons ( +1 charge / 1.67 x 10-27 kg )
Neutrons ( 0 charge / 1.67 x kg )
Electrons ( -1 charge / 9.11 x kg )
All ( most ) elements contain all three
Atoms of the same element contain the same number of protons, but neutrons may vary
Each element has a unique number of protons

6. Distinguishing Elements and Atoms
Atomic number – number of protons in an atom
Mass number – number of protons & neutrons
Isotopes – exist for an element if different atoms ( with the same atomic number ) have varying numbers of protons ( different mass numbers )

7. Hydrogen & Helium
Hydrogen ( H ) contains 1 electron, 1 proton, and 0 neutrons [ Protium isotope ]
Helium ( He ) contains 2 electrons, 2 protons, and 2 neutrons
What is atomic number, mass number of H, He?

8. Isotopes of Hydrogen Protium – Most common form on Earth and Sun
1 electron, 1 proton, 0 neutrons
Deuterium – small fraction of hydrogen; 1 out of every 6,000 hydrogen atoms in Earth’s crust
1 electron, 1 proton, 1 neutron
Trillium – very unstable and so very rare
1 electron, 1 proton, 2 neutrons

9. Isotopes of Chlorine
The common form of chlorine ( Cl ) has 17 protons, 17 electrons, and 18 neutrons
Another form has 20 neutrons
Neutrons can be calculated by using these
How do we know which form is most common?

10. Atomic Mass Units Used to express the mass of such tiny particles
Unified Atomic Mass Unit is 1/12 the mass of a Carbon-12
Carbon-12 isotope has a fairly even mass number
6 protons and 6 neutrons
Remember electrons contribute little mass to atom
Each proton/neutron has mass of 1.0 u
Units given as a u

11. Atomic Mass Units
The value of AVERAGE ATOMIC MASS found in the periodic table tells us which isotope of any element is most common ( HYDROGEN )
weighted average so most common form counts most
Zinc with average atomic mass 65.4 u
65 ( closest mass # ) – 30 ( atomic # ) = 35 neutrons
Most common Zn isotope has 35 neutrons
Cl has average of u , with 35 mass number
Most common Cl form contains 18 neutrons

12. The Mole
In order to count large numbers of small particles, a COUNTING UNIT was devised – THE MOLE ( mol )
Examples ( Avogadro’s number ):
1 mol = 602,213,670,000,000,000,000,000 particles
Equals x 1023 when put into scientific notation
1 mol of marbles = x 1023 marbles
1 mol of atoms = x 1023 atoms ( any atom )
1 mol of He atoms = x 1023 He atoms

13. The Mole How many stars in the universe?
2003, roughly 7 x 1022 stars counted ( within range
Value could increase in future
6.022 x 1023 popcorn kernels would cover the USA in a pile 500 km ( or 310 mi ) tall!!
Avogadro’s number is used only for small particles

14. Relating Moles and Grams
Mass of 1 mol random atoms difficult to find
Each element has a unique MOLAR MASS
The mass of 1 mol of this type of atom ( element )
Usually accounts for several isotopes
Mass of 1 mol of the same atom easy to find
For Carbon-12, molar mass is g
Molar mass ( in g ) equals average atomic mass ( in u )
Carbon-12 has u as well as g/mol
Figure 7 in text

15. Relating Moles and Grams
Mass of 1 mol Carbon-12 atoms is g
USE PERIODIC TABLE
24 g Carbon-12 = 2 mol Carbon-12
3 mol Carbon-12 = 36 g Carbon-12

16. Molar Mass of Compounds, Molecules
As elements ( atoms with same atomic number ) have a MOLAR MASS, so do coumpounds
H2O : 2 Hydrogen with 1.00 g/mol & 1 Oxygen with 16 g/mol
Gives ( ) g/mol = 18 g/mol
CO2 : 1 Carbon with g/mol & 2 Oxygens with 16 g/mol
Gives ( ) g/mol = 44 g/mol
O2 : 2 Oxygen atoms at 16 g each
Gives: 32 g/mol for O2 molecules H O