1. Ch 9 Understanding the Atom

2. 9.1 Discovering Parts of the Atom
Democritus & Aristotle
Can you cut a piece of paper in half and cut those halves in half and continue until you are left with 1 particle?
If you could do this you would end up with what Democritus called an atom= not able to be divided
Aristotle disagreed with democritus’s idea. He believed that You would never end up with a particle that could not be cut.
Who was correct? Aristotle or Democritus??

3. 9.1 Discovering Parts of the Atom
Democritus was correct!
Matter is made of particles which we call atoms.
Atom - the smallest unit of an element that maintains the properties of that element

4. 9.1 Discovering Parts of the Atom
Dalton – 1803
Figured out that compounds are made of elements that are made of single atoms
Dalton’s atomic theory was based on Democritus's ideas!
All substances are made of atoms
Atoms are small particles that cannot be created, divided, or destroyed
Atoms of the same element are exactly alike and atoms of different elements are different.
Atoms join with other atoms to make new substances.

5. 9.1 Discovering Parts of the Atom
New information didn’t quite fit with Dalton’s ideas. His atomic theory had to be changed.
Thomson- 1897
Discovered the negatively charged particles (electrons)
There are small particles inside the atom. Atoms can be broken down further. (Dalton thought atoms were solid spheres – so he’s wrong)

6. 9.1 Discovering Parts of the Atom
Cathode Ray Tube
A positively charged plate was attracted to a source of electric current.
Therefore, the beam must be made up of negative charges.
Thomson concluded these negative charges are present in every kind of atom.

7. 9.1 Discovering Parts of the Atom
Electron- A subatomic particle that has a negative charge.
Thomson described his model of the atom like plum pudding. Today you might call it the chocolate chip cookie model. (with electrons represented by chocolate chips)

8. 9.1 Discovering Parts of the Atom
Rutherford- 1909
Rutherford was a student of Thomson’s. He tried to help prove Thomson theory was correct and ended up proving him wrong!
He aimed a beam of small, positively charged particles (α particles) at a thin piece of gold foil.
Photo paper behind the gold recorded where the charged particles hit.
Surprising Results
He thought the particles would pass right through the gold in a straight line.
Some particles were deflected. Some shot right straight back

9. 9.1 Discovering Parts of the Atom
Rutherford’s Gold Foil Experiment determined an atom must be mostly empty space with a tiny part made of highly dense material.

10. 9.1 Discovering Parts of the Atom
Rutherford’s model of the atom
The nucleus is a small area in the center of an atom where most of an atom’s mass and positive charge is concentrated
The nucleus is positive because of protons, atomic particles that have one positive charge (1+)
Negatively charged electrons move in the empty space surrounding the nucleus

11. 9.1 Discovering Parts of the Atom
Bohr – 1913
Niels Bohr proposed that electrons move around the nucleus in certain paths or energy levels
Electrons can jump from path to path but cannot exist in between paths.
Think of rungs on a ladder. You can stand on the rungs, but not between the rungs.
Or think of planets in our solar system –
they do not stray off the set path or
go in-between paths.

12. 9.1 Discovering Parts of the Atom
Discovering Neutrons
Rutherford’s colleague, James Chadwick discovered in addition to protons, the nucleus also contained neutrons
Neutrons are neutral particles that exist in the nucleus of an atom

13. 9.1 Discovering Parts of the Atom
Modern Atomic Model
Schrodinger & Heisenberg and others
Electrons do not move in paths that are very clear
We cannot predict where electrons are exactly
We’re likely to find them in an electron cloud, which is an area around the nucleus where an electrons is most likely to be located

14. Journal
Why was Thomson’s model of the atom incorrect? Which scientists changed the model and describe how they changed it.
Write neatly
Your journal should take up at least 8 lines

15. 9.2 Protons, Neutrons, and Electrons – How Atoms Differ
How small is an atom?
A penny contains 2 x 1022 atoms. (20,000,000,000,000,000,000,000 atoms)

16. 9.2 Protons, Neutrons, and Electrons – How Atoms Differ
Parts of the atom
Symbol
Size
Charge
Location
Electron e-
0 amu -
Outside nucleus
Proton p+
1 amu +
Nucleus
Neutron n0
No charge
PROTON
+ charge
In nucleus
NEUTRON
No charge
ELECTRON
- charge
Electron cloudnucleus

17. 9.2 Protons, Neutrons, and Electrons – How Atoms Differ
How do atoms of different elements differ?
Atoms of different elements have different numbers of protons.
No two elements have the same number of protons (atomic #).
How do we figure out the number of protons for an atom?
Atomic Number-number of protons in an atom of an element (= # p+)
Atom’s identity
EX: Carbon has 6 protons; its atomic number is 6

18. 9.2 Protons, Neutrons, and Electrons – How Atoms Differ
How do we figure out the number of neutrons for an atom?
Mass Number- sum of the number of protons and neutrons in an atom (= p+ + n0). The mass of electrons is too small, so they are not included in the mass.
EX: Oxygen
The mass on the periodic table is a decimal (15.999), so we have to round it.
Mass # = 16
Atomic # = 8
Number of Neutrons = Mass # - Atomic #
16-8 = Oxygen has 8 neutrons

19. 9.2 Protons, Neutrons, and Electrons – How Atoms Differ
How do we figure out the number of electrons for an atom?
Atoms are overall neutral (+ = -)
# p+ = # e-
EX: Oxygen has 8 electrons to balance the 8 protons
Ions
Ions are when atoms are no longer neutral because they have gained or lost electrons
Lose e-? Become positive (NaNa+) CATION
Gain e-? Become negative (ClCl-) ANION
EX: Oxygen atom and Oxygen ion
O 8p+ 8n0 8e-
O-2 8p+ 8n0 10e-

20. 9.2 Protons, Neutrons, and Electrons – How Atoms Differ
Isotopes
Atoms of the same element HAVE to have same # p+, but not # n0
Isotopes atoms of the same element with different numbers of neutrons
EX: Oxygen-16 and Oxygen-18
O16 8p+ 8n0 8e-
O18 8p+ 10n0 8e
Some isotopes are radioactive, which means they spontaneously emit radiation in a process called nuclear decay

21. HOW DO ATOMS OF DIFFERENT ELEMENTS DIFFER?
Mass #
Atomic #
Protons
Neutrons
Electrons Li Be B O F Na k

22. 9.2 Protons, Neutrons, and Electrons – How Atoms Differ
The atomic number of Calcium is 20, and the mass number is 40
There are _______ protons
There are _______ electrons
There are _______ neutrons
The mass number of Calcium is 41
What changes? Proton Neutron Electron
We call atoms of the same element with different mass number ____________________

23. 9.2 Protons, Neutrons, and Electrons – How Atoms Differ
The atomic number of Sodium is 11, and the mass number is rounded to 23
There are _______ protons
There are _______ electrons
There are _______ neutrons
This sodium has 10 electrons
What changes? Proton Neutron Electron
We call atoms of the same element with electron numbers ____________________

24. 9.2 Protons, Neutrons, and Electrons – How Atoms Differ
Atom 1 has 7 protons and 7 neutrons
Atom 2 has 8 protons and 8 neutrons
How many electrons are in Atom 1? ___________
What is the mass number of Atom 1? __________
How many electrons are in Atom 2? ___________
What is the mass number of Atom 2? __________
Are these different atoms , ions or isotopes? ____________

25. 9.2 Protons, Neutrons, and Electrons – How Atoms Differ
Calculating the mass of an element
The atomic mass is not really a whole number
Hydrogen has an atomic mass of
Where does this number come from?
The average atomic mass of an element is the average mass of the element’s isotopes, weighted according to the abundance of each isotope
Takes into account that not all isotopes are found equally on earth

26. 9.2 Protons, Neutrons, and Electrons – How Atoms Differ
Isotopes- Atoms that have the same number of protons, but have different numbers of neutrons.
Examples:
Hydrogen -1 and Hydrogen -2
Naming Isotopes
Write the name of the element, followed by a hyphened and the mass number of the isotope.
Ex. A hydrogen atom with 1 proton and 0 neutrons
A hydrogen atom with 1 proton and 1 neutron

27. 9.2 Protons, Neutrons, and Electrons – How Atoms Differ
Example Isotope calculation
Skittles consist of Orange – 1 which is 25% of the Skittles and Red – 2 which is 75% of skittles.
Step1: change percents to decimals.
Step 2: multiply the decimal by the mass
Step 3: add the two answers together

28. Solving for Atomic Mass
Ex: Calculate the average atomic mass for Chlorine which is made of Chlorine-35 which is 76% of the Chlorine atoms and Chlorine- 37 which is 24% of Chlorine atoms.
Step1: change percents to decimals.
Step 2: multiply the decimal by the mass
Step 3: add the two answers together

29. Solving for atomic mass
Calculate the atomic mass of silicon, which occurs naturally as 92% silicon-28, 5% silicon-29, and the remaining ___ is silicon-30

30. DRAWING BOHR MODELS OF ATOMS
Draw the nucleus and label with #p & #n
Draw electron orbitals
1st orbital can have 2 electrons only
2nd and 3rd ring can each have 8 electrons
Fill – the lowest energy level first in a North North, East South West, East South West pattern
Inner rings must be filled first before any electron enters a higher ring!!!!!
Examples:
We draw the electrons as dots
H He

31. DRAWING BOHR MODELS OF ATOMS
Li Be
Na O