Chemistry 11 – Unit 5 Forero
Name: ______________________________ Date: ___________ Blk: _____
NOTES: ATOMIC STRUCTURE
I. History of the Atom
Dalton
1803
Thomson
1897
Rutherford
1909
Bohr
1913
1. DALTON
2. THOMSON
- everything is made of atoms
- discovery of subatomic particle (electron)
- different elements combine to form
compounds in simple whole ratios
Cathode ray experiment
- rays are particles, not waves
- each element has its own unique type of
atom with a characteristic weight
- new particles are negative, lighter than
smallest atom (hydrogen)
- small indivisible solid particle model
- plum pudding model
3. RUTHERFORD
Thin gold foil experiment
- alpha particles mostly pass through foil
- but sometimes deflected back when they hit something massive
- thus, atom made mostly of empty space
- core of positively charged material, where most of the mass of the atom is (nucleus)
- why don’t negative electrons give in to the pull of the positive nucleus and collapse
inwards?
Chemistry 11 – Unit 5 Forero
4. BOHR
- negatively charged electrons found in concentric circular orbits around the positive
charged nucleus
- electrons found at fixed energy levels orbiting at fixed distances from the nucleus
- path closest to nucleus = lowest energy level
- energy higher the farther the orbits are from the nucleus
- the farther the electron is from the nucleus, the less attraction it feels
- electrons can jump from one energy level to another, but are not found between levels
- they lose or gain a discrete package of energy (quantum of energy) every time it jumps
levels  quantum leap
- explains why each element has a certain number of electrons available for reactions
- the electrons found on the outer most orbit  valence electrons
- planetary model
II. Atomic Number and Atomic Mass (review!)
Particles which make up the atom are called subatomic particles.
1. Protons: positively charged particles in the nucleus
2. Electrons: small negative particles outside the nucleus
3. Neutrons: neutral particles in the nucleus
PARTICLE
electron
The Properties of the Atomic Particles
SYMBOL
CHARGE
MOLAR MASS
e
-1
0.000549g (=0g)
LOCATION
outside nucleus
proton
p
+1
1.007825g (=1g)
inside nucleus
neutron
n
0
1.008665g (=1g)
inside nucleus
The chemical elements are different from one another by the number of protons in the
nucleus.
Chemistry 11 – Unit 5 Forero
ATOMIC NUMBER = the # of protons in the nucleus
- found on top of element symbol
A NEUTRAL atom has NO OVERALL CHARGE:
NUMBER OF ELECTRONS = NUMBER OF PROTONS
EXAMPLE:
1. Any atom containing 3 protons must be…? Lithium
2. A neutral sodium atom has 11 electrons
If electrons are added to or subtracted from a neutral atom, the resulting particle is called:
ION  # of protons ≠ # of electrons
Electrons have a NEGATIVE charge so:
- SUBTRACTING a NEGATIVE charge produces a POSITIVE ion
- Cation = # of e- < # of p’s
- ADDING a NEGATIVE charge produces a NEGATIVE ion
- Anion = # of e- > # of p’s
Example:
1. If an electron is added to a neutral F atom, then the ion is written as:
2. If two electrons are removed from a neutral Ba atom, then the ion is written as:
Chemistry 11 – Unit 5 Forero
Since both neutrons and protons have a molar mass of approximately 1 g, then:
- Total mass of an atom = total number of protons and neutrons
*the electrons are too light to make an appreciable contribution to the mass of an atom
ATOMIC MASS = the total # of protons and neutrons, thus:
NUMBER OF NEUTRONS = ATOMIC MASS – ATOMIC NUMBER
Example: Find the number of protons, neutron and electrons in the following atoms:
a) Al
b) C
But, then why is it that the atomic mass of some elements is not a whole number?
*See Titanium above…
III. Natural Mixtures: Isotopes and Mass Number
Example: ISOTOPES OF HYDROGEN
What’s the difference between Hydrogen-1, Hydrogen-2 and Hydrogen-3?
The number of neutrons
ISOTOPES: different forms of the same element, with the SAME # of protons but with
DIFFERENT #’s of neutrons, or…
- atomic species having the same atomic number, but different atomic
masses
Chemistry 11 – Unit 5 Forero
MASS NUMBER = the total # of protons and neutrons…
IN AN ISOTOPE OF AN ELEMENT
Hydrogen-3
To find the p’s and n’s from nuclear notation:
Calcium-41
Find the # of protons and # of neutrons in each of the following isotopes:
Chemistry 11 – Unit 5 Forero
Now try the other way!
Now try it with ions!
1)
56Fe3+
2)
76As3-
3)
201Au+
4)
82Br -
Chemistry 11 – Unit 5 Forero
EXTRA PRACTICE:
Write the nuclear notation:
1) An isotope has 46 protons, 58 neutrons and 42 electrons.
2) An isotope has 52 protons, 79 neutrons and 54 electrons.
NATURAL MIXTURES
- The molar mass of chlorine is 35.5 g. Since there can’t be 0.5 of a proton or neutron, then
this atomic mass MUST represent an AVERAGE value of a MIXTURE of isotopes.
Example:
Find the average atomic mass of:
1) Cl, given that it is composed of 75.77% Cl-35 and 24.23% Cl-37.
2) B, given that it is composed of 18.8% B-10 and 81.2% B-11.
BONUS:
3) Naturally occurring silicon consists of 92.23% Si-28 (mass = 27.976927 g), 4.76%
Si-29 (mass = 28.976495 g) and 3.10% Si-30 (mass = 29.973770 g). What is the
expected average molar mass (atomic mass) of a sample of natural silicon,
expressed to 4 decimal places?
(Ans: 28.0855 g)