Dec 22, 2015
HIROSHIMA 1945
August 6th, 1945
ATOMIC THEORY
Part 1: Atomic Structure
The Origin of the Atom
Changes in the Model of the Atom
1. DALTON
- everything is made of atoms - different elements combine to form compounds in simple whole ratios - each element has its own unique type of atom with a characteristic weight - small indivisible solid particle model
2. THOMSON
- discovery of subatomic particle (electron) Cathode ray experiment - rays are particles, not waves - new particles are negative, lighter than smallest atom (hydrogen) - 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?
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
EXAMPLES
1. Any atom containing 3 protons must be…?
2. A neutral sodium atom has _____ electrons
IONS
If electrons are added to or subtracted from a neutral atom, the resulting particle is called an
ION Electrons have a NEGATIVE charge so:- SUBTRACTING a NEGATIVE charge produces a
POSITIVE ion- ADDING a NEGATIVE charge produces a
NEGATIVE ion
EXAMPLES
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:
Atomic Mass
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
EXAMPLES
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?...
or..- atomic species having the same atomic number, but different mass numbers
Now try it with ions!
Find the number of protons, neutron and electrons in the following ions:
1) 56Fe3+
2) 76As3-
3) 201Au+
4) 82Br -
Extra Practice…
Extra Practice…
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 of1) Cl, given that is composed of 75.77% Cl-35 and 24.23% Cl-37
Natural Mixtures
Now you try…
Find the average atomic mass of:2) B, given that is is composed of 18.8% B-10 and 81.2% B-11