HIROSHIMA 1945

August 6 th, 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) 56 Fe 3+ 2) 76 As 3- 3) 201 Au + 4) 82 Br -

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 of 1) 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