Culver City H.S. • Honors Chemistry Name KEY______

4 • Structure of the Atom Period Date

4.1,4.2 NOTES – HISTORY OF ATOMIC THEORY

History of the Atom

Democritus (400 BCE):

·  “The atomists hold that splitting stops when it reaches indivisible particles and does not go on infinitely.”

·  atomos – “without” + “cut”

John Dalton (1805)

1.  Elements are made of tiny, indestructible particles called atoms. (Democritus)

2.  All atoms of the same element are identical and have the same properties.

3.  Atoms of different elements combine in small whole number ratios to form compounds. (Law of Definite Proportions, Proust)

4.  Atoms may combine in more than one ratio to form different compounds.
(Law of Multiple Proportions)

5.  In a chemical reaction, atoms are rearranged; nothing is created nor destroyed.
(Law of Conservation of Mass, Lavoisier)

·  Model: Billiard Ball Model

Sir Joseph John Thomson (1897)

·  Cathode Ray Tube Experiments

o  The Cathode Ray Tube (TVs and computer monitors are examples of CRTs)

Cathode rays (light emanating from positive
electrode) accelerated through electrodes

cathode anode

o  Cathode Rays deflected by a magnet

Cathode rays deflected in magnetic field

à cathode rays are not light

o  Cathode rays with electric fields

Cathode rays attracted to positive plate

à cathode rays are negatively charged.

o  Cathode ray and pinwheel

Cathode rays move pinwheel

à cathode rays must have mass and energy

pinwheel ramp with track

o  Determining charge:mass

Determined how much electric field must be applied
to deflect ray back to original spot: determined charge to mass ratio of the electron

·  Model: Plum Pudding Model

Homogenous POSITIVE sphere (no protons)

Electrons (negative charges)

·  Robert Millikin’s Oil Drop Experiment

Ionized oil drops

anode

oil drops suspended in air (gravity + electric field)

cathode

Determined the mass of the electron = 9.11 × 10–28 g

Ernest Rutherford (1911)

·  Radioactivity

o  In 1896 Bacquerel found Uranium could produce an image on photographic plate without light.

o  Three kinds of radioactive Particles

§  Alpha (a) particles à helium nucleus, 2+ charge

§  Beta (b) particles à high speed electron

§  Gamma (g) rays à high energy light

·  Gold Foil Experiment: “It was as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you!”

If atom were like plum pudding, then a would

go through, maybe slightly deflected; but did

explain large angle deflections

a rays

Rutherford theorized there must be a tiny

dense nucleus

Gold foil would look like this:

thin gold foil Thomson Rutherford

large angle deflection

Size of nucleus compared to golf ball in the Rose Bowl

Density of nucleus compared to a pea weighing 250 million tons

·  Model: Nuclear Model

Atom is mainly empty space with electrons.

Protons in a tiny very dense nucleus.

What an Atom Looks Like

Particle / Symbol / Location / Charge / Mass / Size
Proton / p+ / Makes up nucleus / +1 / 1.67 × 10–24 g
= 1.00728 amu ≈ 1 / 10−15 m
Neutron / n0 / Makes up nucleus / 0 / 01.67 × 10–24 g
= 1.00866 amu ≈ 1 / 10−15 m
Electron / e− / Outside nucleus / −1 / 9.11 × 10–28 g
= 0.00055 amu ≈ / 10−18 m

(1 charge = 1.60 × 10–19 C)

object / actual size / model size / model
proton / 10-15 m / 10 cm / orange
neutron / 10-15 m / 10 cm / apple?
electron / 10-18 m / 0.1 mm / speck
atom / 10-10 m / 6-miles

The atom’s mass is due to: protons and neutrons

The atom’s volume is due to: electrons

Isotopic Notation

Mass Number (A) = # protons + neutrons
Atomic Number (Z) = # protons

·  If you change the number of protons, you get a different element .
Elements are arranged on the Periodic Table in order of their atomic number .

·  If you change the number of neutrons in an atom, you get an isotope of that element.

·  If you change the number of electrons in an atom, you get an ion of that element.
In a neutral atom, # of protons = # of electrons .

The charge of an ion is found by # protons − # electrons.

Example 1: State the number of protons, neutrons, and electrons in Cl–

17 protons, 18 neutrons, 18 electrons

Negative ions are called anions; they are formed by adding electrons.

Example 2: Mass Number = 60, Atomic Number = 26, Charge = +2

State the number of protons, neutrons, and electrons, and write the atom in isotopic notation.

60Fe2+ 26 protons, 34 neutrons, 24 electrons.

Positive ions are called cations; they are removing by adding electrons.

Atomic Mass

6
C
12.011

1 amu (atomic mass unit) = 1/12 mass of C-12 = 1.66 × 10–24g

·  Atomic Mass: weighted average mass of all naturally occurring isotopes by abundance.

atomic mass =

Example 3:

Isotope Abundance Isotopic Mass Product

C-12 98.89% 12 amu (exactly) 11.8668

C-13 1.11% 13.003355 amu + 0.1443

atomic mass = 12.0111 amu ß This is the atomic mass on periodic table