Section 1: Development of the Atomic Theory

CHAPTER 4 NOTES

Section 1: Development of the atomic theory

MATTER

Make up

◦Particles called atoms

First theory-400 B.C.

◦Democritus

Suggested

◦Universe was made of indivisible units

ATOMS

Atom

◦Greek meaning unable to be cut or divided

Convincing

◦He didn’t have evidence

◦1700’s  emphasis was put on making careful and repeated measurements in experiments

DALTON-1808

All atoms of a given element were exactly alike

Atoms of different elements could join to form compounds

A.K.A. Billiard ball model

Law of definite proportions

Chemical compound always contains the same elements in exactly the same proportions by mass or weight

Water

◦2 hydrogen, 1 oxygen

FOUNDATION

For modern atomic theory

◦Dalton’s theory

Changed

◦Scientists continued to do experiments and acquire more information

J.J. THOMSON-1897

Suggested atoms were not indivisible

Experiment  cathode-ray tube-negatively charged particles

Discovered electrons

Electrons negatively charged particles outside the atom

A.K.A. Plum pudding model

◦After a popular dessert

◦Looks like blueberries in a muffin

RUTHERFORD-1911

Most of the mass of the atom was concentrated in the middle

Gold-foil experiment

Thomson-mass and + change evenly distributed

Rutherford-most particles would travel in a straight path

Discovered the nucleus

Electrons orbit the nucleus

Section 2: The structure of atoms

Elements

Defined

By number of protons

Equal number of protons to electrons

Gain or loses electrons

Charged atom  ion

Electric force

Attraction between a positive and negative charged atoms

Holds an atom together

Atomic number

How many protons

Represented by Z

Protons and electrons

Never changes

Mass number

Protons + neutrons

Represented by A

Fluorine  19

9 protons + 10 neutrons

Isotopes

Number of neutrons vary

An atom that has the same number of protons but a different number of neutrons

neutrons

Mass number – atomic number (A-Z)

235-92=143

Atomic mass

Unified atomic mass unit (u)

Equal to 1/12th of the mass of C-12

Atomic mass unit, amu

Average atomic mass

Avg. mass of all isotopes

mole

Used to measure large numbers of small particles

1 mole = 6.022 x 1023

Avogadro’s number

Molar mass

Mass in grams of one mole of a substance

Magnesium

24.3 g = 1 mole

By using molar mass

MOLAR MASS CONT’D

Oxygen

16 g/mol

Add up all the molar masses of all the atoms

Water

18.02 g/mol

Total mass of 6.022 x 1023 =18.02 g of water molecules

SECTION 3: MODERN ATOMIC THEORY

MODERN MODEL

• Electrons can be found only in certain energy levels but not between

• Locations cannot be predicted precisely

NEW MODEL-1925

• Electrons don’t move in a definite path

• Electrons behave more like waves on a vibrating string that like particles

ORBITALS

• Region in an atom where there is a high probability of finding electrons

• Shaded area =orbitals find electrons

VALENCE ELECTRONS

• Filled with electrons

• Number of electrons in the outermost shell

• Electrons in the outer energy level

• Based on group number

• Determine the chemical properties of an atom

• 4 types
• s, p, d, f

JUMPING

• When atoms gains or loses energy

• Ground state
• Lowest state of energy of an electron

• Excited state
• Electrons gaining energy

• Gain energy
• By absorbing particles of light, photons

Lewis dot structures (valence electrons)

Group number Valence electrons