ATOMS: The Building Blocks of Matter

Objectives1. Law of conservation of mass

2. Law of definite proportions

3. Law of multiple proportions

4. Dalton’s Atomic Theory

5. How Dalton’s Atomic Theory relates to 1, 2, & 3

Atomic Theory Foundations

Law of Conservation of Mass – mass is neither created or destroyed during a chemical or physical change

Law of Definite Proportions – a compound contains the same proportions by mass regardless of the size of the sample

Example:

NaCl – always 39.34% Na & 60.66% Cl

Atomic Theory Foundations

Law of Multiple Proportions – if two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers

Example

CO2 and CO : ratio of oxygen is always 2:1

Dalton’s Atomic theory

1. All matter is composed of atoms

2. Atoms of an element have the same size, mass and properties; atoms of a different element have different sizes, masses and properties

3. Atoms cannot be divided, created or destroyed

4. Atoms of different elements combine in simple whole number ratios

5. Chemical reactions combine, separate, or rearrange atoms

Modern Atomic Theory

• Atoms can be divided

• Atoms of the same element can have different masses

• All else remains the same

Structure of the Atom

Objectives

Discovery of the Electron

Rutherford’s Experiments

Protons, Neutrons, Electrons

Atomic Structure

• Electron = no mass; negative charge

• Proton mass = hydrogen atom; positive

• Neutron mass = hydrogen atom; no charge

• Dalton’s Model

• JJ Thompson’s Plum Pudding Model

Rutherford

• Shoots alpha particles (Helium atoms) at gold foil

• Expected to pass right through

• Particles are deflected

• Leads to idea of a dense positively charged center with e- orbiting around it

The electron

• Mass of 9.109 x 10-31 kg

• Negative charge

The Proton

• Mass = 1.673 x 10-27 kg

• Positive charge

The neutron

• Mass = 1.675 x 10-27 kg

• No charge

Comparing Theories

Dalton

See notes

Solid sphere

Thompson

Plum pudding model

Electrons scattered thru positively charged cloud

Rutherford

Concept of the nucleus

Positively charged

Counting Atoms

ObjectivesExplain isotopes

Define atomic number, atomic massDetermine number of protons, neutrons & electrons

Define mole and molar massConvert between grams, moles, and atoms

Atomic Number

• Atomic Number– # of protons in nucleus

• Element Symbol

• Element Name

• Atomic Weight

• Electron Configuration

3

LiLithium

6.914

[He]2s1

Isotopes

Def: atoms of the same element that have different masses

Example: hydrogenprotium – 1 proton in nucleusdeuterium – 1 proton; 1 neutrontritium – 1 proton; 2 neutrons

*Nuclide – general term for any isotope

Mass Number

Def: the number of protons and neutrons in the nucleus of an isotope

mass # - atomic # = # of neutrons

Example – oxygen

Mass # (16) – atomic # (8) = # of neutrons (8)

Writing Isotopes

SymbolElement number massnumber atomic

U23592

Example – Uranium 235