CHEM 12.21_Module 2.1: Modern Atomic Theory

Greek philosophers who created the first concept of the atom

• Leucippus

• Democritus

greek for indivisible

atomos

believed that matter consisted of various combinations of the four “elements”—fire, earth, air, and water

Aristotle

5 postulates of Dalton’s atomic theory

1. matter is comprised of small units called atoms

2. elements consist of one type of atom

3. elements differ in properties

4. compounds have fixed ratios

5. atoms are neither created nor destroyed

if atoms are neither created nor destroyed during a chemical change, then the total mass of matter present when matter changes from one type to another will remain constant

law of conservation of matter

all samples of a pure compound contain the same elements in the same proportion by mass

law of definite proportions

when two elements react to form more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small, whole numbers

law of multiple proportions

J.J. Thomson’s experiment

cathode ray tube

discovered the electron

J.J. Thomson

Robert Millikan’s experiment

oil drop experiment

discovered the charge of the electron

Robert Millikan

charge of the electron

1.6 × 10^–19

J.J. Thomson’s model

plum pudding model

Ernest Rutherford’s experiment

gold foil scattering experiment

discovered the nucleus

Ernest Rutherford

A small, relatively heavy, positively charged body at the center of the atom

nucleus

discovered the proton

Ernest Rutherford

positively charged subatomic particle is located in the nucleus

proton

atoms of the same element that differ in mass

isotopes

discovered isotopes

Frederick Soddy

uncharged, subatomic particles with a mass approximately the same as that of protons

neutrons

discovered neutrons

James Chadwick

contains the majority of an atom’s mass

nucleus

occupy almost all of an atom’s volume

electrons

diameter of an atom

10^–10 m

diameter of a nucleus

10^–15 m

charge of electron

1.602 × 10^–19 C

number of protons in the nucleus of an atom

atomic number

symbol for atomic number

Z

determines the identity of the atom

atomic number

atom that contains the same number of positive and negative charges

neutral atom

total number of protons and neutrons in an atom

mass number

symbol for mass number

A

equation for mass number

• p⁺ - e^-

• A-Z

when the number of protons and electrons are not equal

ion

negative charged ion

anion

positive charged ion

cation

single atom in amu

atomic mass

instrument used to determine the occurrence and natural abundances of isotopes

mass spectrometer

shows the same information as a molecular formula but also shows how the atoms are connected

structural formula

diatomic molecules

• H₂

• N₂

• F₂

• O₂

• I₂

• Cl₂

• Br₂

indicates the simplest whole-number ratio of the number of atoms (or ions) in the compound

emperical formula

indicates the actual numbers of atoms of each element in a molecule of the compound

molecular formula

compounds with the same chemical formula but different molecular structures

isomers

the amount of a substance containing the same number of discrete entities

mole

number of entities composing a mole

Avogadro’s number