Ch. 3 Section 1 The Atom: From Philosophical Idea to Scientific Theory

Democritus

Greek philosopher that proposed that all elements of matter (earth, wind, fire, water) must be made up of the same basic particle (tiny and indivisible), the atom

ATOM

= from the Greek ‘ATOMOS’ meaning indivisible

atom

it is the smallest particle of an element that retains the properties of that element. Also called the building blocks of matter.

chemical reaction

the transformation of a substance or substances into one or more new substances is known as a

law of conservation of mass

mass is neither created nor destroyed during ordinary chemical reactions or physical changes

law of definite proportions

same proportions by mass regardless of the size of the sample or source of the compound

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

All matter is composed of extremely small particles called atoms.

Dalton’s Atomic Theory 1.)

atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.

Dalton’s Atomic Theory 2.)

atoms cannot be subdivided, created, or destroyed.

Dalton’s Atomic Theory 3.)

Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

Dalton’s Atomic Theory 4.)

In chemical reactions, atoms are combined, separated, or rearranged.

Dalton’s Atomic Theory 5.)

• Atoms are divisible into even smaller particles.

• A given element can have atoms with different masses.

• Nuclear reactions can change atoms of one element into different element.

Modern Atomic Theory

Not all aspects of Dalton’s atomic theory have proven to be correct. Now we know that:

• All matter is composed of atoms

• Atoms of any one element differ in properties from atoms of another element

Modern Atomic Theory

Some important concepts remain unchanged: