Section 1

Democritus

• 400BC Greek thinker

• atom- nature’s basic particle

Aristotle

• generation succeeding Democritus

• didn't believe in atoms

• all matter is continuous

Foundations of Atomic Theory

element- substance that cannot be further broken down by chemically/ordinary means

elements combine to form compounds

• different chemical/physical propertied than the elements that make them up

chemical reaction- transformation of a substance to a new substance

law of conservation of mass- mass is neither created nor destroyed during ordinary chemical reaction or physical change

law of definite proportions- chemical compound contains same elements in 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 the ratio of small whole numbers

Dalton’s Atomic Theory

• proposed explanation for law of conservation of mass, definite proportions, and multiple proportions- his 5 postulates

5 Postulates:

1. all matter is composed of extremely small particles called atoms

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

3. atoms cannot be subdivided, created, destroyed

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

5. in chemical reactions, atoms are combined, separated, or rearranged

Modern Atomic Theory

• not all aspects of Dalton’s theory have been proven correct

• atoms can be divided into smaller parts

  • although law of conservation of mass still holds true

• a given element can have atoms with different masses

• Dalton’s theory has been modified

modern atomic theory concepts:

1. all matter is composed of atoms

2. atoms of any one element differ in properties from atoms of another element remain unchanged

Section 2- Structure of the Atom

atom- smallest particle of an element that retains the chemical properties of that element

two regions of an atom:

1. nucleus- small region in the center

   1. at least one proton- positive charge

   2. at least one neutron- neutral charge

2. area around nucleus occupied by electrons- negatively charge particles

• subatomic particles- proton, electron, neutron

Discovery of the Electron

Cathode Rays and Electrons

• current passing through cathode tube causes it to glow

• observations lead to hypothesis- particles were negatively charged

  • experiments support: Thompson measure ratio of particles to mass

    • always the same

  • cathode rays composed of identical negative charge particles- electrons

Charge and Mass of an Electron

• electrons in all atoms

• atoms are divisible

• large charge to mass ratio

• atoms are electrically neutral

  • must have a positive

  • electrons have much less mass than atoms, so other particles must have much more mass

• plum pudding model- electrons evenly spread throughout and the rest is negative

Discovery of Atomic Nucleus

Ernest Rutherford- shoot positive alpha particles at think gold foil

• some deflect

• must be hitting something positive and dense in the center- nucleus

Bhor- propose atomic model

• electrons surround positive nucleus like planets around the sun

Composition of the Atomic Nucleus

atomic nuclei made of protons and neutrons

• protons= positive charge, electrons= negative charge

  • equal in magnitude

• atoms are electrically neutral: same number of protons and electrons

• atoms of different elements have different numbers of protons- affects positive charge

  • protons determine identity of an atom

Forces in the Nucleus

nuclear forces: the interaction that binds protons and neutrons, protons and protons, and neutrons and neutrons together in a nucleus

• usually particles with the same charge repel each other, but the particles are so close that a strong attraction is formed

The Size of Atoms

electron cloud: region outside of the nucleus where electrons are stored

• radius of an atom- from the center of the nucleus to the outer portion of the electron cloud

  • extremely small

  • measured in picometer (pm)

Section 3- Counting Atoms

Atomic Number

atomic number: the number of protons in each atom of an element

• identifies the element because different elements have different atomic numbers

• atoms are neutral, so the atomic number also tell the number of electrons

Isotopes

isotopes: atoms of the same element with different masses

• same number of protons and electrons but different number of neutrons

Mass Number

mass number: total number of protons and neutrons that make up the nucleus of an isotope

Designating Isotopes

identified by specifying the mass number

2 methods:

• hyphen notation

  • mass number written with a hyphen after the element name

  • Hydrogen-3

• nuclear symbol

  • shows composition of the nucleus

  • superscript= mass number

  • subscript= atomic number

  • ³₂ He

Relative Atomic Masses