Honors Chemistry - Unit 3 Lessons 1 and 2

atom

the smallest particles of an element that retains the properties of the element

molecule

two or more atoms bonded together

Dalton’s Atomic Theory

• 1803

• all elements are composed of indivisible atoms

• all atoms of a given element are identical

• atoms of different elements are different, that is they have different masses

• compounds are formed by the combination of atoms of different elements

• Solid Sphere/Cannonball Model

JJ Thomson Model

• 1897

• used a cathode ray tube to discover the atom was made of smaller units(subatomic particles)

• the ray was deflected by a magnetic field and allowed him to conclude these particles were negative

• Thomson called these small negatively charged particles

• his model was a positive sphere with small negative particles embedded in the structure

• Plum Pudding/Chocolate Chip Model

Ernest Rutherford Model

• 1909

• used a gold foil(leaf) experiment

• most of the particles passed through as predicted

• some particles were greatly deflected

• meaning most of the atom is empty space

• the atom must have a densely packed postive core(nucleus)

• most of the mass of the atom is in the nucleus

• Nuclear Model

Bohr Model

• 1913

• Bohr observed that electrons can absorb and release specific amounts of energy(quanta)

• Bohr put the electrons into specific rings orbiting the nucleus, like planets

• Electrons have to be a set distance from the nucleus

• Electrons can absorb energy to move to a higher level(further from the nucleus)

• Energy is released when electrons move to a lower level

• But they only absorb or release the exact amount of energy needed to move(quanta)

• Planetary Model

Wave Mechanical Model

• electrons act more like electromagnetic waves(but can have physical properties too)

• electrons exist in orbitals

• orbital: the region with the highest probability of finding an electron

• each orbital in an atom can hold a maximum of two electrons

Protons

• determine the identity of the atom(element)

• known as the atomic number

• equal to the nuclear charge

• equal to the number of electrons in an atom(neutral)

• mass of 1 atomic mass unit(amu, u)

1 AMU is equal to ½ the mass…

of a Carbon Atom

Electrons

• negligible mass

• occupy orbitals outside the nucleus

• highest energy electrons are called valence electrons(outermost shell)

• valence electrons determine the chemical properties of an element(chemical properties are how an atom reacts)

• gain or losing electrons changes the atom to an ion

  • gaining an electron - negative ion

  • losing an electron - positive ion

Neutrons

• contained in the nucleus of the atom

• mass of 1 u contributes to the mass number

• protons + neutrons = mass number

• atoms of the same element can have different numbers of neutrons(isotopes)

Isotopic Notation

Carbon-14

C-14

14C

14 6 C

a=mass number

z=atomic number

x=element

azX

atomic number

number of protons in an atom

mass number

protons + neutrons

isotope

atoms of the same element with different mass numbers

how to find the number of neutrons

subtract the protons from the mass number

average atomic mass(normal)

average atomic mass = mass(%ab)+mass(%ab)/100

average atomic mass(challenge)

average atomic mass = mass(x)+mass(1-x)