The AtomThe Atom

Two major parts of an atomTwo major parts of an atom

Nucleus (not to scale)

Electron Cloud

Three Major Sub-Atomic Three Major Sub-Atomic ParticlesParticles

Protons Neutrons Electrons

THE PROTONTHE PROTON

p+

•Heavy = 1 amu

• Positive + 1 charge

• Location: Nucleus

THE NEUTRONTHE NEUTRON

N°

• Heavy = 1 amu• No charge, neutral• Location: Nucleus

THE ELECTRONTHE ELECTRON

•Very light = 1/2000 amu

• Negative -1 charge

• Location: electron cloud e-

Where are they located?Where are they located?Nucleus: Protons and Neutrons

Electron Cloud: Only Electrons

ATOMIC NUMBER (Z)

The # of protons in an atomThe # of protons in an atomCan not change for an elementCan not change for an elementAll atoms are neutral, so Z equals the # All atoms are neutral, so Z equals the #

of electronsof electronsFor an ion – the number of electrons For an ion – the number of electrons

may differmay differ

Example: SodiumExample: Sodium

Na11

Atomic # = # of protons

MASS NUMBER (A)

The mass of an atomThe mass of an atomA = protons + neutronsA = protons + neutrons

To determine # of neutronsTo determine # of neutronsNeutrons = A - ZNeutrons = A - Z

ExampleExample

An atom of sodium has a mass of 24 amu, how many protons, electrons and neutrons does it have?

11 p+11 e-

24 - 11 = 13 N°Na

11

ISOTOPES

Atoms of the same Atoms of the same element that differ in element that differ in mass.mass.(They have the same # of (They have the same # of pp++, but different # of , but different # of NN°)°)

Isotope NotationIsotope Notation

Atomic #

Mass #

Isotope NotationIsotope Notation

Can also be written as Can also be written as ELEMENT – MASS #ELEMENT – MASS #

Example: Carbon - 12Example: Carbon - 12

So, why do the So, why do the elements on the PT elements on the PT have masses with have masses with

decimals???decimals???

AVERAGE ATOMIC MASS

The weighted average The weighted average mass of all naturally mass of all naturally occurring isotopes of an occurring isotopes of an element.element.

Example using exam scores!Example using exam scores!Exam Scores can be weighted higher than homework or quiz grades:

What if… And you received:Exam 50% Exam 93Quizzes 20% Quizzes 82.5Homework 30% Homework 85

Your grade =

(93 x .5) + (82.5 x .2) + (85 x .3) = 88.5

ExampleExample

Example:Example: The element hydrogen has three The element hydrogen has three isotopes. It exists in nature 99.41% of the isotopes. It exists in nature 99.41% of the time as Hydrogen-1, 0.4% of the time as time as Hydrogen-1, 0.4% of the time as Hydrogen-2 and 0.19% of the time as Hydrogen-2 and 0.19% of the time as Hydrogen – 3. What is the average atomic Hydrogen – 3. What is the average atomic mass of hydrogen?mass of hydrogen?

0.9995*1 + 0.004*2 + 0.001*3 = 1.0078 u0.9995*1 + 0.004*2 + 0.001*3 = 1.0078 u

Famous Famous Scientists in Scientists in

Atomic TheoryAtomic Theory

Democritus (400 B.C.)Democritus (400 B.C.)

First to First to develop the develop the idea of an idea of an atom.atom.

DALTONDALTONDalton created an atomic theory that Dalton created an atomic theory that

stated:stated:All elements are composed of tiny indivisible All elements are composed of tiny indivisible

particles called atomsparticles called atomsAtoms of the same element are identical. The Atoms of the same element are identical. The

atoms of any one element are different from atoms of any one element are different from those of any other elementthose of any other element

J.J. ThomsonJ.J. Thomson

““Plum Pudding Model”Plum Pudding Model”Solid positively charged sphereSolid positively charged sphereElectrons embedded withinElectrons embedded within

Rutherford’s Gold Foil ExperimentRutherford’s Gold Foil Experiment

RutherfordRutherford

Proved the existence of a tiny, dense, Proved the existence of a tiny, dense, positively charged nucleus.positively charged nucleus.

The rest of the atom is mostly empty The rest of the atom is mostly empty space.space.