Atoms:Atoms:Development of Development of

the Atomic the Atomic TheoryTheory

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Ancient BeliefAncient Belief

Ancient Greeks believed that all Ancient Greeks believed that all things were made up of earth, wind, things were made up of earth, wind, fire, and water.fire, and water.

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DemocritusDemocritus

460 BC - Greek philosopher proposes the 460 BC - Greek philosopher proposes the existence of the existence of the atom atom

said that all matter was composed of tiny said that all matter was composed of tiny particles that he called atomos. Atomos is particles that he called atomos. Atomos is Greek for “not to be divided”. Greek for “not to be divided”.

His theory:His theory: all atoms are small hard particles all atoms are small hard particles made of a single material formed into different made of a single material formed into different

shapes and sizesshapes and sizes always moving, and that they form different always moving, and that they form different

materials by joining togethermaterials by joining together

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John DaltonJohn Dalton 1803 - British chemist; elements combined in 1803 - British chemist; elements combined in

specific proportions to form compounds. specific proportions to form compounds. Rediscovered Democritus’ theory and added four Rediscovered Democritus’ theory and added four

parts.parts. His theory:His theory:

all substances are made of atoms that cannot be all substances are made of atoms that cannot be created, divided, or destroyedcreated, divided, or destroyed

atoms join with other atoms to make new atoms join with other atoms to make new substances called compoundssubstances called compounds

Atoms of different elements are differentAtoms of different elements are different atoms of the same element are exactly alike, atoms of the same element are exactly alike,

and atoms of different elements are different in and atoms of different elements are different in mass and size (mass and size (elementselements))

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Edward FranklandEdward Frankland

1852 - English chemist developed the 1852 - English chemist developed the valence valence theorytheory

His theory:His theory: every atom has a fixed number of bonds every atom has a fixed number of bonds

(chemical links) that it can form(chemical links) that it can form for the atom to be stable, all of these for the atom to be stable, all of these

bonds must be used.bonds must be used.

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J.J. ThomsonJ.J. Thomson 1897 - English chemist and physicist discovered 1897 - English chemist and physicist discovered

11stst subatomic particles subatomic particles His theory: His theory: used a cathode-ray tube

negatively charged particles called negatively charged particles called electrons electrons andand positively charged matterpositively charged matter

Assumed since there was a negatively charged Assumed since there was a negatively charged particle there must be a positively charged particle there must be a positively charged particle: the particle: the protonproton but he did not know how but he did not know how they were arrangedthey were arranged

created a model to describe the atom as a created a model to describe the atom as a sphere filled with positive matter with negative sphere filled with positive matter with negative particles mixed inparticles mixed in

Referred to it as the plum pudding modelReferred to it as the plum pudding model6

Cathode Ray TubeCathode Ray Tube

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Ernest RutherfordErnest Rutherford 1912 - New Zealand physicist1912 - New Zealand physicist Uses the gold-foil experiment and discovers the Uses the gold-foil experiment and discovers the

nucleusnucleus Said the atom was mainly a solid core at the Said the atom was mainly a solid core at the

center (nucleus)center (nucleus)

His theory:His theory: small, dense, positively charged particle small, dense, positively charged particle

present in present in nucleusnucleus called a called a protonproton electronselectrons travel around the travel around the nucleus,nucleus, but their but their

exact places cannot be describedexact places cannot be described8

Niels BohrNiels Bohr

1913 - Danish physicist discovered 1913 - Danish physicist discovered energy levelsenergy levels

It is the form we know todayIt is the form we know today

His theory:His theory: electronselectrons travel around the nucleus in travel around the nucleus in

definite paths and fixed distancesdefinite paths and fixed distances electrons can jump from one level to a electrons can jump from one level to a

path in another levelpath in another level Is called the planetary modelIs called the planetary model

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Erwin ShrodingerErwin Shrodinger

1924 - Austrian physicist developed 1924 - Austrian physicist developed the the electron cloud electron cloud modelmodel

His theory:His theory: electrons exact path cannot be predictedelectrons exact path cannot be predicted regions, referred to as the electron cloud, regions, referred to as the electron cloud,

are areas where electrons can likely be are areas where electrons can likely be found. found.

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James ChadwickJames Chadwick

1932 - English physicist discovered 1932 - English physicist discovered neutronsneutrons

His theory:His theory: neutronsneutrons have no electrical charge have no electrical charge neutrons have a mass nearly equal to neutrons have a mass nearly equal to

the mass of a protonthe mass of a proton unit of measurement for subatomic unit of measurement for subatomic

particles is the particles is the atomic mass unit atomic mass unit ((amuamu))

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Demitiri MendeleevDemitiri Mendeleev 1869-Russian chemist1869-Russian chemist The creator of the first Periodic Table of ElementsThe creator of the first Periodic Table of Elements Predicted the properties of the elements yet to be Predicted the properties of the elements yet to be

discovereddiscovered His periodic table is based on atomic mass.His periodic table is based on atomic mass.

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Henry MoseleyHenry Moseley 1915- English scientist that developed the 1915- English scientist that developed the

periodic table that we use todayperiodic table that we use today Sorted the chemical elements of the periodic Sorted the chemical elements of the periodic

table of the elements in a logical order based on table of the elements in a logical order based on their physics- on their atomic number (how many their physics- on their atomic number (how many protons an element has)protons an element has)

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Modern Theory of the Modern Theory of the AtomAtom

AtomsAtoms are composed of three main are composed of three main subatomicsubatomic particles: the particles: the electronelectron, the , the proton,proton, and the and the neutronneutron. .

Most of the mass of the atom is Most of the mass of the atom is concentrated in the concentrated in the nucleusnucleus of the atom. of the atom.

The The protons protons and and neutrons neutrons are located are located within the within the nucleusnucleus while the while the electronselectrons exist outside of the nucleus. exist outside of the nucleus.

In stable atoms, the number of In stable atoms, the number of protonsprotons is is equal to the number of equal to the number of electronselectrons. .

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Modern Theory of the Modern Theory of the Atom Cont.Atom Cont.

The type of atom is determined by the The type of atom is determined by the number of number of protons protons it has. it has.

The number of protonsThe number of protons in an atom is equal in an atom is equal to the to the atomic number atomic number

The sum of the number of protonsThe sum of the number of protons and and neutrons in a particular atom is called the neutrons in a particular atom is called the atomicatomic massmass

Valence electrons Valence electrons are the outermost are the outermost electrons and are where bonding takes electrons and are where bonding takes placeplace

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IsotopesIsotopes

An element that has the same An element that has the same number of protons but different number of protons but different number of neutrons.number of neutrons.

Example: Example: CC1212, C, C1414

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Chemical SymbolChemical Symbol

The shorthand way of writing an The shorthand way of writing an element. 1element. 1stst Letter is always Letter is always capitalized; the second and third capitalized; the second and third letters are lower-cased.letters are lower-cased.

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ProtonProton (p (p++)-Positively charged particle in the )-Positively charged particle in the atom and is found in the nucleus. It has a mass atom and is found in the nucleus. It has a mass 1840 times greater than an electron. A proton 1840 times greater than an electron. A proton has a charge of +1 and a relative mass of 1amu.has a charge of +1 and a relative mass of 1amu.

NeutronNeutron (n (noo)-Neutral particle in the atom found )-Neutral particle in the atom found in the nucleus. It has a mass nearly equal to in the nucleus. It has a mass nearly equal to that of a proton, but it carries no electrical that of a proton, but it carries no electrical charge. A neutron has 0 (zero) charge and a charge. A neutron has 0 (zero) charge and a relative mass of 1 amu.relative mass of 1 amu.

ElectronElectron (e (e--)-Negatively charged particle in the )-Negatively charged particle in the atom found on the outside of the nucleus (the y atom found on the outside of the nucleus (the y surround the nucleus in an electron cloud). Its surround the nucleus in an electron cloud). Its mass is 1/1840 that of a hydrogen atom. An mass is 1/1840 that of a hydrogen atom. An electron as a charge of -1 and has a relative electron as a charge of -1 and has a relative mass of 0 (zero).mass of 0 (zero).

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Figuring out the Neutron Figuring out the Neutron Number of any elementNumber of any element

Subtract the proton number from Subtract the proton number from the atomic mass number. To the atomic mass number. To calculate mass number, you add calculate mass number, you add the proton number plus the the proton number plus the neutron number.neutron number.

Mass#=Proton # + Neutron #Mass#=Proton # + Neutron #

Neutron #=Mass# - Proton#Neutron #=Mass# - Proton#19

Element/Ion

AtomicNumber

Atomic Mass Mass Number

Protons Neutrons Electrons

1H1

1 1.008 1 1 0 1

1H+

1

1 1.008 1 1 0 0

6 C12

6 12.011 12 6 6 6

17 Cl-35

17 35.453 35 17 18 18

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AtomsAtomsIsotope Atomic

NumberMass

NumberNumber

of ProtonsNumber

of Neutron

Number of

Electrons

32.065 16

24 20

Zn-64

9 10

11 22.990

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Isotopes and average atomic mass-Isotopes and average atomic mass-Elements come in a variety of Elements come in a variety of isotopes, meaning they are made isotopes, meaning they are made up of atoms with the same atomic up of atoms with the same atomic number but different atomic number but different atomic masses. These atoms differ in the masses. These atoms differ in the number of neutrons. The average number of neutrons. The average atomic mass is the weighted atomic mass is the weighted average of all the isotopes of an average of all the isotopes of an element.element.

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Calculating Average Atomic Mass Calculating Average Atomic Mass of an Elementof an Element

Example: A sample of cesium is 75% 133Cs, Example: A sample of cesium is 75% 133Cs, 20% 132 Cs, and 5% 134Cs. What is the 20% 132 Cs, and 5% 134Cs. What is the average atomic mass? (1average atomic mass? (1stst, change all , change all percentages to decimals)percentages to decimals)

Answer: .75 x 133=99.75Answer: .75 x 133=99.75

.20 x 132=26.4.20 x 132=26.4

.05 x 134= .05 x 134= 6.7____ 6.7____

132.85132.85

133 a.m.u. (atomic mass units133 a.m.u. (atomic mass units))

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Radioactivity-the process by which some Radioactivity-the process by which some elements spontaneously emit radiation.elements spontaneously emit radiation.

Radiation is the rays and particles emitted Radiation is the rays and particles emitted by a radioactive material. There are 3 types by a radioactive material. There are 3 types of natural radiation:of natural radiation:

a-alpha radiationa-alpha radiation b-beta radiationb-beta radiation g-gamma radiationg-gamma radiation

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Alpha radiation Alpha radiation consists of particles containing consists of particles containing two protons and two neutrons. Alpha particles two protons and two neutrons. Alpha particles are identical to a helium nucleus. The symbol are identical to a helium nucleus. The symbol is a or is a or 44HeThey are large but do not penetrate. HeThey are large but do not penetrate. The relative mass is 4 and the charge is +2.The relative mass is 4 and the charge is +2.   

Beta radiation Beta radiation consists of fast-moving consists of fast-moving electrons. The symbol for a beta particle is B electrons. The symbol for a beta particle is B or or 00B B or or 00e. They are small and fairly e. They are small and fairly penetrating. The relative mass is 0 and the penetrating. The relative mass is 0 and the charge is -1.charge is -1.   

Gamma radiation Gamma radiation (or gamma rays) is not (or gamma rays) is not particles but extremely high-energy particles but extremely high-energy electromagnetic radiation. They are very electromagnetic radiation. They are very penetrating and dangerous. They have no penetrating and dangerous. They have no mass and no charge.mass and no charge.

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The neutron to proton ration The neutron to proton ration (n(n00:p:p++) determines the stability ) determines the stability of a nucleus. Unstable nuclei of a nucleus. Unstable nuclei will continue to decay until a will continue to decay until a stable ration is achieved.stable ration is achieved.

Nuclear equations show the Nuclear equations show the changes involved with the changes involved with the atomic number and the mass atomic number and the mass number using nuclear symbols. number using nuclear symbols.

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The total mass numbers of the The total mass numbers of the reactants must equal the total reactants must equal the total mass numbers of the products. mass numbers of the products. The total atomic numbers of the The total atomic numbers of the reactants must equal the total reactants must equal the total atomic numbers of the products.atomic numbers of the products.

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Nuclear equations show the Nuclear equations show the changes involved with the changes involved with the atomic number and the mass atomic number and the mass number using nuclear symbols. number using nuclear symbols.

The total mass numbers of the The total mass numbers of the reactants must equal the total reactants must equal the total mass numbers of the products. mass numbers of the products. The total atomic numbers of the The total atomic numbers of the reactants must equal the total reactants must equal the total atomic numbers of the products.atomic numbers of the products.

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