ATOMIC THEORYATOMIC THEORY

Building blocks of matterBuilding blocks of matter

Who are these men?Who are these men?

In this lesson, we’ll learn about the men whose quests for knowledge about the fundamental nature of the universe helped define our views.

DEMOCRITUSDEMOCRITUS

IN 400 BC, DEMOCRITUS IN 400 BC, DEMOCRITUS SAID:SAID:

All matter is made of All matter is made of tiny particles called tiny particles called “atomos”“atomos”

Disputed by AristotleDisputed by Aristotle

Why?Why?Eminent philosophers Eminent philosophers

of the time, of the time, AristotleAristotle and Plato, had a and Plato, had a different idea.different idea.

They favored the earth, fire, air and water theory of matter. They were more popular, so the atomos idea was buried for approximately 2000 years.

AtomosAtomos

To Democritus, atoms were small, hard particles like marbles with different shapes and sizes.

Atoms were infinite in number, always moving and capable of joining together.

For the next 2000 years…For the next 2000 years… Alchemists tried to make gold from other Alchemists tried to make gold from other

metals.metals.

UNTIL…UNTIL…

1808 JOHN DALTON’S NEW ATOMIC 1808 JOHN DALTON’S NEW ATOMIC THEORYTHEORY

LAWS FROM ATOMIC THEORYLAWS FROM ATOMIC THEORY(from last chapter)(from last chapter)

1. Law of conservation of matter—the mass of 1. Law of conservation of matter—the mass of the reactants before the reaction equals the the reactants before the reaction equals the mass of the products after.mass of the products after.

2. Law of definite proportions—Every sample 2. Law of definite proportions—Every sample of the same compound has the same mass of the same compound has the same mass ratio of component elements.ratio of component elements.

3. Law of multiple proportions—In a series of 3. Law of multiple proportions—In a series of compounds of the same two elements, the compounds of the same two elements, the ratio of an element in one compound to ratio of an element in one compound to another is also a small, whole number.another is also a small, whole number.

Dalton’s ModelDalton’s Model

In the early 1800s, In the early 1800s, English chemist English chemist John John Dalton Dalton performed careful performed careful experiments that experiments that eventually led to the eventually led to the acceptance of the acceptance of the idea of atoms.idea of atoms.

Dalton’s atomic theoryDalton’s atomic theory

1. All matter is made up of atoms1. All matter is made up of atoms 2. Atoms of the same element are alike.2. Atoms of the same element are alike. 3. Atoms of different elements are 3. Atoms of different elements are

different.different. 4. Compounds have a definite 4. Compounds have a definite

composition by weight and combine in composition by weight and combine in small whole number ratios.small whole number ratios.

5. Atoms cannot be subdivided.5. Atoms cannot be subdivided.

..

This theory This theory became one became one of the of the foundations of foundations of modern modern chemistry.chemistry.

Thomson’s Plum Pudding ModelThomson’s Plum Pudding Model

In In 18971897, the , the English scientist English scientist J.J. Thomson J.J. Thomson provided the first provided the first hint that an atom hint that an atom is made of even is made of even smallersmaller particles. particles.

Thomson ModelThomson Model

His model of the His model of the atom is sometimes atom is sometimes called the “called the “PlumPlum PuddingPudding” model. ” model.

Atoms were made Atoms were made from a from a positively positively charged substance charged substance with with negativelynegatively charged electrons charged electrons scattered about, like scattered about, like raisins in a pudding.raisins in a pudding.

Thomson ModelThomson Model

Thomson studied Thomson studied passing an passing an electric current electric current through a gas.through a gas.

As the current As the current passed through, it passed through, it gave off rays of gave off rays of negatively negatively charged particles.charged particles.

This surprised This surprised Thomson, Thomson, because the because the atoms of the gas atoms of the gas were uncharged. were uncharged. Where had the Where had the negative charges negative charges come from?come from?

Where did they come from?

He concluded that the negative charges came from within the atom.

A particle smaller than an atom had to exist.

The atom waswas divisible!

Thomson called the negatively charged “corpuscles,” today known as electrons.

Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom.

But he could never find them.

Ernest RutherfordErnest Rutherford

In 1908, the English In 1908, the English physicist Ernest physicist Ernest Rutherford was Rutherford was hard at work on an hard at work on an experiment that had experiment that had little to do with little to do with unraveling the unraveling the mysteries of the mysteries of the atomic structure.atomic structure.

Rutherford’s experiment involved firing a Rutherford’s experiment involved firing a stream of tiny stream of tiny positivelypositively charged charged particles at a thin sheet of particles at a thin sheet of gold foilgold foil (2000 (2000 atoms thick)atoms thick)

RutherfordRutherford– MostMost of the positive of the positive

particles passed through particles passed through the gold atoms in thethe gold atoms in the foil foil without changing course without changing course at all.at all.

– SomeSome of the positive of the positive charges did bounce charges did bounce away from the gold sheet away from the gold sheet as if they had hit as if they had hit something something solidsolid. He . He knew that like charges knew that like charges repel.repel.

RutherfordRutherford

The gold atoms in the The gold atoms in the sheet were sheet were mostly empty mostly empty spacespace. Atoms were not a . Atoms were not a plum pudding. plum pudding.

Atom has a Atom has a small, dense, small, dense, positively chargedpositively charged center center that repelled the positive that repelled the positive “bullets.”“bullets.”

He called the center of the He called the center of the atom the “nucleus”atom the “nucleus”

The The nucleus is tinynucleus is tiny compared to the atom as a compared to the atom as a whole. whole.

http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html

Atomic ParticlesAtomic Particles

Electron—discovered by Thomson in 1890’sElectron—discovered by Thomson in 1890’s– Robert Millikan—determined the charge of an Robert Millikan—determined the charge of an

electron in 1909 w/ oil dropselectron in 1909 w/ oil drops

Proton—discovered by Rutherford in 1911Proton—discovered by Rutherford in 1911 Neutron—discovered by James Chadwick in Neutron—discovered by James Chadwick in

19321932

Particles and ChargeParticles and Charge

MassMass ChargeCharge LocationLocation

Proton (pProton (p++) )

Neutron (nNeutron (noo))

Electron (eElectron (e--))

1 amu +1 1 amu +1 nucleus nucleus

1/18401/1840 amu -1 electron amu -1 electron cloudcloud

1 amu 1 amu 0 0 nucleusnucleus

How Atoms DifferHow Atoms DifferDalton said that all atoms of an element are Dalton said that all atoms of an element are

alike, but we know that is not completely alike, but we know that is not completely true. So what true. So what isis alike? alike?

1.1. All atoms of the same element have the All atoms of the same element have the same number of protons. same number of protons.

2.2. If the atom is neutralIf the atom is neutral, that means they , that means they also have the same number of electrons.also have the same number of electrons.

3.3. The number of neutrons, however, can The number of neutrons, however, can vary.vary.

How Atoms DifferHow Atoms Differ

Atoms with the same atomic number are Atoms with the same atomic number are the same element, but they may have the same element, but they may have different numbers of neutrons.different numbers of neutrons.

Atoms of the same element with a different Atoms of the same element with a different number of neutrons are called isotopes.number of neutrons are called isotopes.

Atoms of the same element with a different Atoms of the same element with a different number of electrons than protons are called number of electrons than protons are called ions.ions.

The Chemists’ Shorthand: The Chemists’ Shorthand: Atomic Symbols for IsotopesAtomic Symbols for IsotopesThe Chemists’ Shorthand: The Chemists’ Shorthand:

Atomic Symbols for IsotopesAtomic Symbols for Isotopes

K Element Symbol39

19

Mass number

Atomic number

#p#p++ + #n + #noo

#p#p+ + oror #e #e--

Mass # - Atomic # = #nMass # - Atomic # = #noo

Atomic MassesAtomic Masses

Elements occur in nature as mixtures Elements occur in nature as mixtures of isotopesof isotopes

Atomic mass is the weighted average Atomic mass is the weighted average of all isotopes for an element.of all isotopes for an element.

Carbon =Carbon = 98.89% 98.89% 1212CC

1.11% 1.11% 1313CC

<0.01% <0.01% 1414CC

Carbon atomic mass = 12.01 amuCarbon atomic mass = 12.01 amu

MASS NUMBER AND AVERAGE MASS NUMBER AND AVERAGE ATOMIC MASSATOMIC MASS

Atomic masses are based on CARBON. Atomic masses are based on CARBON. The The atomic mass unitatomic mass unit is 1/12 of the mass is 1/12 of the mass of one carbon atom. of one carbon atom.

How do we calculate average atomic How do we calculate average atomic mass?mass?

Multiply the % times the mass for each Multiply the % times the mass for each isotope, then add them together.isotope, then add them together.

Average atomic massAverage atomic mass

Calculate the average mass of isotopes Calculate the average mass of isotopes of neptunium with:of neptunium with:

50.0% at 238.05 amu50.0% at 238.05 amu29.4% at 235.1 amu29.4% at 235.1 amu20.6% at 237.98 amu20.6% at 237.98 amu

(.500 x 238.05) + (.294 x 235.1) + (.206 x (.500 x 238.05) + (.294 x 235.1) + (.206 x 237.98) = 237.17amu237.98) = 237.17amu

Another problem:Another problem:

Calculate the average atomic mass of Calculate the average atomic mass of calcium with these isotopes:calcium with these isotopes:

28.4% at 40.06 amu28.4% at 40.06 amu

34.1% at 41.02 amu34.1% at 41.02 amu

22.8% at 40.89 amu22.8% at 40.89 amu

14.7% at 39.98 amu14.7% at 39.98 amu

(.284x40.06)+(.341x41.02)+(.228x40.89)+(.147x39.98)(.284x40.06)+(.341x41.02)+(.228x40.89)+(.147x39.98)

40.5640.56

One more for ArsenicOne more for Arsenic

35.1% of 74.9 amu35.1% of 74.9 amu

18.6% of 74.2 amu18.6% of 74.2 amu

46.3% of 75.02 amu46.3% of 75.02 amu

74.83 amu74.83 amu

Atomic MassAtomic Mass

Atomic mass is the Atomic mass is the weighted average of weighted average of all of the known all of the known isotopes of an isotopes of an element, so will element, so will always be shown as a always be shown as a decimal number.decimal number.

Covalent Chemical BondingCovalent Chemical Bonding

The forces that hold atoms together in The forces that hold atoms together in compoundscompounds. . Covalent bonds result from Covalent bonds result from atoms sharing electrons between nonmetal atoms sharing electrons between nonmetal atoms.atoms.

MoleculeMolecule: : a collection of covalently-bonded a collection of covalently-bonded atoms.atoms.

AtomAtom: : representative particle for a representative particle for a monatomic elementmonatomic element

Ionic Chemical BondingIonic Chemical Bonding

CationCation: : A positive ionA positive ion

MgMg2+2+, NH, NH44++

AnionAnion: : A negative ionA negative ion

ClCl, SO, SO4422

Ionic BondingIonic Bonding: : Force of attraction between Force of attraction between oppositely charged ions. Smallest particle oppositely charged ions. Smallest particle called a formula unit.called a formula unit.

Stupendous SevenStupendous Seven

Periodic TablePeriodic Table

Elements classified by:Elements classified by: propertiesproperties

atomic numberatomic number

GroupsGroups (vertical columns)—also called families(vertical columns)—also called families 1A = alkali metals1A = alkali metals 2A = alkaline earth metals2A = alkaline earth metals 7A = halogens7A = halogens 8A = noble gases8A = noble gases

PeriodsPeriods (horizontal rows)(horizontal rows)

Periodic TablePeriodic Table

Antoine Lavoisier , 1790’s Antoine Lavoisier , 1790’s made first list of known elements, made first list of known elements, 23 total. By 1870, there were 70!23 total. By 1870, there were 70!

John Newlands, 1864—John Newlands, 1864—Law of Octaves: When element were Law of Octaves: When element were placed in order of increasing atomic mass, every 8placed in order of increasing atomic mass, every 8thth element repeated properties.element repeated properties.

Lothar Meyer, 1869—Lothar Meyer, 1869—Periodic table based on physical Periodic table based on physical characteristics only and increasing atomic mass.characteristics only and increasing atomic mass.

Dmitri Mendeleev, 1869—Dmitri Mendeleev, 1869—Periodic table based on physical Periodic table based on physical and chemical characteristics and increasing atomic mass. and chemical characteristics and increasing atomic mass. Predicted new elements.Predicted new elements.

Henry Moseley, 1913—Henry Moseley, 1913—Modern periodic law based on Modern periodic law based on subatomic particles: subatomic particles: There is a periodic repetition of There is a periodic repetition of chemical and physical properties of the elements when chemical and physical properties of the elements when they are arranged by increasing atomic number (protons).they are arranged by increasing atomic number (protons).

Periodic TrendsPeriodic Trends

Periodic TrendsPeriodic Trends

GroupsGroups Periods Periods

Periodic TrendsPeriodic Trends

Periodic TrendsPeriodic Trends

Periodic TrendsPeriodic Trends