Chapter 4Chapter 4

Structure of the AtomStructure of the Atom

HistoryHistory

In the 1800’s, early In the 1800’s, early philosophers believed all philosophers believed all matter consisted of matter consisted of either air, earth, water, either air, earth, water, or fire.or fire.

HistoryHistoryDemocritusDemocritus

Coined the term atomCoined the term atomBelieved atoms consisted mostly of empty Believed atoms consisted mostly of empty

spacespaceDifferent properties of matter are due to Different properties of matter are due to

the size shape and movement of the the size shape and movement of the atoms atoms

HistoryHistoryDaltonDalton

Proposed first Atomic TheoryProposed first Atomic Theory1. 1. Atoms Atoms All matter is composed of very All matter is composed of very

small particles called atomssmall particles called atoms2. 2. Identical Identical All atoms of a given element All atoms of a given element

are identical are identical 3. 3. Created Created Atoms cannot be created, Atoms cannot be created,

divided into smaller particles, or destroyeddivided into smaller particles, or destroyed

HistoryHistoryDaltonDalton

4. 4. CompoundsCompounds Atoms combine in simple Atoms combine in simple whole number ratios to form compoundswhole number ratios to form compounds

Dalton was a school teacherDalton was a school teacherLead to advancement in chemistryLead to advancement in chemistryPart of his theory was proven incorrect Part of his theory was proven incorrect

upon later experimentationupon later experimentation

Law of Definite ProportionsLaw of Definite Proportionsby Joseph Proustby Joseph Proust

Same as 4Same as 4thth postulate of Dalton’s Atomic postulate of Dalton’s Atomic TheoryTheory

Regardless of the amount, a compound of Regardless of the amount, a compound of the same elements is in the same the same elements is in the same proportion by massproportion by mass

Subatomic Particles and the Subatomic Particles and the Nuclear AtomNuclear Atom

The electron was discovered by J.J. The electron was discovered by J.J. Thomson in the 1890’s.Thomson in the 1890’s.

1. used a cathode ray tube determine the 1. used a cathode ray tube determine the electronelectron

2. was able to determine charge to mass 2. was able to determine charge to mass ratio but was unable to calculate exact ratio but was unable to calculate exact chargecharge

Subatomic Particles and the Subatomic Particles and the Nuclear AtomNuclear Atom

Millikan determine the Millikan determine the charge of an electron using charge of an electron using an oil drop and a charged an oil drop and a charged plateplate

Millikan’s results were within Millikan’s results were within one percnet of today’s one percnet of today’s accepted valueaccepted value

Subatomic Particles and the Subatomic Particles and the Nuclear AtomNuclear Atom

Thomson proposed the plum Thomson proposed the plum pudding model of the atompudding model of the atom

Atom model was spherically Atom model was spherically shaped with uniformly shaped with uniformly distributed positive charge distributed positive charge with individual negatively with individual negatively charged electrons attachedcharged electrons attached

Rutherford Proposes a New AtomRutherford Proposes a New Atom

Rutherford experimented with alpha Rutherford experimented with alpha particle reacting with matterparticle reacting with matter

Other scientists tried to see if alpha Other scientists tried to see if alpha particles would pass through a gold foil particles would pass through a gold foil sheet.sheet.

almost all of the alpha particles went almost all of the alpha particles went through the gold foil as if it were not even through the gold foil as if it were not even there.there.

Rutherford Proposes a New AtomRutherford Proposes a New Atom

some of the alpha some of the alpha particles were deflected particles were deflected only slightly, usually 2° or only slightly, usually 2° or less.less.

a very, very few (1 in a very, very few (1 in 8000 for platinum foil) 8000 for platinum foil) alpha particles were alpha particles were turned through an angle turned through an angle of 90° or more. of 90° or more. (Rutherford cites 1 in (Rutherford cites 1 in 20,000 for gold in his 20,000 for gold in his 1911 paper.)1911 paper.)

Rutherford Proposes a New AtomRutherford Proposes a New Atom

From these results he found he concluded:From these results he found he concluded:Atoms consisted mostly of empty space Atoms consisted mostly of empty space

through which electrons movethrough which electrons moveThere is a tiny dense region called the There is a tiny dense region called the

nucleus which has a positive chargenucleus which has a positive chargeConcludes most of the atom’s mass is Concludes most of the atom’s mass is

contained in the nucleuscontained in the nucleus

Nucleus of the AtomNucleus of the Atom

1920 - Rutherford coined the term proton 1920 - Rutherford coined the term proton for the positively charged part of the for the positively charged part of the nucleusnucleus

1932 - James Chadwick showed that the 1932 - James Chadwick showed that the nucleus also contained a neutrally charged nucleus also contained a neutrally charged part of the nucleus called the neutronpart of the nucleus called the neutron

Subatomic ParticlesSubatomic Particles

ParticleParticle SymbolSymbol ChargeCharge Mass Mass (amu)(amu)

NeutronNeutron nn00 00 11

ProtonProton pp++ +1+1 11

ElectronElectron ee-- -1-1 00

How Atoms DifferHow Atoms Differ

Atomic Number- The number of protons Atomic Number- The number of protons in an atom is the atomic number. In an in an atom is the atomic number. In an element, the number of protons equals element, the number of protons equals the number of electronsthe number of electrons

A = P = EA = P = EAll atoms of same element have same All atoms of same element have same

number of protonsnumber of protons

How Atoms differHow Atoms differ

Isotopes – Atoms of the same element Isotopes – Atoms of the same element with different number of neutronswith different number of neutrons

Mass Number – The sum of number of Mass Number – The sum of number of protons and neutronsprotons and neutrons

How to find the number of How to find the number of neutrons.neutrons.

Mass # = # of nMass # = # of n0 0 + # of p+ # of p++

Hence, nHence, n0 0 = mass # - # of p= mass # - # of p++

Example: How many neutrons are in Mercury-Example: How many neutrons are in Mercury-204?204?

First find the atomic # to find the number of First find the atomic # to find the number of protonsprotons

Next use the equation nNext use the equation n0 0 = mass # - # of p= mass # - # of p++

nn0 0 = 204 – 80= 204 – 80 nn0 0 = 124= 124

Example ProblemExample Problem

How many protons, electrons, and How many protons, electrons, and neutrons are in a Carbon isotope with a neutrons are in a Carbon isotope with a mass number of 12?mass number of 12?

If a Zinc isotope has 30 protons and 34 If a Zinc isotope has 30 protons and 34 neutrons, what is the atomic number and neutrons, what is the atomic number and mass number?mass number?

Nuclide numberNuclide number

is a symbol that notes the number of is a symbol that notes the number of protons and neutrons of a specific atomprotons and neutrons of a specific atom

1414 Mass #Mass #

CC ElementElement

66 Atomic #Atomic #

Atomic MassAtomic Mass

The atomic mass is the bottom number The atomic mass is the bottom number below the element on the periodic table.below the element on the periodic table.

The atomic mass is the weighted average The atomic mass is the weighted average mass of the isotopes of that elementmass of the isotopes of that element

Atomic MassAtomic Mass

Example: Chlorine’s atomic mass is 35.453 amuExample: Chlorine’s atomic mass is 35.453 amu Chlorine exists naturally as a mixture of 75% Chlorine exists naturally as a mixture of 75%

chlorine-35 and 25% chlorine-37chlorine-35 and 25% chlorine-37 To calculate atomic mass multiply the mass of To calculate atomic mass multiply the mass of

each isotope by it’s relative abundance and add each isotope by it’s relative abundance and add them together.them together.

0.75(35amu) + 0.25(37amu)= atomic mass0.75(35amu) + 0.25(37amu)= atomic mass 26.25amu + 9.25amu = atomic mass26.25amu + 9.25amu = atomic mass 35.5amu = atomic mass35.5amu = atomic mass

IonsIons

Ion : atom which have gained or lost Ion : atom which have gained or lost electronselectrons

Cation : contains fewer electrons than a Cation : contains fewer electrons than a neutral atom, therefore, it has a positive neutral atom, therefore, it has a positive charge.charge.

Anion: contains more electrons than a Anion: contains more electrons than a neutral atom, therefore, it has a positive neutral atom, therefore, it has a positive charge.charge.

Nuclear ForcesNuclear Forces

You know protons and neutrons make up You know protons and neutrons make up the nucleus,the nucleus,

You also know protons are positively You also know protons are positively charged, and like charges repel each charged, and like charges repel each other.other.

Therefore, some force must hold the Therefore, some force must hold the nucleus together.nucleus together.

Nuclear ForcesNuclear Forces

Strong Force Strong Force – Force which holds the protons – Force which holds the protons and neutrons in the nucleus together.and neutrons in the nucleus together.

Relatively strong force which acts over a very Relatively strong force which acts over a very short range short range

100 times stronger than electromagnetic force100 times stronger than electromagnetic force Electromagnetic force has no limit on distance, Electromagnetic force has no limit on distance,

unlike strong force, therefore, protons which are unlike strong force, therefore, protons which are far apart repel each other.far apart repel each other.

Nuclear ForcesNuclear Forces

The larger the atom the weaker the strong The larger the atom the weaker the strong force.force.

Therefore, larger atoms tend to have more Therefore, larger atoms tend to have more nuclear decaynuclear decay

Also, atoms with certain numbers of Also, atoms with certain numbers of protons tend to decay. If they have the protons tend to decay. If they have the same number of protons and neutrons same number of protons and neutrons they tend to be more stable.they tend to be more stable.

Unstable Nuclei and Radioactive Unstable Nuclei and Radioactive DecayDecay

Nuclear Reactions : Reactions which Nuclear Reactions : Reactions which involve a change in an atom’s nucleusinvolve a change in an atom’s nucleus

Radiation : Rays and particles emitted by Radiation : Rays and particles emitted by the radioactive elementsthe radioactive elements

Radioactivity: Substances spontaneously Radioactivity: Substances spontaneously emitted radiation emitted radiation

Types of RadiationTypes of Radiation 1. Alpha Radiation : an alpha particle emitted from 1. Alpha Radiation : an alpha particle emitted from

radioactive nuclei, consists of 2 protons and 2 neutrons, radioactive nuclei, consists of 2 protons and 2 neutrons, but no electronsbut no electrons

Not very harmfulNot very harmful Large atoms are not very stable and need to decrease Large atoms are not very stable and need to decrease

massmass What is the charge and mass of an alpha particle?What is the charge and mass of an alpha particle? +2 and 4amu+2 and 4amu ExampleExample 226226

8888Ra Ra → → 2222228686Rn + Rn + 44

22HeHe RadiumRadium Radon Radon Alpha ParticleAlpha Particle

Types of RadiationTypes of Radiation Beta Radiation: fast moving electron emitted from a Beta Radiation: fast moving electron emitted from a

radioactive element called a Beta Particle.radioactive element called a Beta Particle. Can cause serious health problems especially in bonesCan cause serious health problems especially in bones Atoms want to have a 1 : 1 neutron to proton ratioAtoms want to have a 1 : 1 neutron to proton ratio Beta emission is used to decrease the neutron to proton Beta emission is used to decrease the neutron to proton

ratio.ratio. What is the charge and mass on a beta particle?What is the charge and mass on a beta particle? -1 and 0-1 and 0 1414

66C C → → 141477N + N + 00

-1-1ββ

Types of RadiationTypes of Radiation

Gamma Radiation: Gamma rays are Gamma Radiation: Gamma rays are released from radioactive nuclei. released from radioactive nuclei.

Gamma rays have no mass or charge.Gamma rays have no mass or charge.Gamma rays are very harmful and have a Gamma rays are very harmful and have a

very high energyvery high energy