To Understand The Atom Is To Understand The Atom Is To Understand ChemistryTo Understand Chemistry

Learning ObjectivesLearning Objectives

Describe the three particles in the atomDescribe the three particles in the atom Define atomic number and mass numberDefine atomic number and mass number Describe isotopesDescribe isotopes Write symbols for elementsWrite symbols for elements Determine the numbers of particles in any Determine the numbers of particles in any

atom from the element symbolatom from the element symbol Determine average atomic mass from isotope Determine average atomic mass from isotope

distributiondistribution

Atoms are not the Atoms are not the smallest thingsmallest thing

Growing evidence for the divisibility Growing evidence for the divisibility of the indivisibleof the indivisible

Electrostatics and electricityElectrostatics and electricity

Static electricity was observed by Static electricity was observed by Thales (300 BC). Some Thales (300 BC). Some “charged” objects repel and “charged” objects repel and others attractothers attract

The voltaic cell (Volta, 18The voltaic cell (Volta, 18thth century) generated electrical century) generated electrical current from chemical reactionscurrent from chemical reactions

Mechanical electrical generation Mechanical electrical generation was achieved in 1825was achieved in 1825

The point: Atoms are neutral. If The point: Atoms are neutral. If indivisible, where do electrical indivisible, where do electrical charges come from?charges come from?

Faraday’s prescienceFaraday’s prescience

““Although we know nothing of what an atom Although we know nothing of what an atom is, we cannot resist forming some idea of a is, we cannot resist forming some idea of a small particle; and though we are in equal small particle; and though we are in equal ignorance of electricity, there is an immensity ignorance of electricity, there is an immensity of facts which justify us in believing that the of facts which justify us in believing that the atoms of matter are associated with electrical atoms of matter are associated with electrical powers to which they owe their most striking powers to which they owe their most striking qualities, and amongst them their chemical qualities, and amongst them their chemical affinity.”affinity.”

Ray of hopeRay of hope

18581858 Discovery of cathode rays by Julius PluckerDiscovery of cathode rays by Julius Plucker

Application of a large voltage across an evacuated Application of a large voltage across an evacuated tube causes a current to flow. The current flow is tube causes a current to flow. The current flow is accompanied by radiation from the excited gas accompanied by radiation from the excited gas moleculesmolecules

How does the neutral and indivisible atom create a How does the neutral and indivisible atom create a charge?charge?

Cathode rays are negatively charged particlesCathode rays are negatively charged particles18971897.J. Thomson demonstrates that cathode rays consist of negatively charged .J. Thomson demonstrates that cathode rays consist of negatively charged

particles. The first sighting of the electron: a particle much smaller than an atom.particles. The first sighting of the electron: a particle much smaller than an atom.

The Thomson model of the atomThe Thomson model of the atom

"I regard the atom as containing a large number of smaller bodies "I regard the atom as containing a large number of smaller bodies which I will call corpuscles, these corpuscles are equal to each which I will call corpuscles, these corpuscles are equal to each other.... In the normal atom, this assemblage of corpuscles other.... In the normal atom, this assemblage of corpuscles forms a system which is electrically neutral. Though the forms a system which is electrically neutral. Though the individual corpuscles behaveindividual corpuscles behave

like negative ions, yet when they arelike negative ions, yet when they are

assembled in a neutral atom the negativeassembled in a neutral atom the negative

effect is balanced by something which effect is balanced by something which

causes the space through which the corpuscles are spread to act causes the space through which the corpuscles are spread to act as if it had a charge of positive electricity equal in amount to as if it had a charge of positive electricity equal in amount to the sum of the negative charges of the corpuscles…”the sum of the negative charges of the corpuscles…”

X-rays and atoms’ invisible raysX-rays and atoms’ invisible rays

18951895 Wilhelm Conrad Roentgen discovers X-rays Wilhelm Conrad Roentgen discovers X-rays

while doing experiments with cathode rayswhile doing experiments with cathode rays

Radioactivity: the search for Radioactivity: the search for invisible raysinvisible rays

18961896 Henri Becquerel discovered radioactivity, Henri Becquerel discovered radioactivity,

which suggested that some atoms were capable which suggested that some atoms were capable of decomposing to give smaller particles.of decomposing to give smaller particles.

19031903 Frederick Soddy and William Ramsey Frederick Soddy and William Ramsey

demonstrated that uranium decayed to give demonstrated that uranium decayed to give helium. Direct proof that atoms were helium. Direct proof that atoms were divisible.divisible.

1909 1909 Ernest Rutherford’s gold foil Ernest Rutherford’s gold foil experiment experiment

The atom’s inner secrets exposed by its The atom’s inner secrets exposed by its own offspringown offspring

Hierarchy of discoveryHierarchy of discovery

The path to knowledge occurs in steps, each The path to knowledge occurs in steps, each depending on a previous advancedepending on a previous advance

The battery and electricity: Volta 1799

Cathode rays: Plucker 1858

X-rays: Roentgen 1895

Radioactivity: Becquerel 1896

The nucleus: Rutherford 1909

The nucleusThe nucleus

TinyTiny Incredibly dense – contains all the mass Incredibly dense – contains all the mass

of the atomof the atom Positively chargedPositively charged Contains protons (charged) and neutrons Contains protons (charged) and neutrons

(neutral) – not discovered until much (neutral) – not discovered until much laterlater

Summary of Atom PicturesSummary of Atom Pictures Dalton: Indivisible atomDalton: Indivisible atom Thomson: ElectronsThomson: Electrons Rutherford: NucleusRutherford: Nucleus

Comparison of subatomic particlesComparison of subatomic particles

Atoms are neutral:Atoms are neutral: # electrons = # protons # electrons = # protons

The number of protons in the nucleus of an atom The number of protons in the nucleus of an atom determines the atomic number (Z) and indicates the determines the atomic number (Z) and indicates the element's identity. For a neutral atom, the atomic element's identity. For a neutral atom, the atomic number also describes the number of electrons around number also describes the number of electrons around the nucleus.the nucleus.

Variations on the number of neutrons in the Variations on the number of neutrons in the nucleus give rise to different nucleus give rise to different isotopesisotopes of the of the

same element.same element.

Element notation: Element notation: AtomicAtomic number number and and massmass number number

136C

Mass number: number of protons

+ neutrons

Atomic number: number of protons

Element symbol

Counting particles:Counting particles: Number of electrons = number of protons = 6Number of electrons = number of protons = 6 Number of neutrons = mass number – atomic number (13 – Number of neutrons = mass number – atomic number (13 –

6 = 7)6 = 7)

Atomic massAtomic mass

Atoms are very small and so normal units of Atoms are very small and so normal units of mass are inconvenient for describing atomic mass are inconvenient for describing atomic massmass

Atomic Mass UnitAtomic Mass Unit Mass of one atom of carbon-12 = 12 amuMass of one atom of carbon-12 = 12 amu 1 amu = 1.660 x 101 amu = 1.660 x 10-24-24 g g

Similarity of atomic mass and mass Similarity of atomic mass and mass numbernumber

Almost all mass is Almost all mass is concentrated in the concentrated in the nucleonsnucleons

Means mass Means mass number (integer) number (integer) and atomic mass are and atomic mass are very similarvery similar

Isotopes cause disparityIsotopes cause disparity

Carbon has two isotopes: C-12 and C-13Carbon has two isotopes: C-12 and C-13 Atomic mass C-12 = 12 amu; C-13 = 13.0034 Atomic mass C-12 = 12 amu; C-13 = 13.0034

amuamu Atomic mass C is weighted average of the Atomic mass C is weighted average of the

masses of the two isotopes (98.89 % C-12 + masses of the two isotopes (98.89 % C-12 + 1.11 % C-131.11 % C-13

= 12.011 amu= 12.011 amu

What of the electrons?What of the electrons?

We now understand the atom to contain a tiny We now understand the atom to contain a tiny positively charged massive nucleus surrounded positively charged massive nucleus surrounded by a comparatively vast empty space by a comparatively vast empty space containing the electronscontaining the electrons

When atoms combine the electrons must When atoms combine the electrons must interactinteract

We need to understand the arrangement of We need to understand the arrangement of electrons in the atomelectrons in the atom

Rudiments of elementsRudiments of elements All of matter is made from elements, in combination with All of matter is made from elements, in combination with

other elements or alone.other elements or alone. There are some ninety There are some ninety naturallynaturally occurring – have “always” occurring – have “always”

been.been. More than twenty have been created artificiallyMore than twenty have been created artificially

Elements organized – a triumph of Elements organized – a triumph of chemistrychemistry

The periodic table of the elements is organized into 18 The periodic table of the elements is organized into 18 groupsgroups and 7 and 7 periodsperiods. Elements are represented by one- or two-letter symbols and are . Elements are represented by one- or two-letter symbols and are arranged according to atomic number.arranged according to atomic number.

The Periodic Table: Groups and PeriodsThe Periodic Table: Groups and PeriodsGroups: columns of

elements

Periods: rows of elements

Elements in groups have similar Elements in groups have similar propertiesproperties

One of the eight main groups of elements, Group 1A One of the eight main groups of elements, Group 1A is known as the alkali metals group. Elements in this is known as the alkali metals group. Elements in this group are highly reactive and form alkaline (basic) group are highly reactive and form alkaline (basic) solutions with water.solutions with water.

All groups have characteristic All groups have characteristic propertiesproperties

One of the eight main groups of elements, Group 8A is known One of the eight main groups of elements, Group 8A is known as the noble gas group. Elements in this family have such a as the noble gas group. Elements in this family have such a low reactivity that they were formerly known as the inert low reactivity that they were formerly known as the inert gases.gases.

Although their chemical properties are very uninteresting, Although their chemical properties are very uninteresting, their importance to understanding chemistry cannot be their importance to understanding chemistry cannot be understatedunderstated

Group 7A – the halogensGroup 7A – the halogens

Reactive nonmetalsReactive nonmetals Fluorine and chlorine are gasesFluorine and chlorine are gases Bromine liquidBromine liquid Iodine solidIodine solid