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

The phenomenon of static electricity was The phenomenon of static electricity was known since ancient times. Certain “charged” known since ancient times. Certain “charged” objects repel and others attractobjects repel and others attract

The discovery of the voltaic cell allowed the The discovery of the voltaic cell allowed the harnessing of electrical current from chemical harnessing of electrical current from chemical activityactivity

Electrical generation was achieved in 1825Electrical generation was achieved in 1825 Atoms are neutral. If indivisible, with what Atoms are neutral. If indivisible, with what

are the electrical charges associated?are the electrical charges associated?

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 Cathode rays are negatively charged particlescharged particles

18971897.J. Thomson demonstrates that cathode rays .J. Thomson demonstrates that cathode rays consist of negatively charged particles, which have a mass consist of negatively charged particles, which have a mass very much less than that of the atom. The first sighting of very much less than that of the atom. The first sighting of

the electron.the electron.

The “oil drop” experiment The “oil drop” experiment gave a measurement of gave a measurement of

electron chargeelectron charge

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…”

A hierarchy of discoveryA hierarchy of discovery

Investigations of cathode rays led to the Investigations of cathode rays led to the discovery of X-rays in 1895 (Wilhelm discovery of X-rays in 1895 (Wilhelm Roentgen)Roentgen)

This new discovery inspired search for other This new discovery inspired search for other types of invisible emission by substancestypes of invisible emission by substances

The world would never be the same again…The world would never be the same again…

RadioactivityRadioactivity

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 Ernest Rutherford’s gold foil experiment with gold foil experiment with alphaalpha particles and the particles and the

discovery of the nucleus. discovery of the nucleus.

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

Atoms are mostly nothingAtoms are mostly nothing

Comparison of subatomic particlesComparison of subatomic particles

ParticleParticle

MassMass ChargeCharge

gramsgrams amuamu coulombscoulombs ee

ElectronElectron 9.1x109.1x10-28-28 5.5x105.5x10-4-4 -1.6x10-1.6x10-19-19 -1-1

ProtonProton 1.67x101.67x10-24-24 1.0071.007 +1.6x10+1.6x10-19-19 +1+1

NeutronNeutron 1.67x101.67x10-24-24 1.0091.009 00 00

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

different different isotopesisotopes of the same of the same element.element.

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

C126

Mass number – protons + neutrons

Atomi number – protons

Element symbol

Isotopes and relative atomic massesIsotopes and relative atomic masses It is important to be able to calculate the It is important to be able to calculate the

average mass of a mixture of isotopes of a average mass of a mixture of isotopes of a given elementgiven element We need the distribution of the isotopes and their We need the distribution of the isotopes and their

mass numbersmass numbers For chlorine there are isotopes with mass numbers For chlorine there are isotopes with mass numbers

35 and 37, with relative abundances of 75.8 % and 35 and 37, with relative abundances of 75.8 % and 24.2 % respectively24.2 % respectively

Average atomic mass =Average atomic mass =5.35)242.037()758.035( xx

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