Atoms are not the Atoms are not the smallest thingsmallest thing

Growing evidence for the Growing evidence for the divisibility of the indivisibledivisibility of the indivisible

Atoms are not the Atoms are not the smallest thingsmallest thing

Growing evidence for the Growing evidence for the divisibility of the indivisibledivisibility of the indivisible

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

Electrostatics and electricityElectrostatics and electricity Static electricity was observed by Thales (300 BC). Static electricity was observed by Thales (300 BC).

Some “charged” objects repel and others attractSome “charged” objects repel and others attract The voltaic cell (Volta, 18The voltaic cell (Volta, 18thth century) generated century) generated

electrical current from chemical reactionselectrical current from chemical reactions Mechanical electrical generation was achieved in Mechanical electrical generation was achieved in

18251825 The point: Atoms are neutral. If indivisible, where do The point: Atoms are neutral. If indivisible, where do

electrical charges come from?electrical 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 electronsCathode “rays” are electrons18971897. J.J. Thomson demonstrates that cathode rays are negatively charged . J.J. Thomson demonstrates that cathode rays are negatively charged

particles, which have a much smaller mass than an atom. The first sighting of the particles, which have a much smaller mass than an atom. The first sighting of the electron.electron.

The Thomson :Plum Pudding” The Thomson :Plum Pudding” modelmodel

"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 Wilhelm Conrad

Roentgen discovers X-Roentgen discovers X-rays while doing rays while doing experiments with experiments with cathode rayscathode rays

Radioactivity: invisible rays and Radioactivity: invisible rays and unstable atomsunstable atoms

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 own The atom’s inner secrets exposed by its own offspringoffspring

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 advanceThe 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

Atomic number (Z) = number of protons in nucleusAtomic number (Z) = number of protons in nucleus Each element has unique atomic number Each element has unique atomic number For neutral atom, atomic number equals number of For neutral atom, atomic number equals number of

electrons around nucleus.electrons around nucleus.

Isotopes and the mass numberIsotopes and the mass number

Mass number = protons Mass number = protons plusplus neutrons neutrons IsotopesIsotopes have same atomic number, different have same atomic number, different

mass numbermass number

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)

Ions: losing and gaining electronsIons: losing and gaining electrons

Atoms can lose or gain electronsAtoms can lose or gain electrons Atomic number remains the sameAtomic number remains the same Loss: Positive ions have fewer electrons than Loss: Positive ions have fewer electrons than

protonsprotons Gain: Negative ions have more electrons than Gain: Negative ions have more electrons than

protonsprotons

What of the electrons?What of the electrons? We now understand atom contains tiny We now understand atom contains tiny

positively charged massive nucleus surrounded positively charged massive nucleus surrounded by vast empty space containing electronsby vast empty space containing 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