Chapter 4Chapter 4Atomic TheoryAtomic Theory

Democritus hypothesis of the atomDemocritus hypothesis of the atomDalton’s atomic theoryDalton’s atomic theorySubatomic particlesSubatomic particlesRutherford’s atomic modelRutherford’s atomic modelAtomic mass of an elementAtomic mass of an elementMass number of an elementMass number of an elementIsotopesIsotopes

The AtomThe AtomIn chapters 1-3, we described matter in terms of In chapters 1-3, we described matter in terms of

physical and chemical propertiesphysical and chemical propertiesSpecifically, we explained matter with regards Specifically, we explained matter with regards

to its compositionto its compositionWhen we referred to composition, we were When we referred to composition, we were

actually referring to the actually referring to the ATOMATOM..Definition of an atom: A particle of matter that Definition of an atom: A particle of matter that

uniquely defines a chemical elementuniquely defines a chemical elementTherefore, Therefore, an atom is the basis for the an atom is the basis for the

composition of mattercomposition of matter..

Early Models of the AtomEarly Models of the AtomDemocritus (Greek: ca. 400 B.C.)Democritus (Greek: ca. 400 B.C.) Matter---composed of tiny Matter---composed of tiny

indestructible particles indestructible particles (Democritus’s hypothesis)(Democritus’s hypothesis)

Atoms remain unaltered, but, Atoms remain unaltered, but, move about in space to combine move about in space to combine in various ways to form all in various ways to form all macroscopic objectsmacroscopic objects

No No experimentsexperiments to prove to prove existence of atomsexistence of atoms

Hypothesis did not explain Hypothesis did not explain chemical behaviorchemical behavior

Among the first to claim atoms Among the first to claim atoms are realare real

Dalton (English: ca. 1800)Dalton (English: ca. 1800) Transformed Democritus’s Transformed Democritus’s

hypothesis into theoryhypothesis into theory Used an experimental approachUsed an experimental approach Linked macroscopic changes to Linked macroscopic changes to

activity at atomic levelactivity at atomic level Developed a system of chemical Developed a system of chemical

symbols based on atomic masssymbols based on atomic mass Proposed atoms determine the Proposed atoms determine the

composition of mattercomposition of matter Chemical combination of Chemical combination of

different elements occur in different elements occur in simple numerical ratios by simple numerical ratios by weightweight

Dalton’s atomic theory is the Dalton’s atomic theory is the basis for modern atomic theorybasis for modern atomic theory

Structure of the nuclear atomStructure of the nuclear atomSubatomic particlesSubatomic particles

Subatomic particlesSubatomic particlesOne major change in Dalton’s theory One major change in Dalton’s theory

is that atoms can be broken down into:is that atoms can be broken down into:1. Electrons (1. Electrons (negatively chargednegatively charged) )

2. Protons (2. Protons (positively chargedpositively charged))

3. Neutrons (3. Neutrons (neutralneutral))

ElectronsElectrons

Discovery of the electron is attributed to J. J. Discovery of the electron is attributed to J. J. Thomson (English physicist; 1856-1940)Thomson (English physicist; 1856-1940)

He discovered electrons using a Cathode ray He discovered electrons using a Cathode ray tubetube

Measured mass-to-charge ratio (m/z) of Measured mass-to-charge ratio (m/z) of particles emitted after using a variety of gases particles emitted after using a variety of gases and electrodesand electrodes

He found that the m/z ratio of the particle did He found that the m/z ratio of the particle did not change in all the different gases he usednot change in all the different gases he used

Cathode Ray Tube (CRT)Cathode Ray Tube (CRT)

ElectronsElectrons (continued) (continued)

Conclusion:Conclusion: electrons are parts of atoms electrons are parts of atoms of all elementsof all elements

Showed electrons were negatively Showed electrons were negatively chargedcharged

Proposed “Plum pudding” model of the Proposed “Plum pudding” model of the atom: electrons are embedded in a sphere atom: electrons are embedded in a sphere of positive chargeof positive charge

Ernest Rutherford (ca. 1870)Ernest Rutherford (ca. 1870)• Student of J. J. ThomsonStudent of J. J. Thomson

• J. J. Thomson: an atom was a solid sphere of J. J. Thomson: an atom was a solid sphere of positive chargepositive charge

• Electrons, embedded in the sphere like the seeds Electrons, embedded in the sphere like the seeds in a watermelon.in a watermelon.

• Rutherford disproved the Plum pudding modelRutherford disproved the Plum pudding model

                                                                                                  

                                                                            

Rutherford Gold Foil ExperimentRutherford Gold Foil Experiment

Ernest RutherfordErnest Rutherford (continued) (continued)Theory: i) Theory: i) Atoms are mostly empty spaceAtoms are mostly empty space

ii) ii) Most of mass centralized in a very small area Most of mass centralized in a very small area called nucleus called nucleus

Rutherford’s model known as the Rutherford’s model known as the NUCLEAR ATOMNUCLEAR ATOMNuclear atom: protons and neutrons in nucleusNuclear atom: protons and neutrons in nucleusElectrons are distributed around the nucleusElectrons are distributed around the nucleusElectrons occupy almost all the volume of the atomElectrons occupy almost all the volume of the atomBased on the model, nucleus is very tiny compared to the Based on the model, nucleus is very tiny compared to the

entire atom (football stadium vs. a marble)entire atom (football stadium vs. a marble)

The Atom (Rutherford Model)The Atom (Rutherford Model)

AtomicAtomic and and Mass numberMass number Atomic numberAtomic number: Number of protons (and electrons) in an : Number of protons (and electrons) in an

atomatom Mass numberMass number: Total number of particles (protons and : Total number of particles (protons and

neutrons) in an atom's nucleus. neutrons) in an atom's nucleus. Mass #Mass # = (# of Protons) + (Number of Neutrons) = (# of Protons) + (Number of Neutrons) For krypton, this equation becomes:For krypton, this equation becomes:

83.80 = 84 = (Number of Protons) + (Number of Neutrons)83.80 = 84 = (Number of Protons) + (Number of Neutrons) # of Neutrons = 84 – (# of Protons) # of Neutrons = 84 – (# of Protons) # of Neutrons = 84 – 36 = 48# of Neutrons = 84 – 36 = 48

shorthand notation

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AtomicAtomic and and Mass numberMass number

In the shorthand notation, in Krypton for In the shorthand notation, in Krypton for example, the atomic number is usually example, the atomic number is usually listed on the top left corner of the box and listed on the top left corner of the box and the mass number below the element the mass number below the element symbol symbol

Note:Note: Mass number is always larger Mass number is always larger than the atomic numberthan the atomic number

IsotopesIsotopes IsotopesIsotopes = atoms with the same atomic number but different = atoms with the same atomic number but different

neutronsneutrons Have different mass numbersHave different mass numbers Because isotopes contain the same # of electrons and protonsBecause isotopes contain the same # of electrons and protons

they have similar chemical propertiesthey have similar chemical properties This is because electrons determine the chemical reactivity of This is because electrons determine the chemical reactivity of

elementselements

To calculate the relative mass (mass #) of an element, e.g. chlorine, based on the abundance of its isotopes:Mass # = (35 * 0.7577) + (37 * 0.2423) = 26.5195 + 8.9651 = 35.4846 = 35.49 (35.5) amu

Atomic, Mass number, Isotopes (cont.)Atomic, Mass number, Isotopes (cont.)

In Summary...In Summary...• For any element:For any element:• Number of Protons = Atomic NumberNumber of Protons = Atomic Number• Number of Electrons = Number of Protons = Atomic NumberNumber of Electrons = Number of Protons = Atomic Number• Number of Neutrons = Mass Number - Atomic NumberNumber of Neutrons = Mass Number - Atomic Number• For krypton:For krypton:• Number of Protons = Atomic Number = 36Number of Protons = Atomic Number = 36• Number of Electrons = Number of Protons = Atomic Number Number of Electrons = Number of Protons = Atomic Number

= 36= 36• Number of Neutrons = Mass Number - Atomic Number = 84 - Number of Neutrons = Mass Number - Atomic Number = 84 -

36 = 4836 = 48