Atomic StructureAtomic StructureBelieving What You Cannot SeeBelieving What You Cannot See

Early Models of the AtomEarly Models of the AtomDemocritus (4Democritus (4thth Century BC) Century BC)

Was the first to propose the idea of atoms.Was the first to propose the idea of atoms.Not an experimental model.Not an experimental model.

Dalton’s Atomic TheoryDalton’s Atomic TheoryAll matter is composed of atomsAll matter is composed of atomsAtoms of different elements differAtoms of different elements differChemical change involves a rearrangement of Chemical change involves a rearrangement of

atomsatomsAtoms are indivisibleAtoms are indivisible

What is an Atom?What is an Atom?

AtomAtom= the smallest = the smallest particle of an element particle of an element that retains the that retains the properties of that properties of that element. The atom is the element. The atom is the representative particle of representative particle of any element.any element.

Just How Small is an Atom?Just How Small is an Atom? If an apple was magnified to the size of the If an apple was magnified to the size of the

Earth, then the atoms in the apple would be Earth, then the atoms in the apple would be approximately the size of the original apple.approximately the size of the original apple.

In 27 grams of aluminum there are 6.02 X 10In 27 grams of aluminum there are 6.02 X 1023 23

atoms of aluminum atoms of aluminum (602,000,000,000,000,000,000,000 atoms)(602,000,000,000,000,000,000,000 atoms)*This is equal to one mole of aluminum *This is equal to one mole of aluminum

(avogodro’s number)!!(avogodro’s number)!!

What are the component particles What are the component particles that scientists have discovered that scientists have discovered

within atoms?within atoms?

ElectronsElectrons

Electrons Electrons = negatively charged = negatively charged subatomic particles with nearly zero subatomic particles with nearly zero mass.mass.Located in orbitals within the electron cloud Located in orbitals within the electron cloud

around the nucleusaround the nucleusJ.J. Thomson experimentJ.J. Thomson experiment

-cathode ray tube -cathode ray tube

Cathode ray tube (CRT)Cathode ray tube (CRT)

Electrons were introduced to one end (cathode) and Electrons were introduced to one end (cathode) and were flung to the opposite end (anode) producing the were flung to the opposite end (anode) producing the cathode ray (a beam of electons). cathode ray (a beam of electons).

At first, many thought it was light, but the ray in the tube At first, many thought it was light, but the ray in the tube could be deflected by magnets disproving it was light. could be deflected by magnets disproving it was light.

Different metals and gases were used in the CRT, but Different metals and gases were used in the CRT, but the result was always the same; a cathode ray was the result was always the same; a cathode ray was produced. produced.

Scientists inferred that the particles making up the Scientists inferred that the particles making up the cathode ray were a part of all matter. This is how cathode ray were a part of all matter. This is how

electrons were discoveredelectrons were discovered..

Cathode Ray tubeCathode Ray tube

ProtonsProtons

ProtonsProtons= positively charged subatomic = positively charged subatomic particles (1,840 X the mass of an electron)particles (1,840 X the mass of an electron)Located in the nucleusLocated in the nucleusWith the neutron, accounts for the mass of the With the neutron, accounts for the mass of the

atomatomHas a mass of nearly 1 a.m.u.Has a mass of nearly 1 a.m.u.

NeutronsNeutrons

NeutronsNeutrons = subatomic particles with no = subatomic particles with no charge (mass nearly equal to a proton charge (mass nearly equal to a proton which is nearly 1 a.mu.)which is nearly 1 a.mu.)Located in the nucleusLocated in the nucleusWith the proton, accounts for the mass of the With the proton, accounts for the mass of the

atomatom

Chadwick confirmed the existence of the Chadwick confirmed the existence of the neutron (1932)neutron (1932)

The Atomic NucleusThe Atomic Nucleus

Nucleus= the central core of the atom, is Nucleus= the central core of the atom, is composed of protons and neutrons.composed of protons and neutrons.

Ernest Rutherford discovered the nucleus Ernest Rutherford discovered the nucleus in his famous gold foil experiment. (in his famous gold foil experiment. (video link)

Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

Positively charged radioactive particles Positively charged radioactive particles (alpha radiation) were shot at a thin piece (alpha radiation) were shot at a thin piece of gold foil. of gold foil. Why did some of the alpha particles bounce Why did some of the alpha particles bounce

back?back?Why did most of the alpha particles go Why did most of the alpha particles go

through?through?What did these findings prove about the atom?What did these findings prove about the atom?

The size of a nucleus to the electron cloud of an atom is similar to the size of one blade of grass in

reliant stadium to the size of the stadium

Distinguishing Between AtomsDistinguishing Between Atoms

All atoms are composed of electrons, All atoms are composed of electrons, protons, and neutrons. Therefore, how do protons, and neutrons. Therefore, how do you differentiate between atoms?...you differentiate between atoms?...

Atomic NumberAtomic Number

Each element is different due to varying Each element is different due to varying numbers of protonsnumbers of protons

Atomic NumberAtomic Number = the number of protons in = the number of protons in the nucleus of an atom.the nucleus of an atom.Example – because Oxygen has eight Example – because Oxygen has eight

protons, the atomic number is eightprotons, the atomic number is eight (If it has eight protons, how many electrons (If it has eight protons, how many electrons

are there? Why?)are there? Why?)

Atoms (with the exception of ions) do Atoms (with the exception of ions) do not have a charge, thus if you have 8 not have a charge, thus if you have 8 positively charged protons then you positively charged protons then you

must have 8 negatively charged must have 8 negatively charged electronselectrons

8+ protons8+ protons8- electrons8- electrons

0 charge0 charge

• Charged particleCharged particle = atom that has lost or = atom that has lost or gained electrons gained electrons

* When an atom loses an * When an atom loses an electron a positive ion (cation) is electron a positive ion (cation) is formedformed

* When an atom gains an * When an atom gains an electron a negative ion (anion) is electron a negative ion (anion) is formedformed

IonIon

PracticePractice

Boron has an atomic number of 5, how many Boron has an atomic number of 5, how many protons does it have? Electrons?protons does it have? Electrons?

Nitrogen has 7 electrons, how many protons Nitrogen has 7 electrons, how many protons does it have? What about neutrons? What is the does it have? What about neutrons? What is the net charge (total charge)?net charge (total charge)?

What is the net charge of an ion with 15 protons What is the net charge of an ion with 15 protons and 18 electrons?and 18 electrons?

What is the charge of an ion with that had a -3 What is the charge of an ion with that had a -3 charge and then lost 2 electrons?charge and then lost 2 electrons?

Mass NumberMass Number

Most of the mass of an atom is in the Most of the mass of an atom is in the nucleus where the protons and neutrons nucleus where the protons and neutrons are located.are located.

Mass NumberMass Number = = total number of protonstotal number of protons + + total number of neutronstotal number of neutrons If you are given the number of protons and If you are given the number of protons and

the mass number you can determine the the mass number you can determine the number of neutronsnumber of neutrons- Ex. Determine the number of protons and neutrons - Ex. Determine the number of protons and neutrons

in an atom of C-13in an atom of C-13

IsotopesIsotopes

Not all atoms of the same Not all atoms of the same element have the same element have the same

number of neutrons!number of neutrons!(These are called isotopes)(These are called isotopes)

IsotopesIsotopes

IsotopesIsotopes = atoms that have the same = atoms that have the same number of protons but different numbers of number of protons but different numbers of neutrons (thus they have different mass neutrons (thus they have different mass numbers)numbers)

Note: sometimes they refer to an isotope by Note: sometimes they refer to an isotope by it’s name and mass numberit’s name and mass number-Example: Carbon-14 (how many neutrons does this -Example: Carbon-14 (how many neutrons does this

particular isotope have?)particular isotope have?)

Atomic MassAtomic Mass

Atomic MassAtomic Mass = = average massaverage mass of the atoms of the atoms in a naturally occurring sample of the in a naturally occurring sample of the elementelement

(The (The average massaverage mass includes the mass and includes the mass and the natural percent abundance of the the natural percent abundance of the different isotopes).different isotopes).

Calculating Average atomic Calculating Average atomic massmass

Knowing that the atomic mass is an Knowing that the atomic mass is an average of the masses of it’s isotopes, you average of the masses of it’s isotopes, you can calculate the atomic mass based on can calculate the atomic mass based on its abundance.its abundance.Nitrogen-14 has an exact atomic mass of Nitrogen-14 has an exact atomic mass of

14.003074 and has an abundance of 99.63%. 14.003074 and has an abundance of 99.63%. Nitrogen-15 has an exact atomic mass of Nitrogen-15 has an exact atomic mass of 15.000108 and has an abundance of 0.37%. 15.000108 and has an abundance of 0.37%. Find its average atomic mass.Find its average atomic mass.

AnswerAnswer

(14.003074 x 0.9963) + (15.000108 x (14.003074 x 0.9963) + (15.000108 x 0.0037) = 14.007 0.0037) = 14.007

On the periodic table, what is the average On the periodic table, what is the average atomic mass of Nitrogen?atomic mass of Nitrogen?