Chapter Chapter 55

Atomic Atomic Structure & the Structure & the Periodic TablePeriodic Table

Sec 1: AtomsSec 1: AtomsDemocritus was the first to suggest the idea of Democritus was the first to suggest the idea of atoms.atoms.

The modern idea of the atom started with John The modern idea of the atom started with John Dalton.Dalton.

Dalton’s Atomic Dalton’s Atomic TheoryTheoryDalton’s Atomic Theory:Dalton’s Atomic Theory:

1.1. All elements are composed of tiny All elements are composed of tiny particles called atomsparticles called atoms

2.2. Atoms of the same element are identical, atoms of Atoms of the same element are identical, atoms of one element are different than other elements.one element are different than other elements.

3.3. Atoms of different elements can physically mix or Atoms of different elements can physically mix or chemically combine in whole-number ratios to chemically combine in whole-number ratios to form compounds.form compounds.

4.4. Chemical reactions occur when atoms are Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of 1 separated, joined, or rearranged. Atoms of 1 element are never changed into another element.element are never changed into another element.

Dalton’s Atomic Dalton’s Atomic TheoryTheory

AtomsAtomsA sample of an element can A sample of an element can be broken down into smaller be broken down into smaller and smaller pieces—and smaller pieces—eventually it can’t be broken eventually it can’t be broken down any more.down any more.

AtomAtom—smallest particle of an —smallest particle of an element that retains the element that retains the properties of that element.properties of that element.

Sec 2: Nuclear Sec 2: Nuclear AtomAtomAtoms are now known to be divisible.Atoms are now known to be divisible.

Atoms are composed of electrons, protons, and Atoms are composed of electrons, protons, and neutrons.neutrons.

ElectronsElectrons—negatively charged subatomic particles.—negatively charged subatomic particles.

Electrons were discovered by J.J. Thomson using a Electrons were discovered by J.J. Thomson using a cathode ray.cathode ray.

Negative electrons are attracted to positively charged metal plates.

Protons & Protons & NeutronsNeutrons

Atoms are neutral; they have Atoms are neutral; they have no net charge.no net charge.

Protons—positively charged Protons—positively charged subatomic particles.subatomic particles.

Neutrons—subatomic Neutrons—subatomic particles with no charge.particles with no charge. Mass of neutron is about equal Mass of neutron is about equal

to proton.to proton.

electron

neutron

proton

NucleusNucleusNucleusNucleus—the central core of an atom —the central core of an atom composed of protons & neutrons.composed of protons & neutrons.

The nucleus was discovered by Ernest The nucleus was discovered by Ernest Rutherford using the gold foil experiment.Rutherford using the gold foil experiment.

Sec 3Sec 3: Distinguishing : Distinguishing Between AtomsBetween Atoms

Atomic Number—the number of protons Atomic Number—the number of protons in the nucleus of an atom of an element.in the nucleus of an atom of an element.

Every element has a different number of Every element has a different number of protons.protons.

RememberRemember: atoms are neutral.: atoms are neutral.

The number of electrons equal the The number of electrons equal the number of protons in a neutral atom.number of protons in a neutral atom.

Mass NumberMass NumberMass number—the total number of protons Mass number—the total number of protons and neutrons in an atomand neutrons in an atom Example: a helium atom has 2 protons and 2 Example: a helium atom has 2 protons and 2

neutrons. What is the mass number?neutrons. What is the mass number? 44

To find the number of neutrons in an atom:To find the number of neutrons in an atom: Number of neutrons = mass # – atomic #Number of neutrons = mass # – atomic #

Shorthand notation:Shorthand notation:

You can use the mass number to name specific atoms. Example: Helium-4

IsotopesIsotopesIsotopesIsotopes—Atoms that have the same —Atoms that have the same number of protons, but different numbers number of protons, but different numbers of neutrons.of neutrons.

Isotopes have different numbers of Isotopes have different numbers of neutrons, so they have different mass neutrons, so they have different mass numbers.numbers.

Atomic MassAtomic MassThe mass of a proton or neutron is The mass of a proton or neutron is 1.67 x 101.67 x 10-24-24 g. g.

The mass of an electron is 9.11 x 10The mass of an electron is 9.11 x 10-28-28 g. g. This is negligible in comparison.This is negligible in comparison.

A mass spectrometer can A mass spectrometer can be used to determine the be used to determine the tiny masses of atoms.tiny masses of atoms.

Atomic MassAtomic MassAtomic massAtomic mass—a weighted average mass —a weighted average mass of the atoms in a naturally occurring of the atoms in a naturally occurring sample of the element.sample of the element.

The atomic mass is usually closest to the The atomic mass is usually closest to the most abundant isotope of that element.most abundant isotope of that element.

Atomic MassAtomic MassTo calculate atomic mass, you need to knowTo calculate atomic mass, you need to know

1.1. The number of stable isotopes of the elementThe number of stable isotopes of the element2.2. The mass of each isotopeThe mass of each isotope3.3. The natural percent abundance of each isotope.The natural percent abundance of each isotope.

Multiply the atomic mass of each isotope by Multiply the atomic mass of each isotope by its abundance, then add the results.its abundance, then add the results.ExampleExample: Element B has 2 isotopes. The : Element B has 2 isotopes. The isotope with a mass of 10.012 amu has an isotope with a mass of 10.012 amu has an abundance of 19.92%. The isotope with a abundance of 19.92%. The isotope with a mass of 11.009 amu has an abundance of mass of 11.009 amu has an abundance of 80.09 %. What is the atomic mass?80.09 %. What is the atomic mass?

Calculating Atomic Calculating Atomic MassMass

10.012 amu x 0.1991 = 1.993 amu10.012 amu x 0.1991 = 1.993 amu

11.009 amu x 0.8009 = 11.009 amu x 0.8009 = 8.817 amu8.817 amu

atomic mass = 10.810 amuatomic mass = 10.810 amu

Done with

Section 3