Nivaldo J. Tro

http://www.cengage.com/chemistry/tro

Mark Erickson • Hartwick College

Chapter 3Atoms and Elements

The connection between the microscopic atom and the macroscopic element is the key to understanding the chemical world.

Understanding the Macroscopic World

Atoms• Atoms are incredibly small.

• An atom is the smallest identifiable unit of an element.

• Similarities and differences on the atomic scale correlate with similarities and differences on the macroscopic scale.

Atoms

Protons Determine the Element• The atomic number, Z, represents the number of protons in

the nucleus of an atom.

• The charge of a proton is assigned numerical value of +1.

• An amu is a unit of mass equivalent to 1/12 the mass of a carbon-12 nucleus and is equivalent to 1.66 × 10-24 g.

• Protons constitute a significant part of the mass of an atom.

– The mass of a proton is 1.0 amu.

Concept Check 2.1The graphite in a pencil and the gemstone diamond are composed entirely of pure carbon atoms. Even though these substances appear and behave very differently, what about the nucleus do these carbon atoms have in common that identify them as carbon?

Concept Check 2.1 Solution• Diamonds and graphite are composed of carbon

atoms with six protons in the nucleus.

• It is the number of protons in the nucleus that determine an element’s identity.

Chemical Symbols• Symbols are an abbreviation of the

chemical name which can be based on:

– The English name for the element: hydrogen H

– The Greek or Latin name: ferrum (iron) Fe

– Their place of discovery: europium Eu

– The scientist honored by the naming: curium Cm

Periodic Table of Elements

Electrons• A neutral atom has as

many electrons outside its nucleus as protons within its nucleus.

• Opposing charges of protons and electrons hold electrons within a spherical region surrounding the nucleus.

• Atoms can lose and gain one or more electrons.

Determining Protons and Electrons

Concept Check 3.2How many protons and electrons are in the Li+ ion?

Concept Check 3.2 Solution• Lithium (Li) has an atomic number of 3, therefore it

has a 3 protons in its nucleus.

• A nucleus with 3 protons (1+ charge each) has a charge of 3+. Neutral Li needs needs 3 electrons outside of the nucleus (1− charge each) for a total charge of 3− resulting in a neutral atom.

• Li+ ion has 3 protons in the nucleus and 2 electrons outside of the nucleus for a total charge of 1+.

Neutrons• Neutrons are nearly the same

mass as protons but carry no electrical charge.

• The number of neutrons in the atoms of an element can vary resulting in ISOTOPES.– Can be naturally occurring

or man-made

• The sum of neutrons and protons in an atom is called the mass number (A) of the atom.

Carbon-13 isotope

How Many Protons, Neutrons, and Electrons?

• Decoding an element’s symbol:

• mass number = sum of protons + sum of neutrons• atomic number = number of protons• number of neutrons = A – Z• Neutral atoms have equal numbers of protons and

electrons.

ZAXC

whereZ is the the atomic numberA is the mass numberC is the chargeX is the symbol of the element

Concept Check 3.3• There are three isotopes of the element hydrogen:

• How many protons, electrons, and neutrons are in each isotope of hydrogen?

protium 11H

deuterium 12H

tritium 13H

Concept Check 3.3 Solution

• Each isotope of hydrogen has an atomic number of 1, and therefore has 1 proton in the nucleus and the neutral atoms have 1 electron outside the nucleus.

• Subtracting the atomic number from the mass number gives the number of neutrons in each isotope.

protium 11H 1 1 = 0 neutrons

deuterium 12H 2 1 = 1 neutron

tritium 13H 3 1 = 2 neutrons

Concept Check 3.4How many protons, neutrons, and electrons are in the following radioactive ion of potassium:

1940K+

Concept Check 3.4 Solution

• protons = Z = 19• neutrons = A – Z = 40 – 19 = 21• electrons = Z – C = 19 – 1 = 18

1940K+

Atomic Mass• Atomic masses listed on the periodic table are weighted

averages of the masses of each naturally occurring isotope for that element.

• Silver has two naturally occurring isotopes. One has a mass of about 107 amu and the other has a mass of about 109 amu.

– Silver-107 has an abundance in nature of 51.84%.

– Silver-109 has an abundance in nature of 48.16%.

Atomic MassCalculate the average atomic mass of silver.

Silver has two naturally occurring isotopes. • One has a mass of 106.90509 amu (abundance in nature of

51.84%.)• The other has a mass of 108.90476 amu (abundance in

nature of 48.16%.)

106.90509 amu (0.5184) = 55.42 amu

108.90476 amu (0.4816) = 52.45 amu 107.87 amu

Concept Check 3.5Chlorine has two principle isotopes, 35Cl with a mass

of 34.969 amu and 37Cl with a mass of 36.965 amu. The atomic weight of chlorine is 35.453 amu. Which of the two isotopes is most abundant in a sample of chlorine?

Concept Check 3.5 SolutionThe atomic weight of chlorine (35.453 amu) is closer to the mass of 35Cl (34.969 amu) than that of 37Cl (36.965 amu), therefore the sample contains more 35Cl than 37Cl. On the planet Earth, chlorine contains 77.77% 35Cl and 24.23% 37Cl.

Periodic Law• Mendeleev listed the

known elements in order of increasing atomic mass, grouping those with similar properties.

• He noticed that certain similar properties would recur in a periodic fashion.

• His tabulation is the precursor to our modern periodic table.

Mendeleev and the Periodic Table• Proposed that elements would be discovered to fill in gaps in

the table.

• Proposed that some measured atomic masses were in error.

• Summarized a large number of observations– The underlying reasons for periodic behavior were as yet

unknown– The Bohr model and the quantum mechanical model were

needed to explain the observations.

Bohr Diagrams/Electron Configurations• Based on the behavior of

electrons as particles• Electron orbits

– Specified with n, the orbit’s quantum number

• Fixed energies• Fixed radii• Maximum number of electrons

based on n• This model is an

oversimplification (as are most models).

• Atoms with full outer orbits are extremely stable.

• Atoms with outer orbits that are not full are unstable and will undergo chemical reactions attempting to fill the outer orbit.

The Bohr Model• This is a MODEL of the atom that links electron behavior

(microscopic) to the periodic law (macroscopic).

Concept Check 3.6Based on electron configurations, would you expect Li, O, or Ne to be the least reactive.

Concept Check 3.6 SolutionOf the three elements, only Ne has a filled octet. Elements that have atoms with filled octets are inert.

Concept Check 3.7Which pair of elements would expect to show similar chemical properties?

(a) C and Al(b) K and Na(c) F and Ar(d) Li and Mg

Concept Check 3.7 Solution• Answer (b)

(a) C and Al(b) K and Na(c) F and Ar(d) Li and MgBoth K and Na have the same number of valence electrons and therefore belong to the same group in the Periodic Table of Elements. They are expected to show similar chemical properties.

The Quantum Mechanical Model• Based on the behavior of electrons as waves• Replace Bohr’s orbits with orbitals, a representation of

electron location as the probability of finding it in a certain region of space.

• Orbitals are grouped into shells and fill similarly to Bohr’s orbits.

Determinism and Quantum Mechanics• Specifics of orbital filling are beyond the scope of this text…

BUT– The statistical nature of the quantum mechanical model

rocked the scientific establishment.• An indeterminate universe• Both models are useful even though Bohr’s, by experiment,

has been shown to be invalid.

Concept Check 3.8Which statement is true for Bohr model but not of the quantum mechanical model?

a) Electrons move in circular orbits around a nucleus.

b) Electrons can exhibit wave behavior.c) The exact path of an electron within an atom

cannot be specified.

Concept Check 3.8 SolutionWhich statement is true for Bohr model but not of the quantum mechanical model? Ans: (a)

The Bohr model describes electrons moving in circular orbits of specific fixed positions around a nucleus. Although rather simple, the Bohr model does help explain periodic properties. The quantum mechanical model better describes the behavior of electrons in the atom.

Families of Elements• Also called groups

• Based on outer electron configurations

• Important vertical columns– Alkali metals (Group 1A)– Alkaline earth metals (Group 2A)– Chalcogens (Group 6A)– Halogens (Group 7A)– Noble gases (Group 8A)

Element Families

Metals, Nonmetals, and Metalloids

Molecular Elements• Some elements occur as diatomic molecules.

The Mole Concept• Counting particles as small as atoms is impractical.• The mole concept allows us to relate the mass of a sample

of an element to the number of atoms it contains.

• The unit of comparison is called a mole and corresponds to 6.022 × 1023.

Avogadro’s Number• Determined such that the

numerical value of the atomic mass of an element in amu is equal to the molar mass of that element in grams per mole.

• These relationships of grams, moles, and atoms can be used as conversion factors.

Concept Check 3.9The average pre-1982 U.S. penny contained 3.11 g of copper. How many moles of copper did the pre-1982 penny contain?

Concept Check 3.9 SolutionNumber of moles of Cu in a pre-1982 U.S. penny:

Cu 63.55 g/mol

3.11 g Cu 1 mol Cu

63.55 g Cu = 0.0489 mol Cu

Concept Check 3.10• An 85.0 kg human contains about 4.25 g of the

element magnesium. How many atoms in 4.25 g of Mg?

Concept Check 3.10 Solution• How many atoms in 4.25 g of Mg?

Mg 24.305 g/mol

4.25 g Mg 1 mol Mg

24.305 g Mg

236.022 10 Mg atoms 1 mol Mg

23 = 1.05 10 Mg atoms

Chapter SummaryMolecular Concept

• Atomic Number and Mass Number

• Isotopes

• Molar Mass

• Bohr Model

• Quantum Mechanical Model

• Orbitals

Societal Impact

• Chemical processes that cause change are caused by changes in atoms or molecules.

• Pollution results from “misplaced” atoms. Atoms that are not where they are supposed to be located.

• Models are used to describe and interpret nature.