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Atoms,Molecules,
and Ions
Warm Up
A measured mass of an unreactive metal was dropped into a small graduated cylinder half filled with water. The following measurements were made.
Mass of metal = 19.611 gramsVolume of water before addition of metal = 12.4 millilitersVolume of water after addition of metal = 14.9 milliliters
The density of the metal should be reported as
(A) 7.8444 grams per mL(B) 7.844 grams per mL(C) 7.84 grams per mL(D) 7.8 grams per mL(E) 8 grams per mL
Atoms,Molecules,
and Ions
Chapter 2Atoms, Molecules,and Ions
John D. Bookstaver
St. Charles Community College
Cottleville, MO
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Subatomic Particles
Protons and electrons are the only particles that have a charge.
Protons and neutrons have essentially the same mass.
The mass of an electron is so small we ignore it.
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Symbols of Elements
Elements are symbolized by one or two letters.
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Atomic Number
All atoms of the same element have the same number of protons:
The atomic number (Z)
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Atomic Mass
The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom.
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Isotopes
Isotopes are atoms of the same element with different masses.
Isotopes have different numbers of neutrons.
© 2009, Prentice-Hall, Inc.
116C
126C
136C
146C
Same number of protons Different masses
Atoms,Molecules,
and Ions
Average Mass
Because in the real world we use large amounts of atoms and molecules, we use average masses in calculations.
Average mass is calculated from the isotopes of an element weighted by their relative abundances.
© 2009, Prentice-Hall, Inc.
Average mass is shown on periodic table
Atoms,Molecules,
and Ions
Check for Understanding
How many protons, neutrons and electrons are there in an atom of oxygen-17 ( )? 8 protons (equal to atomic number) 9 neutrons (mass number – number of protons, 17 – 8 = 9 8 electrons (equal to number of protons in a neutral atom)
Use your periodic table to determine the number of protons, neutrons, and electrons in a potassium (K) atom. 19 protons, 20 neutrons (39 – 19 = 20), 19 electrons
© 2009, Prentice-Hall, Inc.
€
817O
Atoms,Molecules,
and Ions
Metals and Nonmetals
Metals are on right side of periodic table (with the exception of hydrogen)
Nonmetals are on the right.
Metalloids are in the middle.
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Chemical Formulas
The subscript to the right of the symbol of an element tells the number of atoms of that element in one molecule of the compound.
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Chemical Formulas
Molecular compounds are composed of covalently bonded atoms and contain only nonmetals.
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Diatomic Molecules
These seven elements occur naturally as molecules containing two atoms.
© 2009, Prentice-Hall, Inc.
So if someone says we need oxygen to survive, that oxygen is really O2 not just O. Single oxygen atoms are not found alone in the air.
Atoms,Molecules,
and Ions
Ions
When atoms lose or gain electrons, they become ions. Cations are positive and are formed when
atoms lose electrons. Anions are negative and are formed when
atoms gain electrons.
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Ionic Bonds
Ionic compounds (such as NaCl) are generally formed between metals and nonmetals.
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Check for understanding
Are the following molecular compounds or ionic compounds? KF CH4
FeSO4
Explanation: Both KF and FeSO4 contain a metal ion, K+ and Fe2+, and thus are ionic compounds.
CH4 contains only nonmetals so it is molecular.
© 2009, Prentice-Hall, Inc.
IonicMolecularIonic
Atoms,Molecules,
and Ions
Common Cations
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Common Anions
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Writing Formulas
Because compounds are electrically neutral, one can determine the formula of a compound this way: The charge on the cation becomes the
subscript on the anion. The charge on the anion becomes the
subscript on the cation. If these subscripts are not in the lowest
whole-number ratio, divide them by the greatest common factor.
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Polyatomic Ions
Polyatomic ions are “many atom” ions, OR charged compounds. Examples: SO4
2- (sulfate)
NH4+ (ammonium)
CH3COO- (acetate)
To write formulas follow the same procedure as before. For magnesium sulfate we have
Mg2+ and SO42- giving us:
MgSO4
Notice the subscript on the polyatomic ion remains.
If you use more than one polyatomic ion, as in magnesium acetate, you must put parentheses around the polyatomic ion:
Mg(CH3COO)2
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Inorganic Nomenclature
Write the name of the cation.
If the anion is an element, change its ending to -ide; if the anion is a polyatomic ion, simply write the name of the polyatomic ion.
If the cation can have more than one possible charge, write the charge as a Roman numeral in parentheses.
© 2009, Prentice-Hall, Inc.
Atoms,Molecules,
and Ions
Examples
Write the formulas for the following ions: Calcium hydroxide Iron (III) chloride Potassium carbonate
Ca(OH)2
FeCl3 K2CO3
Name the following: Pb(NO3)2
Na2S
Mg3(PO4)2
lead (II) nitrate sodium sulfide magnesium phosphate
© 2009, Prentice-Hall, Inc.