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.

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.

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Atoms,Molecules,

and Ions

Symbols of Elements

Elements are symbolized by one or two letters.

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Atoms,Molecules,

and Ions

Atomic Number

All atoms of the same element have the same number of protons:

The atomic number (Z)

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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.

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Atoms,Molecules,

and Ions

Isotopes

Isotopes are atoms of the same element with different masses.

Isotopes have different numbers of neutrons.

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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.

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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

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€

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.

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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.

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Atoms,Molecules,

and Ions

Chemical Formulas

Molecular compounds are composed of covalently bonded atoms and contain only nonmetals.

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Atoms,Molecules,

and Ions

Diatomic Molecules

These seven elements occur naturally as molecules containing two atoms.

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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.

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Atoms,Molecules,

and Ions

Ionic Bonds

Ionic compounds (such as NaCl) are generally formed between metals and nonmetals.

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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.

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IonicMolecularIonic

Atoms,Molecules,

and Ions

Common Cations

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Atoms,Molecules,

and Ions

Common Anions

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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.

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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

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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.

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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

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