Forming Chemical Bonds

Why do atoms form chemical bonds ?

so that the system can achieve the lowest possible potential energy

H • H•

Example covalent bonding in H2

Distance of separation between atoms

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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.

Ions and Ionic Compounds

Ionic compounds

Any compound when melted that conducts electricity is considered ionic

NaCl( s) is made up of Na+ and Cl- ions

Example : NaCl(s) NaCl( l)

Ionic Bonds

especially prevalent in compounds formed between group 1A and 2A elements with group 6A and 7A elements.

between Elements with the biggest difference in electronegativity

Ionic compounds are usually formed between metals and nonmetals.

Ionic and Molecular Compounds

Molecular compounds are usually formed between two nonmetals.

When electrons are removed from or added to a neutral atom or molecule, a

charged particle called an ion is formed.

Ions

Positively charged ions are calledcations

Negatively charged ions are calledanions

Atoms vs Ions

Na: 11 protons; 11 electrons

Cl: 17 protons; 17 electrons

Na+: 11 protons; 10 electrons

+

Cl–: 17 protons; 18 electrons

–

Ions

positively charged: cations

negatively charged: anions

most common type is metal cationNa+, Ca2+, Al3+, Fe2+, Fe3+

most common type is nonmetal anionCl–, O2–

Ionic Bonding

electrostatic attraction between oppositely charged ions

+ –

cation anion

discrete molecules are not present, so ionic compounds are represented by their empirical formulas

Ionic Compounds

some times referred to as formula units

Chemical FormulasExpress the composition of moleculesand ionic compounds in terms of thesymbols for the elements they contain.

Empirical formula tells us which elements are present and the simplest whole-number ratio of their atoms.

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Atoms,Molecules,and Ions

Types of Formulas

• Structural formulas show the order in which atoms are bonded.

• Perspective drawings also show the three-dimensional array of atoms in a compound.

Structure of solid NaCl. In reality, the cations are in contact with the anions. The smaller spheres (red) represent Na+ ions and the larger spheres (purple) the Cl– ions.

Structure of solid NaCl.

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Atoms,Molecules,and Ions

Ions

• When atoms lose or gain electrons, they become ions.– Cations are positive and are formed by elements

on the left side of the periodic chart.– Anions are negative and are formed by elements

on the right side of the periodic chart.

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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

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

Common Cations

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Atoms,Molecules,and Ions

Common Anions

Molecules:Atoms in Combination

Molecule —is an aggregateof at least two atoms in a definitearrangement held together by chemical forces.

Definition

structurebonds

Chemical FormulasExpress the composition of moleculesand ionic compounds in terms of thesymbols for the elements they contain.

Empirical formula tells us which elements are present and the simplest whole-number ratio of their atoms.Molecular formula gives the exact number of atoms each element in the compound

© 2012 Pearson Education, Inc.

Atoms,Molecules,and Ions

Chemical Formulas

Molecular compounds are composed of molecules and almost always contain only nonmetals.

A molecule is not necessarily a compound.

ExampleH2 O2 Cl2

A compound is not necessarily molecular.

ionic compounds

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Atoms,Molecules,and Ions

Diatomic Molecules

• These seven elements occur naturally as molecules containing two atoms:– Hydrogen– Nitrogen– Oxygen– Fluorine– Chlorine– Bromine– Iodine

Hydrogen

stable form of element is diatomic molecule H2

Hydrogen

the stable form of the element is diatomic a molecule H2

ball-and-stick modelother diatomic elements include

O2, N2, F2, Cl2, Br2, and I2

The subscript indicates the number of atoms in the formula

space-filling model

is a polyatomic molecule (contains three atoms connected in the order HOH).

Water (H2O)

MethaneMolecular formula: CH4

ball-and-stick model space-filling model

shows the way in which the atoms are joined

structural formula

C

H

H

HH