Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
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
0
Pote
ntia
l ene
rgy
© 2012 Pearson Education, 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.
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.
© 2012 Pearson Education, Inc.
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.
19
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
Atoms,Molecules,and Ions
Ionic Bonds
Ionic compounds (such as NaCl) are generally formed between metals and nonmetals.
© 2012 Pearson Education, 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.
© 2012 Pearson Education, Inc.
Atoms,Molecules,and Ions
Common Cations
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
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