Geneva High School Integrated Science

Geneva High School Integrated Science

Chapter 6 – Chemical Bonds

Standards Covered:

1.  Atoms may be bonded together by losing, gaining or sharing electrons;

2.  Ionic bonds involve the attraction of two oppositely charged ions, typically a metal cation and a nonmetal anion, and is the result of a transferring of electrons;

3.  Covalent bonds result from the sharing or electrons between two atoms, usually nonmetals;

4.  Covalent bonding can result in the formation of structures ranging from small individual molecules to three-dimensional lattices (e.g. diamonds);

5.  Use ionic charges to predict ionic compound formulas;

6.  Given a chemical formula, name a compound using conventional systems that include Greek prefixes where appropriate.

6.1 – Ionic Bonding

Stable Electron Configurations

KEY: When the highest occupied energy level of an atom has eight valence electrons (or two in the case of helium), the atom is stable and not likely to react.

·  The chemical properties of an element depend on the number of valence electrons.

·  All atoms, except hydrogen and helium, need 8 valence electrons in order to be stable.

·  Hydrogen and helium, because they only have one energy level, only require 2 valence electrons.

·  An electron dot diagram is a model of an atom in which each dot represents a valence electron.

examples: see page 158 in textbook and examples done in class

Ionic Bonds

Key: Some elements achieve stable electron configurations through the transfer of electrons between atoms. This is an ionic bond.

Transfer of Electrons

·  The formation of sodium chloride is a good example of an ionic bond. (See example in book on page 159.)

·  Sodium gives up an electron, and chlorine accepts it, giving both atoms stable electron configurations.

Formation of Ions

·  When an atom gains or loses an electron, the number of electrons is no longer equal to the number of protons.

·  The charge on the atom is not balanced and the atom is not neutral.

·  An atom that has a net positive or negative electric charge is called and ion.

®  The charge on an ion is represented by a plus or a minus sign.

®  Ions that gain electrons have a negative charge, and are called anions.

®  Ions that lose electrons have a positive charge, and are called cations.

Formation of Ionic Bonds

·  A chemical bond is the force that holds atoms or ions together as a unit.

·  An ionic bond is the force that holds cations and anions together.

·  A chemical formula is a notation that shows what elements a compound contains and the ratio of the atoms or ions in the compounds.

·  A subscript indicates how many atoms of a specific element there are.

·  A coefficient indicates how many of the compounds/molecules there are.

Examples: 3Al2O3

6.2 – Covalent Bonding

Covalent Bonds

·  A covalent bond is a chemical bond in which two atoms share one or more pairs of valence electrons.

example: a hydrogen atom has one valence electron, but wants two. If two hydrogen atoms share their electrons, they can achieve a stable electron configuration.

®  Covalent bonds can be single bonds, double bonds, or triple bonds, and are represented by dashed lines between the element symbols.

·  A molecule is a neutral group of atoms that are joined together by one or more covalent bonds.

Key: The attractions between the shared electrons and the protons in each nucleus hold the atoms together in a covalent bond.

Many nonmetal elements exist as diatomic molecules.

®  Diatomic molecules consist of two atoms of the same element.

®  F2, N2, Cl2, Br2, I2, H2, O2

Polar Covalent Bonds

·  A polar covalent bond is one in which electrons are not shared equally.

KEY: When atoms form a polar covalent bond, the atom with the greater attraction for electrons has a partial negative charge. The other has a partial positive charge.

example: water

·  Because water is polar, water molecules are attracted to each other and require energy to pull them apart.

·  Remember our activity when we dropped water on the penny.

·  Also accounts for the cohesiveness of water.

Writing Binary Compound Formulas

·  binary compound – a compound composed of only two elements.

·  When writing binary compound formulas, you need to follow these steps:

1.  Write down the symbols of the elements, along with their oxidation numbers.

2.  There will always be an element with a positive oxidation number, and an element with a negative oxidation number.

3.  Use subscripts to balance out the oxidation numbers so that the positive number equals the negative one. (See our website for more help on this topic.)

·  oxidation number – the number of electrons an atom will lose, gain, or share in order to become stable.

See page 174 for more practice on writing binary formulas.

Naming Binary Compounds

1.  Write down the name of the first element in the compound.

2.  Write down the root of the second element in the compound.

3.  Add –ide to the root.

Example: MgCl2

1.  - Write down the name of the first element in the compound.

Magnesium

2.  Write down the root of the second element in the compound.

Chlor

3.  Add –ide to the root.

Magnesium Chloride

For the test, you also need to know how to count the number of atoms in a chemical formula. Please see our worksheets for that.