Chemical Bonding

What is Chemical Bonding?Chemical Bonding is two or more atoms combining by sharing electrons so that a new substance is produced that has different physical and chemical properties than its component elements

Chemical bonding occurs when atoms share valence electrons

Atoms want to share electrons so that their valence shells are full and they are chemically stable

-- for most elements, this means having 8 valence electrons

-- for hydrogen, helium, lithium, and beryllium (who can have a full 1st energy level), this means having 2

valence electrons

Lewis StructuresOther than the Bohr Model, there is a much simpler way that we can show the valence electrons of an atom. . .

This is known as either an electron-dot diagram or, more scientifically, a Lewis Structure

When drawing a Lewis Structure, you place the atomic symbol at the center and put dots around the symbol to indicate the valence electrons.

-- DO NOT pair electrons until all four sides have at least one electron

Examples:

C Na F

The Octet RuleAtoms themselves can gain or lose electrons so that they have a full valence shell of 8 (or 2) electrons. . .

-- when atoms gain or lose electrons, they are called ions

Depending on whether or not the atom gains or loses an electron, it can be positively-charged (cation) or negatively-charged (anion)

This need to be stable and have 8 electrons is called the octet rule

In addition to forming ions, atoms can also “share” their valence electrons with other atoms, giving each atom 8 valence electrons

Types of Chemical BondingThere are two ways in which atoms can share electrons to satisfy the octet rule:

Ionic Bonding – occurs when two or more ions combine to form an electrically-neutral compound

-- the positive cation “loses” an electron (or 2 or 3)

-- the negative anion “gains” the electron (or 2 or 3)

-- the electrons ARE NOT shared between the two atoms, as the anion steals the electrons from the cation

Covalent Bonding – occurs when two or more atoms combine to form an electrically-neutral compound

-- the electrons are shared between the two atoms

-- neither atom had a charge to begin with, and the compound remains with zero charge

Ionic BondingIonic bonding is the combining of two or more ions to form an

electrically-neutral compound

How Ionic Bonding Works

1) The giving atom loses a valence electron (or 2 or 3) so that it has a full valence shell, but a positive charge

2) The gaining atom gains a valence electron (or 2 or 3) so that it has a full valence shell, but a negative charge

3) The negative and positively-charged ions are attracted to each other (like a magnet) based on their opposite charge

If you look at your periodic table, you will see that ionic bonding usually occurs in compounds formed between one metal and one nonmetal

Metals in Ionic BondsIn ionic bonds, metals are always the cation

-- metals are on the left side of the periodic table and only have 1, 2, or 3 valence electrons

-- it is very easy to lose these electrons and become positively charged

Nonmetals in Ionic BondsNonmetals (with the exception of hydrogen) are always the anion in ionic bonds

-- nonmetals all have 4 or more valence electrons, so it is very easy for them to gain electrons to get a full

valence shell

-- when nonmetals gain electrons, they get a negative charge

Drawing Ionic BondsWhen a nonmetal is exposed to a metal, the metal will lose electrons and the nonmetal will gain electrons, forming a new compound

-- that new compound is held together by an ionic bond

The individual atoms will have a charge, but the overall compound will be neutral

When we draw the compound, using Lewis Structures, we show the extra electron around the anion and the cation as having no valence electrons

-- each ion in the compound is shown as being charged

Example:

Na + Cl Na Cl

Diagram of Ionic Bonding

Polyatomic IonsThere are some ions that are made up of more than one type of atom, these are called polyatomic ions

For example, the polyatomic ion known as ammonium NH4+

has 4 atoms of hydrogen and one atom of nitrogen, HOWEVER, the whole ion has an overall charge of +1

-- you will be given the charges of any polyatomic ions

Naming Ionic CompoundsIonic compounds all have two-word names

The first word in the name is the same as the name of the first ion (for example, sodium, ammonium, potassium, etc)

The second word in the name is either:

1. If the second ion is polyatomic, it is just the name of the polyatomic ion

2. If the second ion is an element, the end of the element’s name changes to –ide

Example: chlorine chloride

oxygen oxide

Practice Naming Ionic Compounds

• Na2CO3 -- • CaSO4 --

• KBr --

• MgS --

• BeCl2 --

• NH4F --

Writing Ionic Compounds From NameIf I have an ionic compound, for example, potassium sulfate,

how do I know what the formula is?

REMEMBER THAT THE COMPOUND MUST BE NEUTRAL

So, I have Potassium, K, which will lose 1 electron and gain a positive charge, making it K+

And, I have Sulfate, a polyatomic ion, which has a charge of -2

Therefore, I need 2 potassium ions to balance out my one sulfate ion, making my formula, K2SO4

-- this is why we have subscripts in chemistry

What About. . .What if I have to put multiple polyatomic ions in a compound, for example, magnesium nitrate?

Magnesium has a charge of +2

Nitrate is polyatomic and has a charge of -1

Therefore, we need 2 nitrate ions for each magnesium

In this case, we put the entire polyatomic ion in parenthesis and put the subscript outside the parenthesis

Our answer would be Mg(NO3)2

Properties of Ionic CompoundsIonic compounds have the following properties:

Crystal Pattern – every ion is attracted to all other ions with the opposite charge

-- this results in a repeating 3-dimensional crystal pattern

Properties of Ionic Compounds cont’dHigh Melting Point – the attraction in the crystal pattern

leads to very strong bonds, making it hard to break apart ionic compounds

-- ionic compounds melt at high temperatures

Conductivity – when dissolved in water, ionic compounds conduct electricity

-- in water, the bonds dissociate (fall apart), leaving lots of ions to carry charge

Solid ionic compounds do not conduct electricity very well

-- melted ionic compounds do conduct electricity fairly well