Chemistry 1.2

ChemistryChapter 1 - Section 2

How Elements

Monday, July 9, 2012

Bonding

✦Atoms form bonds with other atoms using the electrons in their outer energy levels.

Bonding

✦Atoms have four ways to do form bonds.

Bonding

✦Atoms have four ways to do form bonds. 1.by losing electrons

Bonding

✦Atoms have four ways to do form bonds. 1.by losing electrons2.by gaining electrons

Bonding

✦Atoms have four ways to do form bonds. 1.by losing electrons2.by gaining electrons3.by pooling electrons

Bonding

✦Atoms have four ways to do form bonds. 1.by losing electrons2.by gaining electrons3.by pooling electrons4.by sharing electrons with another element

Bonding

✦Atoms have four ways to do form bonds. 1.by losing electrons2.by gaining electrons3.by pooling electrons4.by sharing electrons with another element

✦We will be studying each of these in Section 2.

1 & 2. Gaining and Losing

Electrons

Sodium and Chlorine

Sodium is a soft, silvery metal.

Sodium and Chlorine

It can react violently when added to water or to chlorine.What makes sodium so reactive?

Sodium and Chlorine

Sodium has only one electron in its outer level. Removing this electron empties this level and leaves the completed level below. Sodium is then stable.

Sodium and Chlorine

Sodium has only one electron in its outer level. Removing this electron empties this level and leaves the completed level below. Sodium is then stable.

Chlorine forms bonds in a way that is the opposite

of sodium—it gains one electron. When chlorine accepts an electron, its electron configuration becomes stable.

Forming Ions

As sodium atoms lose an electron they become more stable.

Forming Ions

But by losing an electron, the balance of electric charges changes.

Forming Ions

11 protons11 electrons(neutral)

Forming Ions

after losing one electron= but

Forming Ions

after losing one electron= but 11 protons

10 electrons(positively charged)

Forming Ions

Sodium becomes a positively charged ion because there is now one fewer electron than there are protons in the nucleus.

after losing one electron= but 11 protons

10 electrons(positively charged)

Forming Ions

In contrast, chlorine becomes an ion by gaining an electron.

Forming Ions

after gaining one electron= but

Forming Ions

after gaining one electron= but 17 protons

18 electrons(negatively charged)

Forming Ions

after gaining one electron= but 17 protons

18 electrons(negatively charged)

It becomes negatively charged because there is one more electron than there are protons in the nucleus.

Forming Ions

An atom that is no longer neutral because it has

lost or gained an electron is called an ion.

Forming Ions

Sodium ion is represented by the symbol Na+

Forming Ions

Sodium ion is represented by the symbol Na+

A chloride ion is represented by the symbol Cl-

Bond Formation

The positive sodium ion and the negative chloride ion are strongly attracted to each other.

Bond Formation

This attraction, which holds the ions close together, is a type of chemical bond called an ionic bond.

Bond Formation

This attraction, which holds the ions close together, is a type of chemical bond called an ionic bond.

Bond Formation

Sodium and chloride ions form an ionic bond.

Bond Formation

The compound sodium chloride, or table salt, is formed. A compound is a pure substance containing two or more elements that are chemically bonded.

Bond Formation

The compound sodium chloride, or table salt, is formed. A compound is a pure substance containing two or more elements that are chemically bonded.

More Gains and Loses

Can elements lose or gain more than one electron?

Can elements lose or gain more than one electron? Yes!!

Magnesium, Mg, in Group 2 has two electrons in its outer energy level. Magnesium can lose these two electrons and

achieve a completed energy level.

Magnesium, Mg, in Group 2 has two electrons in its outer energy level. Magnesium can lose these two electrons and

achieve a completed energy level.

Oxygen, O, in Group 16 has six electrons in its outer energy level. Oxygen can gain two electrons and achieve a

completed energy level.

The two electrons, in magnesiums outer energy level, can be gained by two chlorine atoms.

The two negatively charged chloride ions are attracted to the positively charged magnesium ion forming ionic bonds.

The compound magnesium chloride (MgCl2) is produced.Monday, July 9, 2012

3.Pooling

Electrons

Swimming in a Pool of Metal

Metallic Bonding - Pooling

We have just looked at how metal atoms form ionic bonds with atoms of nonmetals.

Metals can form bonds with other metal atoms, but in a different way.

In a metal, the electrons in the outer energy levels of the atoms are not held tightly to individual atoms. Instead, they move freely among all the ions in the metal, forming a shared pool of electrons.

Metallic bonds form when metal atoms share their pooled electrons.

This bonding affects the properties of metals.

This bonding affects the properties of metals. For example, when a metal is hammered into sheets or

drawn into a wire, it does not break. Instead, layers of atoms slide over one another.

An ounce of gold can be stretched into a wire 50 miles long.

A lump of pure gold the size of a matchbox can be flattened into a sheet the size of a tennis court.

The pooled electrons tend to hold the atoms together.

Metallic bonding also is the reason that metals conduct electricity well.

The outer electrons in metal atoms readily move

from one atom to the next to transmit current.

The outer electrons in metal atoms readily move

from one atom to the next to transmit current.

4.Sharing

Electrons

Covalent Bonds - Sharing

Some atoms are unlikely to lose or gain electrons because the number of electrons in their outer levels makes this difficult.

Carbon has six electrons, four of the six electrons are in its outer energy level.

To obtain a more stable structure, carbon would either have to gain or lose four electrons.

This is difficult because gaining & losing so many electrons takes so much energy.

The alternative is sharing electrons.

The Covalent Bond

The Covalent BondThe chemical bond that forms between nonmetal

atoms when they share electrons is called a

covalent bond.

covalent bond. Shared electrons are attracted to the nuclei of both

atoms.

atoms. They move back and forth between the outer energy

levels of each atom in the covalent bond.

Each atom has a stable outer energy level some of the

time.Covalently bonded compounds are called molecular

compounds.Monday, July 9, 2012

The Covalent Bond

The atoms in a covalent bond form a neutral particle.

The Covalent Bond

The neutral particle formed when atoms share electrons is called a molecule.

The Covalent Bond

No ions are involved in covalent bonding because no electrons are gained or lost. Ionic compounds, such as sodium chloride (NaCl), are not referred to as molecules, because their basic units are

ions, not molecules.

The Covalent Bond

Double and Triple Bonds

Sometimes an atom shares more than one electron with another atom.

When two pairs of electrons are involved in a

covalent bond, the bond is called a double bond.

Here is the sharing of three pairs of electrons between two nitrogen atoms in the nitrogen molecule.

When three pairs of electrons are shared by two atoms, the bond is called a triple bond.

Ionic vs. Covalent

Polar and Nonpolar Molecules

Do atoms always share their electrons equally?

Do atoms always share their electrons equally?The answer is no.

Some atoms have a greater attraction for

electrons than others do.

Why?????

Some atoms have more protons, therefore more

positive pull on the electrons.

Chlorine attracts electrons more strongly than hydrogen does.

When hydrogen and chlorine covalently bond, the shared pair of electrons tends to spend more time near the chlorine atom than the hydrogen atom.

Chlorine - 17 protons Hydrogen - 1 proton

The unequal sharing makes one side of the bond more negative than the other. Such bonds are called polar bonds.

A polar bond is a bond in which electrons are

shared unevenly.

A polar bond is a bond in which electrons are

shared unevenly.

The bonds between the oxygen atom and hydrogen atoms in the water molecule are another example of polar bonds.

“Get Away, Water!”

When water molecules are exposed to a negative charge, the water molecules line up like magnets

with their positive ends facing the negative charge.

They are drawn to the negative charge on the balloon.

Water molecules also are attracted to each other. This attraction between water molecules accounts for many of the physical properties of water.

Molecules that do not have these uneven charges are called nonpolar molecules.

Because each element differs slightly in its ability to attract electrons, the only completely nonpolar bonds are bonds between atoms of the same element.

One example of a nonpolar bond is the triple bond in the nitrogen molecule.

Chemical Shorthand

In medieval times, alchemists were the first to explore the world of chemistry.

They used symbols to represent elements.

Symbols for Atoms

Modern chemists also use symbols to represent elements.

The symbols are universal.

Each element is represented by a one letter-, two letter-, or three-letter symbol.

Many symbols are the first letters of the element’s name, such as H for hydrogen and C for carbon.

Others are the first letters of the element’s name in another language, such as K for potassium, which stands for kalium, the Latin word for potassium.

Symbols for Compounds

Compounds can be described using element symbols and numbers. The figure below shows how two hydrogen atoms join together in a covalent bond.

The resulting hydrogen molecule is represented by the symbol H2. The subscript 2 means that two atoms of hydrogen are in the molecule.

Don’t get confused!

2 H H2

Chemical Formula

A chemical formula is a combination of chemical symbols and numbers that shows which elements

are present in a compound and how many atoms of each element are present.

When no subscript is shown, the number of atoms is understood to be one.

Chemical Formula

H2S Ag2S

Chemical Formula

H2S Ag2S NH3

Chemical Formula

H2S Ag2S NH3

H2SO4Monday, July 9, 2012

Chemical Formula

H2S Ag2S NH3

H2SO4 NaOHMonday, July 9, 2012

Chemical Formula

H2S Ag2S NH3

H2SNaOH

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