chapter 1: chemical bonds

ChemistryChapter 1: Chemical Bonds

Vocab

• Electron cloud• Energy levels• Periodic Table• Electron dot diagram• Chemical bond

Atom Structure• Everything is made up of atoms• At the center of every atom is a

nucleus containing protons (+ charge) and neutrons (no charge)

• The nucleus represents most of the atom’s space

• The rest of the atom’s space is empty except for the atom’s electrons (- charge) which travel in an area of space around the nucleus called the electron cloud

Electrons

• Electrons have negative charges• Electrons travel in predictable areas

(electron clouds) but not predictable patterns

Element Structure

• Each element has a different atomic structure made up of a specific number of protons, electrons, and neutrons

• The number of protons and electrons is always the same for a neutral element

Electron Arrangement

• The number and arrangement of electrons in the electron cloud are responsible for many of the chemical and physical properties of that element.

Electron Energy

• All electrons in an atom are in the electron cloud but some electrons are closer to the nucleus than others

• Energy levels: The different positions of electrons in an atom

• Each level represents a different amount of energy

Number of Electrons

• Each energy level can hold a specific number of electrons

• The farther away an energy level is from the nucleus the more electrons it can hold– Ex. The first can hold 2, the second up to 8,

and the third up to 18 electrons

Second Energy LevelFirst Energy Level

Energy Steps• A stairway is an excellent model to show

the maximum number of electrons each energy level can hold in the electron cloud.

• Electrons nearest the nucleus have the lowest amounts of energy and are the hardest to remove

• Electrons farthest from the nucleus have the most amounts of energy and are the easiest to remove

Energy Steps

• Ex. Picture a magnet with a chain of paperclips hanging off of it.

• The easiest paperclip to remove is the one at the very end of the chain because the pull of the magnet isn’t as strong

Hardest to remove

Easiest to remove

Periodic Table and Energy Levels• Periodic Table: A way of representing all

of the elements in a way that shows their similarities and differences– First discovered by Dmitri Mendeleev in 1869– Horizontal rows are called periods– Vertical columns are called groups or families– The atomic number is the same as

the number of protons and electrons

Today’s periodic table is still based on the one I created almost 140 years ago!

Reading the Periodic Table

Electron Configuration

• The number of electrons in a neutral atom increases from left to right across a period

• Atoms whose outer energy levels contain one of the levels in the stair step analogy are considered stable

• Each period ends with a stable element

Element Families

• Each column of the Periodic Table contains one family– Hydrogen is considered separate from the

rest of the families• Just like human families, element families

have similar traits (chemical properties)• Mendeleev was inspired by the repeating

patterns he heard in music and saw similar repeating patterns in the elements

Noble Gases

• Made up of the elements in Group 18– Helium, Neon, Argon, Krypton, Xenon, Radon

• All are stable and have 8 electrons in their outer energy levels

• At one time these gases were thought to be inert (non reactive) but this was later found to be untrue and the name changed to the Noble gases

• Used to protect filaments in light bulbs and produce colored light in signs

Halogens• Made up of the elements in Group 17

– Fluorine, Chlorine, Bromine, Iodine, Astatine• Unstable – Halogens need one electron to

obtain a stable outer energy level– The easier it is for a Halogen to form a bond,

the more reactive it is• Fluorine is the most reactive because its outer

energy level is closest to the nucleus

• Used to disinfect drinking water, in clothing bleach, production of paper, and removal of germs during sterilization

Alkali Metals

• Made up of the elements in Group 1– Lithium, Sodium, Potassium, Rubidium,

Caesium, Francium (Hydrogen is not included)• Unstable - have one electron in their outer

energy level Highly reactive• Used to create flavorings and

preservatives and in the manufacturing of paper, soap, and ceramics

Electron Dot Diagrams• Electron dot diagram: a diagram that

shows the number of electrons in the outer energy level– Valance Electrons: electrons in the outer

energy level– Helpful in determining the chemical properties

of an atom and when showing how atoms bond

• Also called Lewis Structures• Created by Gilbert N. Lewis

How to Write Them• For groups 1, 2 and 13-18 see pg. 11 in

your book.• Dots are written in pairs on the four sides

of the element symbol• The dots are written in this order

Ne1

23

45

6

78

Using Dot Diagrams

• Can be used to show how atoms bond with each other

• Chemical Bond: the force that holds two atoms together

• Atoms bond with each other in a way that makes them become more stable by filling up their outer energy levels

Section 2: How Elements Bond

Vocab• Ion• Ionic bond• Compound• Covalent bond• Metallic bond• Molecule• Polar bond• Chemical formula

How Elements Bond

• When atoms are joined together they do not fall apart easily

• Atoms form bonds with other atoms using the electrons in their outer energy level

• There are 4 ways this can be done:– Losing electrons– Gaining electrons– Pooling electrons– Sharing electrons

Ionic Bonds – Loss and Gain

• Occurs between metals and nonmetals• Atoms gain and lose electrons to form

bonds

Ions – A Question of Balance• Atoms lose or gain electrons and become

more stable• When an electron is lost, the electric charge

changes because there is now one more proton than neutron in the nucleus and it is called a positive ion

• When an electron is gained you create a negative ion

• Ion: an atom that is no longer neutral because it has lost or gained an electron

• Examples of ions:– Sodium loses one electron to become a Na+ ion

– Chlorine gains one electron to become a Cl- ion

Bond Formation• Negatively charged ions and positively

charged ions are attracted to each other• Ionic Bond: attraction that holds oppositely

charged ions together– Ex. Sodium (Na) loses an electron and

becomes a positive ion. Chlorine (Cl) gains an electron from Sodium (Na) and becomes a negative ion.

– The two ions are attracted together by an ionic bond

• Pure substances are divided into two categories:– Element: cannot be separated into any

simpler substances• Ex. Sodium and Chlorine

– Compound: a substance containing two or more elements that are chemically bonded

• Ex. Sodium Chloride

More Gains and Loses

• Some atoms need to lose/gain more than two electrons to become stable

• Atoms can do this by bonding to another atom that needs to gain/lose as many electrons as they need to lose/gain or by bonding to more than 1 atom

• Ex: – Magnesium needs to lose 2 electrons– Oxygen needs to gain 2 electrons

• The 2 in the superscript represents the number of electrons that were gained/lost

• If only one electron is gained or lost no number is needed in the superscript

• Ex:– Magnesium needs to lose 2 electrons– 2 Chlorine atoms that each need to gain 1

electron

Metallic Bonding - Pooling• Occurs between two metals• Metallic Bonding: Metal atoms pool their

electrons to form bonds– In metals, electrons are not held tightly to the

individual atoms. Instead they move freely among all the atoms in the metal forming a shared pool of electrons

– This is why metal can be hammered into sheets without breaking and is a good conductor of electricity

Covalent Bonds - Sharing• Occurs between 2 nonmetals• Some atoms need to gain/lose too many

electrons (this needs too much energy) to create ionic bonds

• Instead two nonmetals share their electrons• Covalent Bond: chemical bond that forms

between the shared electrons– Electrons are attracted to the nuclei of both atoms

and move back and forth between them– Molecule: neutral particle formed in a covalent bond

– Ex.• Two chlorine atoms form a stable molecule by

sharing electrons to fill their outer energy level

Double and Triple Bonds• Sometimes an atom shares more than one

electron with another atom– When two pairs of electrons are shared it is

called a double bond

– When three pairs of electrons are shared it is called a triple bond

Polar and Nonpolar Molecules• While atoms share electrons to become stable

they do not always share electrons equally– Some atoms have a stronger attraction to

electrons than others– This unequal sharing makes one side of the bond

more negative than the other– Polar bond: bond where the electrons are shared

unevenly• Ex. Water molecules

– Nonpolar bonds: bond where electrons are shared evenly

The Polar Water Molecule• 2 hydrogen atoms and 1 oxygen atom

bond together in a covalent bond• The oxygen has a greater share of the

electrons (sharing one electron with each hydrogen)

• The oxygen end has a negative charge

• The hydrogen has a positive charge

Chemical Shorthand• In medieval times alchemists were the first

to explore the world of chemistry– They believed in magic and mythical

transformations– Discovered many properties of the elements– Created many symbols to represent the

elements• Many are still used today

Alchemists, like me, were mainly concerned with transforming

ordinary metals into gold

Symbols for Atoms• Modern chemists use symbols to represent

atoms– These symbols can be understood by chemists

all over the world– Each element is represented by a one or two

letter symbol– Many symbols are created by using the common

name of the element • Ex. H for Hydrogen

– Some symbols are creating by using the name of the element in another language

• Ex. K for Potassium (which is called Kalium in Latin)

Symbols for Compounds

• Compounds can be described using element symbols and numbers– Ex. H2

• This tells us that there are 2 Hydrogen atoms in this molecule

Chemical Formulas• Chemical Formula: combination of

chemical symbols and numbers which shows which elements are present in a compound– Ex. Ammonia or NH3

• This tells you that the ratio is one nitrogen atom to three hydrogen atoms