Atomic Theory

• Ancient Greeks believed that elements must be made up of small indivisible particles they called atomos, meaning indivisible…where the word “atom” comes from

• John Daulton, a school teacher, came up with the following hypothesis:

• Atoms are the basic building blocks of matter

• They are the smallest particles of an element that retain its properties

• Atoms are extremely small about 10-10 m in diameter…you can fit a million atoms in a dot on your paper

• The central core, or nucleus, consists of two types of particles, protons & neutrons, and is about 10,000 times smaller than the atom

• The majority of the atom’s volume is where the electrons reside…called the electron cloud!

Atoms• Protons have a positive electrical charge• Neutrons, have no electrical charge…they

are neutral• Both Protons and Neutrons are located

in the NUCLEUS, which is located at the center of the atom…the nucleus moves!

• Most of the mass of an atom is concentrated in the nucleus…the nucleus is held together by strong nuclear force

• In an atom with no charge, the number of positively charged protons is balanced by an equal number of small, negatively charged particles called electrons.

• Electrons have a high kinetic energy but little mass…they are about 2,000 times smaller than a proton

• Remember, an atom that has an equal number of protons & electrons will have no overall charge

• Electrons move around in a cloud outside the nucleus at different energy levels

• Each energy level holds a certain number of electrons…The first energy level can hold 2 electrons…The second energy level can hold up to 8 (atoms like to have a filled outer shell!!)

• Electrons do not orbit the nucleus in a defined path, it is impossible to know exactly where an electron is at any given time

• Electrons, which take up most volume of the atom, play a major role in chemical reactions

• All atoms of the same element will have the same amount of protons…the amount of protons is the atomic number of that particular element

• The atomic mass of an atom is the number of protons PLUS the number of neutrons…an electron’s mass is so small it is not included in the mass

• Atomic Mass – Atomic Number (Protons) = Neutrons• Atomic Mass – Neutrons = Atomic Number (protons)• Atomic weight (average atomic mass) is the average of

the atomic masses of the isotopes of each element…it is listed for each element of the periodic table

Atomic Number & Atomic Mass

• Elements are pure substances that cannot be broken down chemically into simpler kinds of matter… EACH HAVE THEIR OWN PROPERTIES (CHARACTERISTCS)

• They are made of only one type of atom; each element is made of a different atom.

• There are over 100 known elements; they are to molecules as the alphabet is to words; All are located on The Periodic Table of the Elements which is in order of increasing atomic number

• Each element has a different chemical symbol that is composed of one or two letters

• Some of the words are abbreviations of their Latin names

Elements

Isotope

s•Atoms of the same element with different numbers of neutrons are called isotopes

Ions

• Electrons, which take up most volume of the atom, play a major role in chemical reactions and bonding

• Electrons move around in a cloud outside the nucleus at different energy levels, or distances from the nucleus

• Each energy level holds a certain number of electrons…The first energy level can hold 2 electrons…The second energy level can hold up to 8 (atoms like to have a filled outer shell!!)

• Electrons like to have a valence (outer) shell with 8 electrons!• An Ion is an atom or a molecule where the total number of electrons does

not equal the total number of protons, giving it an overall charge• Ions form when an atom gains or loses one or more electrons, giving them an

overall charge…(+) ions are called cations, while (-) ions are called anions• In general, metal atoms tend to lose electrons or form cations; nonmetal atoms

tend to gain electrons or form anions

Common Ions

Chemical Formulas and

Chemical Reactions

• Chemical formulas show the proportion of each element in each compound (H2O) for example 2 Hydrogen : 1 oxygen atom

• Properties of a compound differ from the properties of the elements that compose it…depends on the type of bond

• Most elements undergo chemical reactions to become stable…bonds break, form, and reform

• In nature, almost all elements do not exist by themselves as illustrated on the periodic table

• The same elements can make different compounds…they have different chemical formulas (CO, CO2); (H2O, H2O2)…look at the elements involved!

• In a chemical formula coefficients show how many molecules are present

• Subscripts show how many of each element are present…don’t write 1

• In a chemical reaction, the molecules or elements to the left of the arrow are reactants, or what is present before the reaction…to the right of the arrow are products, or what is formed from the reaction

NH3

Subscripts

Bonding

• In nature, most elements do not exist by themselves• They are reactive, so they combine with other elements to form

molecules and compounds• A pure substance that is made up of atoms of two or more

different elements is called a compound• An element’s ability to form compounds depends on the amount

and arrangement of electrons in their outer energy levels• An atom’s outer energy level electrons are called valence

electrons• Atoms are stable when their outer energy level is filled!!

They bond with each other to make them filled

• Electrons move around in a cloud outside the nucleus at different energy levels• Each energy level holds a certain number of electrons…The first energy level can

hold 2 electrons…The second energy level can hold up to 8 (atoms like to have a filled outer shell!!)

• Sodium has one electron in its outer shell• Chlorine has seven• What needs to be done for each to become stable?• Sodium becomes a positive ion, because it gives up an electron, while chlorine becomes a

negative ion because it accepts an electron…opposite charges attract, resulting in an ionic bond…forms sodium chloride (table salt)

• Common between metals and non metals• Ionic compounds are electrically neutral, so the charges will predict the ratio of atoms

Bond…Ionic Bond Taken not shared

COVALENT Bond • Forms by the sharing of

electrons between atoms…between two nonmetals!!

• In the bond, no atom gains or loses electrons…no ions are formed!

• Water, for example has one oxygen atom and two hydrogen atoms held together by covalent bonds

• If you recall, hydrogen needs two electrons to become stable in the outer energy level, and oxygen needs two (because it has six electrons in the outer shell)

• Therefore, in the presence of one another, they share electrons

• Compounds that don’t share the electrons equally form POLAR COVALENT BONDS in which the electrons spend more time around one element, creating a polarized molecule

Metallic

Bonds

• Metal atoms share electrons equally in all directions• They are basically a nucleus in a sea of electrons• This allows metals to be good conductors of electric

current and heat• This also gives them their high melting points• What are some other characteristic properties of metals?