Atomic Structure, Periodic Table, & Lewis Structures

Basic Definitions Atom – smallest unit of an element that retains the properties of that element Atoms - made up of several subatomic particles: protons, neutrons, and electrons

Protons, Neutrons, & Electrons +1 charge found in the nucleus of the atom Neutrons no charge found in the nucleus of an atom Electrons -1 charge found on the outside of the nucleus Nucleus made up of protons and neutrons overall + charge

Atomic Structure

Atomic Numbers Atomic number (Z) - number of protons in the nucleus of an atom of that element. The number of protons determines the identity of an element The number of protons for an element CANNOT be changed.

Atomic Numbers Atoms have no overall electrical charge so the number of protons must equal the number of electrons. So, the atomic number of an element also tells the number of electrons in a neutral atom of that element. The number of electrons can be changed when determining the charge of an ion.

Masses The mass of a neutron is almost the same as the mass of a proton. The sum of the protons and neutrons in the nucleus is the mass number (A) of that particular atom. Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they all have the same atomic number (number of protons & electrons)

Isotopes When writing isotopes, the atomic number (or number of protons) will appear at the bottom left of the formula The mass number (number of protons plus neutrons will appear at the top left of the formula. The element symbol will appear to the right of the numbers The different number of neutrons has NO bearing on chemical reactivity

Writing the Names of Isotopes When writing the name of an isotope, you will write the name of the element – the mass number For example 126 C would be named: Carbon - 12

Try the following Name Symbol # Protons # Neutrons # Electrons Mass # Carbon – 11 197 Au 79 1 2 25 55 Oxygen - 15

Try the following Name Symbol # Protons # Neutrons # Electrons Mass # Carbon – 11 11 C 6 5 Gold - 197 197 Au 79 118 Hydrogen – 3 3 H 1 2 Manganese - 55 55 Mn 25 30 Oxygen - 15 15 O 8 7

Try this one Name Symbol # Protons # Neutrons # Electrons Mass # Iodine -1 - 130

Try this one Name Symbol # Protons # Neutrons # Electrons Mass # Iodine -1 - 130 130 I -1 53 77 54

Atomic Mass Atomic mass –the weighted average mass of all the naturally occurring isotopes of that element. The number is usually located at the bottom of the periodic table and has decimal places

Calculating Atomic Mass

Calculating Atomic Mass Copper exists as a mixture of two isotopes. The lighter isotope (Cu-63), with 29 protons and 34 neutrons, makes up 69.17% of copper atoms. The heavier isotope (Cu-65), with 29 protons and 36 neutrons, constitutes the remaining 30.83% of copper atoms.

Calculating Atomic Mass First, calculate the contribution of each isotope to the average atomic mass, being sure to convert each percent to a fractional abundance.

Calculating Atomic Mass The average atomic mass of the element is the sum of the mass contributions of each isotope.

Try this one… Calculate the atomic mass of germanium. 72.59 amu

You can tell many things from an isotope formula Hydrogen has three naturally occurring isotopes in nature: Hydrogen – 1, Hydrogen – 2, and Hydrogen – 3. Which is the most abundant in nature? Hydrogen – 1 Which is the heaviest? Hydrogen - 3

Periodic Table Periodic Table – arrangement of elements in order of increasing atomic number with elements having similar properties in vertical columns Groups – vertical columns Periods – horizontal rows

Group Names Group Name 1A Alkali Metals 2A Alkaline Earth Metals 6A Chalcogens Halogens Noble Gases

Groups The group tells you the number of valence electrons that the element has Valence electrons - electrons in the outermost shell of the atom All group 1A elements have 1 valence electron. Likewise, all group 8A elements have 8 valence electrons.

Characteristics Elements in the same group exhibit similar chemical characteristics due to the fact that they all have the same number of valence electrons. The most stable number of valence electrons is 8 This is called an octet

Charges Every element wants 8 valence electrons to become stable. They will gain or lose valence electrons to form an octet For example…Group 1A elements have 1 valence electron. They can either gain 7 electrons to have an octet or lose 1. What is easier? Lose 1 If an element loses 1 electron (1 negative charge) what charge will the resulting ion have? +1

Charges Let’s go to group 7A. This group has 7 valence electrons It can either loose 7 or gain 1 What is the easiest? Gain 1 What will be the resulting charge if the element gain 1 electron (1 negative charge)? -1

Physical States and Classes of the Elements The majority of the elements are metals. They occupy the entire left side and center of the periodic table. Nonmetals occupy the upper-right-hand corner. Metalloids are located along the boundary between metals and nonmetals.

Metals Metals are elements that have luster, conduct heat and electricity, and usually bend without breaking. All metals except mercury are solids at room temperature; in fact, most have extremely high melting points. The periodic table shows that most of the metals are not main group elements.

Transition Metals The elements in Groups 3 through 12 of the periodic table are called the transition elements. All transition elements are metals. Many transition metals can have more than one charge

Non Metals Most nonmetals don’t conduct electricity, are much poorer conductors of heat than metals, and are brittle when solid. Many are gases at room temperature; those that are solids lack the luster of metals. Their melting points tend to be lower than those of metals.

Metalloids Metalloids have some chemical and physical properties of metals and other properties of nonmetals. In the periodic table, the metalloids lie along the border between metals and nonmetals.

Lewis Structure Lewis Structures – shows how the valence electrons are arranged among the atoms of a molecule There are rules for Lewis Structures that are based on the formation of a stable compound Atoms want to achieve a noble gas configuration

Octet & Duet Rules Octet Rule – atoms want to have 8 valence electrons Duet Rule – H is the exception. It wants to be like He & is stable with only 2 valence electrons

Steps for drawing Lewis Structures Sketch a simple structure with a central atom and all attached atoms Add up all of the valence electrons for each individual atom If you are drawing a Lewis structure for a negative ion add that many electrons to create the charge If you are drawing a Lewis structure for a positive ion subtract that many electrons to create the charge

Steps for drawing Lewis Structures Subtract 2 electrons for each bond drawn Complete the octet on the central atom & subtract those electrons Complete the octet on the surrounding atoms & subtract those electrons Get your final number If 0  you are done! If +  add that many electrons to the central atom If -  need to form multiple bonds to take away that many electrons

Examples CCl4 Sketch a simple structure with a central atom and all attached atoms Cl │ Cl – C – Cl

Examples Add up all of the valence electrons for each individual atom 4 + 4(7) = 32 Subtract 2 electrons for each bond drawn 32-8 = 24 Complete the octet on the central atom & subtract those electrons Done

Examples Complete the octet on the surrounding atoms & subtract those electrons 24 – 24 = 0 Final number = 0…DONE! Final structure is… __ │Cl │ __ │ __ │Cl – C – Cl │ │

Examples HF

Examples NH3

Examples NO+

Exceptions to the octet rule Sometimes the central atom violates the octet rule and has more or less than 8 valence electrons Keep using the same rules to draw Lewis Structures

Examples SF4 ICl3 XeF4 ICl-

Resonance When more than one Lewis Structure can be written for a particular molecule Resonance structure – all possible Lewis structures that could be formed The actual structure is the average of all of the structures You MUST show all structures!

Examples SO3 NO2- NO3-