The Structure of the Atom The Building Block of Matter

Atoms are the smallest particles of an element that retains the chemical properties of that element Parts:  Nucleus – small region in the center of the atom. The nucleus contains protons and neutrons  Electron Cloud – large region, compared to nucleus, where electrons are found.

Size of nucleus vs. electron cloud If an atom was the size of a sports stadium, the nucleus would be the size of a marble!

Subatomic Particles - Proton Is found inside the nucleus Has a positive (+) charge Has a relative mass of 1 Determines the atomic number

Subatomic Particles - Neutron Is found inside the nucleus Has neutral (0) charge Has a relative mass of 1 Determines the isotope

Subatomic Particles - Electron Found outside the nucleus  Bohr model – electrons are in specific energy levels  Electron cloud model – electrons are in a random cloud surrounding the nucleus Has a negative (-) charge Has a relative mass of 0 Determines the ion

Subatomic Particles Summary Subatomic Particles: Protons : + charge, relative mass = atomic mass units (amu); round to 1 Neutrons: = charge, relative mass = atomic mass units (amu); round to 1 Electrons: - charge, relative mass = atomic mass units (amu); round to 0 (not factored in when figuring total mass of an atom)

What is the structure of an atom? Bohr Model “Planetary Model” Schrödinger Model “Electron Cloud Model”

How are protons, neutrons and electrons related? # protons = atomic # # electrons = # protons in a neutral atom # protons + # neutrons = atomic mass

Atomic Number Number of protons in the nucleus (also the number of electrons) Typically the number on top of the symbol on the Periodic Table Atomic Number

Atomic Mass Number of protons + number of neutrons in an atom. When solving for number of neutrons: round mass number properly, subtract atomic number from mass number.  For Al:atomic mass – atomic number = # of neutrons 27-13=14 neutrons Atomic Mass

Let’s practice! (Round to whole #s) ElementSymbol# P# E# NAtomic #Atomic Mass Lithium7 Br Li3343 Bromine Potassium K Sodium Na Argon Ar18 40 SiliconSi14

Element Symbols Hyphen Notation: Element symbol – Mass # (rounded) Example: Helium  He-4 Nuclear Symbol: Superscript Mass Number, Subscript Atomic Number, Symbol Example: He 4 2

Isotopes Atoms with the same number of protons & electrons but a different number of neutrons. They are the same element, but have different masses. All isotopes are used to calculate atomic mass (reason why weight is a decimal). Most elements consist of a mixture of isotopes.

Average Atomic Mass An element can exist in a number of forms, called isotopes. Isotopes are forms of the same atom that vary in mass. For example, there are two different types (isotopes) of copper atoms. One type of copper atoms (copper- 63) weighs in at amu, the other (copper-65) has a mass of amu. The lighter isotope is more common with 69.09% of the naturally occurring copper having a mass of amu per atom. The remainder of the atoms, %, have a mass of amu.

Average Atomic Mass To find the AVERAGE ATOMIC MASS of an atom, we take into account all of the isotopes that exist and the percentage of each type. The calculation of the average atomic mass is a WEIGHTED AVERAGE. Average atomic mass = Σ (mass of isotope × relative abundance)

Average Atomic Mass The bottom line is that to find the average atomic mass of copper, we insert the information about copper’s isotopes into the formula and solve. There are two isotopes, so we will be adding the contributions of 2 isotopes. (That’s where the Σ sign comes in.) The relative abundance is simply the percentage of the isotope, but in decimal format % corresponds to a relative abundance of

Average Atomic Mass Let’s look at our copper again: One type of copper atoms weighs in at amu, the other has a mass of amu. The lighter isotope is more common with 69.09% of the naturally occurring copper having a mass of amu per atom. The remainder of the atoms, %, have a mass of amu. Average atomic mass of copper = (62.93 amu × ) + (64.94 amu × )= amu From the calculation, we know that an AVERAGE atom of copper has a mass of amu. Notice that in this problem, we would predict that the average is closer to the weight of the lighter isotope. This is because the lighter form of copper is more abundant.

Let’s Practice What is the percentage for Silver-109? Find the average atomic mass of an atom of Ag. Isotope NameIsotope MassIsotope Percentage Silver Silver Average atomic mass = ( x.5186) + ( ) Average atomic mass =