Section 4.2—Atomic Structure

What are atoms? Atom - smallest piece of matter that has the chemical properties of the element.

What’s in an atom? An atom is made of three sub-atomic particles ParticleLocation Nucleus Outside the nucleus Mass 1 amu = 1.67  kg amu 9.10  kg Charge +1 0 Proton Neutron Electron 1 amu (“atomic mass unit”) = 1.66  kg

What gives an atom its identity? What makes an atom “carbon” as opposed to “oxygen”? Every atom has a different number of protons. The number of protons determines the identity of the atom The atomic number shows the number of protons. Atomic number = protons

The Nucleus & Mass Since the nucleus has protons & neutrons, and the mass of each one is 1 amu… The mass of the nucleus (in amu’s) is the number of protons + neutrons Since electrons have relatively no mass (0.054% of one proton or neutron), we don’t need to worry about them when determining mass of an atom Mass # = protons + neutrons

Charges Protons have a positive charge Electrons have a negative charge Neutrons have no charge Overall charge = protons + (-1)×electrons Charge = protons - electrons

How do we show information about an element?

X AC Z# Element symbols Charge # protons - # electrons (assumed to be “0” if blank) Element Symbol 1 or 2 letters, found on the periodic table Mass number # protons + # neutrons Atomic number # of protons Number How many atoms do you have?

Example: Element symbols O Charge -2 Element Symbol O = Oxygen Mass number 16 Atomic number 8 Number Assumed to be “1” if blank

Let’s Practice Example: Fill in the missing values SymbolNameAtomic # Mass # ChargeProtonNeutronElectron Magnesium

Let’s Practice Example: Fill in the missing values SymbolNameAtomic # Mass # ChargeProtonNeutronElectron Magnesium Remember: Atomic number is the identity Atomic number = protons Charge = proton - electrons Mass # = protons + neutrons Lead

Isotopes

What are isotopes? Isotopes - n. Atoms of the same element with a different number of neutrons Some isotopes are radioactive—but not all…many are quite stable!

Isotopes Example If they have different number of neutrons, and neutrons have a mass of 1 amu… Then isotopes of the same element will have different masses! But because their protons are the same, they are the same element! Hydrogen-2 Hydrogen-1 Mass # = 1 amu Mass # = 2 amu

Identifying Isotopes 12 C 13 C Carbon-12Carbon-13 Isotopes can be differentiated by their different mass numbers in the element symbol Or by the mass number following their name.

Mass Number versus Atomic Mass Mass NumberAverage Atomic Mass # of protons + # of neutronsAverage of actual masses For one specific isotope only Is not found on the periodic table Weighted average of all isotopes Is found on the periodic table Always a whole numberNot a whole number

Calculating Average Atomic Mass Average atomic mass =  Abundance of isotope Mass of isotope  () Average atomic mass is a weighted average (it takes into account how often each isotope occurs). “Sum of” What fraction of the time is that isotope present? Actual mass (not mass number)

Example of Finding Avg Atomic Mass Example: Find the atomic mass of chlorine if Chlorine-35 has a mass of amu and Chlorine-37 has a mass of amu and is present 24.22% of the time. Remember that percents add up to 100. So they said the second isotope is present 24.22% of the time. This means that the first isotope is present = 75.78% of the time IsotopeMassPercentDecimal amu amu This chart summarizes the information in the problem: = amu(this is what’s on the periodic table for Cl!)