Section 4.2—Atomic Structure

What are atoms? Atom - smallest piece of matter that still has the chemical properties of the element. One Atom in a solid pure elemental sample. Note how it represents or lookes like everyone else!

THE ATOM’s Nucleus DID YOU KNOW?????? The nucleus holds nearly all the mass of the atom! YET – it is sooo tiny! Look below and consider that IF a nucleus were the size of a pea – then the rest of the atom would be the size of a football stadium!! A nucleus is so dense that a nucleus the size of a pea has a mass of 250 million tons!!!! SO, if it is SO small – How do the protons stay together!! Don’t they repel! The STRONG force of the nucleus holds it together!!!!!! Small dense nucleus Electron Cloud – mostly empty space

What’s in an atom? An atom is made of three sub-atomic particles Location Mass Charge Proton Nucleus 1 amu = 1.6710-27 kg +1 Neutron Nucleus 1 amu = 1.6710-27 kg Electron Outside the nucleus ~0 amu = 9.1010-31 kg -1 1 amu (“atomic mass unit”) = 1.66  10-27 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 is therefore only due to the protons and electrons. Overall Charge = #protons - #electrons

How do we show information about an element?

X A C Z Element symbols Element Symbol Charge Mass number 1 or 2 letters, found on the periodic table X A C Z Charge # protons - # electrons (assumed to be “0” if blank) Mass number # protons + # neutrons Atomic number # of protons

Example: Element symbols O = Oxygen O 16 -2 8 Charge -2 Mass number 16 Atomic number 8

Fill in the missing values Let’s Practice Example: Fill in the missing values Symbol Name Atomic # Mass # Charge Proton Neutron Electron Magnesium-25 +2 82 126

Fill in the missing values Let’s Practice Remember: Atomic number is the identity Atomic number = protons Charge = proton - electrons Mass # = protons + neutrons Example: Fill in the missing values Symbol Name Atomic # Mass # Charge Proton Neutron Electron Magnesium-25 +2 82 126 12 25 12 13 10 Lead-208 208 82

Isotopes

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

Isotopes Example Hydrogen-2 Hydrogen-1 Mass # = 1 amu Mass # = 2 amu If they have a 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! How is the Density of Water Effected by the isotope?

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

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

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

Example of Finding Avg Atomic Mass Find the atomic mass of chlorine if Chlorine-35 has a mass of 34.969 amu and Chlorine-37 has a mass of 36.966 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 100-24.22 = 75.78% of the time Isotope Mass Percent Decimal 1 34.969 amu 75.78 0.7578 2 36.966 amu 24.22 0.2422 This chart summarizes the information in the problem: = 35.45 amu (this is what’s on the periodic table for Cl!)