 # Atomic Structure. Modern Atomic Theory Atom – smallest particle of an element that retains the properties of the element Subatomic Particles –Protons.

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Atomic Structure

Modern Atomic Theory Atom – smallest particle of an element that retains the properties of the element Subatomic Particles –Protons –Neutrons –Electrons

Protons Positive Charge (+1) Located in the nucleus Mass = 1 amu (atomic mass unit) –1 amu = 1.66 x 10 -24 grams Symbol: p + or H +

Neutrons Neutral Charge Located inside the nucleus Mass = approximately 1 amu Symbol: n

Electrons Negative Charge (-1) Located outside the nucleus Mass = 1/1836 of a proton (basically mass less) Symbol: e - or e -1

Nucleons Particles located inside the nucleus –Protons and Neutrons All mass is located in the center of the atom (inside the nucleus), nucleus is very dense

Atomic Number The number of protons in an atom Bold number on the periodic table For a neutral atom: Number of protons = number of electrons Why???

Atomic Mass/Mass Number The mass of an atom (expressed in amu) Equal to the number of protons + the number of neutrons in an atom Why aren’t electrons included in the mass?

Nuclear Symbol The nuclear symbol consists of three parts –the symbol of the element –the atomic number of the element –the mass number of the specific isotope

Examples Indicate how many protons, neutrons and electrons are in each of the following atoms. 1.Aluminum-27 2.Xe-131 3. 56 Fe

Gram Atomic Mass The mass of 1 mole of the element –1 mole = 6.02 x 10 23 particles Expressed in grams instead of amu

Mass Examples 1.1 atom of K = 2.1 mole of K = 3.1 CO 2 molecule = 4.1 mole of CO 2 =

Isotopes Same element, same atomic number, number of protons are the same Different masses, because the number of neutrons varies Example: indicate the number of protons, neutrons, and electrons in carbon-12 and carbon-14

Weighted Average Atomic Mass Mass found on the periodic table is the weighted average of the naturally occurring isotopes Accounts for the percent abundance and mass of each of the elements isotopes

How to Calculate Each exact atomic mass is multiplied by its percent abundance (expressed as a decimal). Then, add the results together and round off to an appropriate number of significant figures. Example: Nitrogen Mass Number Exact Mass (amu) Percent Abundance 1414.00307499.63 1515.0001080.37

Examples Calculate the weighted average atomic mass for Magnesium. Mass Number Exact Mass (amu) Percent Abundance 2423.98504378.99 2524.98583710.00 2625.98259311.01

Ions Occur when a neutral atom gains or loses a electrons Caused by the transfer of electrons Results in a charged atom Protons are never lost or gained

Cation Positive Ion Results from the loss of an electron Elements on the left side of the table (metals) tend to lose electrons to form cations

Anion Negative ion Results from the gain of an electron Elements on the right side of the table (nonmetals) tend to gain electrons to form anions

Ion Examples Indicate the number of protons, neutrons and electrons in each: 1. 35 Cl atom: 2. 35 Cl - ion: 3. 39 K atom: 4. 39 K + ion: 5. 40 Ca atom: 6. 40 Ca 2+ ion:

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