Atoms

Modern Atomic Theory atoms composed of: protons – neutrons – (+) charge, located in nucleus neutrons – (0) charge, located in nucleus electrons – (-) charge, located outside the nucleus in electron cloud

Atomic number based on the number of protons 11 Na Sodium 22.989707

Mass Number The sum (+) of the protons and neutrons in an atom. Must be a whole # (No decimals) Can’t have ½ of a proton or neutron Not found on the periodic table!!

Nuclear Symbols Give the mass # and atomic # for an element How many protons? Just use atomic # =35 How many neutrons? Mass# - atomic # 80-35 = 45 80 Br 35

Ions atom or a group of atoms that has a positive or negative charge because it has gained or lost electrons charge of ion = # of protons - # of electrons Na - as a neutral atom 11 protons 11 electrons if the atom loses 1 electrons, it becomes an ion with a 1 + charge # of protons 11 - # of electrons -10 charge of ion 1+

How many protons and electrons are in: Cr 2 + ion S2- ion 16 p+ 18 e-

Isotopes Second postulate of Dalton’s theory states that all atoms of a given element are identical. Not always true. Isotopes are atoms that have the same number of protons (so they are the same element) but a different number of neutrons. All bromine atoms have 35 protons (atomic #) but some atoms of Br have 45 neutrons= Bromine 80 some atoms of Br have 47 neutrons= Bromine 82

the most common isotope of H has other isotopes of Hydrogen 1 p+, 0 neutrons  protium other isotopes of Hydrogen 1 p+, 1 no  deuterium 1 p+, 2 no  tritium isotopes of the same element differ in the number of neutrons basically the same chemical properties major difference is in the mass

Atomic Mass the weighted average of all of the isotopes of that element to find atomic mass – must look at the fractional (or percentage) abundance of the individual isotopes

Isotopes of silicon