1. Atoms

2. 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

3. Atomic number
based on the number of protons
11 Na Sodium

4. 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!!

5. 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

6. 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+

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

8. 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

9. 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

10. 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

11. Isotopes of silicon