Transcript Basic structure of atoms

Basic structure of
atoms
Basic Structure of an Atom
• Nucleus and electron cloud
• Nucleus: very small, dense core which
consists of protons and neutrons
(nucleons)
Nucleons
• Protons:
– Positive charge +1
– Mass= 1.763 x10-24 grams
– Symbols include +, p, 1H1
• Neutrons:
– Neutral charge 0
– Mass= 1.765 x10-24 grams
– Symbols include n, 0n1
Electron cloud
• Electrons move very rapidly in complicated
paths called orbitals.
• Because of this motion, they appear to
form a cloud.
– Negative charge -1
– Mass: 9.1 x10-28 grams
– Symbols include e-, -1e0
• Normally, atoms are electrically neutral.
– This means that for each proton in the
nucleus, there is one electron on the outer
surface
• Atoms which don’t have this balance are
called ions
– Atoms with extra electrons are negatively
charged and called anions
– Atoms with a deficiency of electrons are
positively charged and called cations
Atomic Number (Z)
• Number of protons which an atom
contains
– Always a whole number
– Determines which type of element an atom is
Atomic Mass Number (A)
• Found by adding mass of protons and
neutrons
– Whole number
Determining the number of
neutrons
• Atomic mass number= neutrons + protons
• Atomic mass- atomic number = neutrons
40.078
Ca
20
How many protons are in …
7
Li
3
35
207
Cl
Pb
82
?
Isotopes
• Atoms of the same element that have
different numbers of neutrons
• Isotopes of an element
do not exhibit chemical
differences
• They do exhibit
physical differences
(i.e. mass)
Atomic Weight
• Represents the average weight of an
element taking into account the different
weights of any naturally occurring isotopes
for that element
Calculating Atomic Weights
• Example: Chlorine has 2 naturally
occurring isotopes- Cl-35 and Cl-37
– 75% Cl-35
(.75)(35) + (.25)(37)= 35.5
Chromium occurs in 4 isotopic forms with
the following approximate distribution: 6%
Cr-50, 85% Cr-52, 8% Cr-53, and 1% Cr54. Calculate the atomic weight.