Transcript chem chapter 04c notes

Useful
Element
Notations
Atomic Number (Z)
• The number of protons in the
nucleus
• Determines the identity of the
atom (which element it is)
If you change the number
of protons, the atom changes
to another element.
Mass Number (A)
• The total number of particles
in the nucleus—protons and
neutrons
• The total number of particles
that make up the mass of the
atom
Isotopes:
Count Those Neutrons
• Atoms of an element which
have various amounts of
neutrons but the same number
of protons
Since the number of
neutrons changes, the mass of
the isotopes is different.
Isotopic Notation
• It shows the mass number,
atomic number, and symbol
for the element.
Isotopic Notation
Example:
9
4
Be
9 protons and neutrons
4 protons
Therefore, there must be
5 neutrons.
Question
If isotopic notation for an
40
element is 18 Ar , how many
neutrons are in the nucleus?
1. 18
2. 40
3. 22
4. 58
Atomic Mass
• Most elements consist of a
mixture of different isotopes,
which have different masses.
• Therefore, the mass of the
element is the average of the
weights of the isotopes.
Unified Atomic
Mass Units
• Dalton tried to find the
masses of the atom but could
only find the relative masses.
• We now know the exact
masses.
Unified Atomic
Mass Units
• A unified atomic mass unit (u)
is approximately the mass of
a proton.
Example
Calculate the average atomic
mass of a chlorine atom.
In nature, 75.77% of chlorine
atoms have a mass of 34.969 u
per atom (Cl-35).
24.23% of chlorine atoms have
a mass of 36.966 u per atom
(Cl-37).
Example
1. Convert the percents to
decimals.
75.77% = 0.7577
24.23% = 0.2423
Example
2. Multiply the decimal
percentage by the mass of
each isotope to find the
total mass of each isotope
in the sample.
Cl-35: 0.7577 x 34.969 u = 26.50 u
Cl-37: 0.2423 x 36.966 u = 8.957 u
Example
3. Add the results to get the
weighted average mass of
the sample.
26.50 u + 8.957 u  35.46 u
The weighted average mass
of a typical chlorine atom:
 35.46 u
Valence Electrons:
Last But Not Least
• Outer level electrons
• In the s and p sublevels
• Maximum # in each atom: 8
 2 in the s
 6 in the p
Valence Electrons:
Last But Not Least
• Emit spectra
• Participate in chemical bonds
• Give elements physical
properties
Electron Dot
Diagrams
A shorthand method
of showing the valence
electrons
Electron Dot
Diagrams
Na
valence electrons: 1
Electron Dot
Diagrams
Mg
valence electrons: 2
Electron Dot
Diagrams
Al
valence electrons: 3
Electron Dot
Diagrams
Si
valence electrons: 4
Electron Dot
Diagrams
P
valence electrons: 5
Electron Dot
Diagrams
S
valence electrons: 6
Electron Dot
Diagrams
Cl
valence electrons: 7
Electron Dot
Diagrams
Ar
valence electrons: 8
Ions:
Charged Atoms
Changing Numbers
• Changing the number of
protons gives a different
element.
• Changing the number of
neutrons gives a different
isotope of the same
element.
Changing Numbers
• Changing the number of
electrons gives an ion of
that element.
Ions
• Are atoms which have
gained or lost electrons
• Occur when an electron
gains enough energy to
“jump” off an atom
Ions
• Are charged, rather than
neutral like atoms, because
the number of electrons
and protons are no longer
equal
Anions
• Are ions that have gained
extra electrons
• Are negatively charged
• Have gained negative
charges
Cations
• Are ions that have lost
some of their electrons
• Are positively charged
• Have lost negative charges