Download atomic number - Net Start Class

Unit 1 notes 2
Intro to parts of the atom
Synopsis of Structure
• The atom is the smallest part of an element
that retains its properties.
• It is made of mostly empty space,with the
majority of the mass concentrated in the
middle (the nucleus).
• The nucleus contains the positively
charged protons and the chargeless
neutrons.
• The electrons are situated outside of the
nucleus in the empty space in
energy levels (more on this later).
Warning:
Oversimplification Ahead
Atomic number, mass number,
atomic mass & isotopes
Atomic Number
• The atomic number of an element tells
you the number of protons in the nucleus.
• Atoms are electrically neutral when found
in elemental form, so the number of
protons will be the same as the number of
electrons.
• Each element has a different atomic
number.
Atomic
number
Answer the following
•What element has atomic number
32?
•How many protons does helium
have?
•What element has 79 protons?
•What is uranium’s atomic number?
•How many electrons does a neutral
atom of calcium have?
Mass number
• Most of the mass of an atom is in the
nucleus; so much that the electrons can be
neglected.
• The mass number is the number of protons
and neutrons in the nucleus.
• If you know the mass number and the
atomic number, you can determine the
composition of an atom.
# of neutrons = mass number – atomic
number
Mass nuMber, cont’d.
• Composition can be written in shorthand by
putting the symbol for the element with the
mass number and atomic number to the left.
12
C
6
• Or, the mass number and chemical
element can designate atoms, such as in:
Carbon-12
Answer the following
•How many neutrons are in
potassium-39?
•How many protons are in lead-208?
•If an element has 22 protons and 26
neutrons, what is its mass number?
•What element is in the previous
question?
201
•How many neutrons are in 80 Hg ?
Isotopes
• Atoms of the same element always have
the same number of protons, but can have
different numbers of neutrons.
• An atom with the same number of protons
but different number of neutrons are
called isotopes.
• Isotopes are chemically alike, because it is
the protons which are responsible for the
chemical behavior.
Answer the following
•What is the difference between
iodine-127 and iodine-125?
•Is an element with 25 protons and
30 neutrons an isotope of one
with 26 protons and 30 neutrons?
Why or why not?
Elements in Compounds
• When atoms chemically combine
to make compounds, many times
they lose or gain electrons.
• Unequal numbers of protons and
electrons means that the particle is
charged.
• Instead of calling these atoms, we
call these ions.
Elements in Compounds
• Metals tend to lose electrons in
Noncompounds, making them positive.
metals
• Nonmetals tend to gain electrons
Metals
in compounds, making them
negative
Answer the following
•How many electrons are in Ca2+?
•How many electrons are in O2-?
•How many protons are in P3-?
Atomic mass
• Because there can be several isotopes of an
element, there is another number, the
atomic mass, that is the weighted average
mass of all an element’s isotopes.
• Carbon-12 was used as a reference to
measure mass of atoms, and the carbon-12
atom was said to have a mass of exactly 12
amu’s.
Atomic mass
Finding Weighted Avg.
• Take into account both the mass of all
isotopes as well as their relative
abundances. For example:
• There are two known stable isotopes of
carbon; carbon-12 and carbon-13. If 99.0% is
carbon-12 and the rest is carbon-13, what is
the average atomic mass of carbon?
Finding Weighted Avg.
• Zinc has five naturally occurring isotopes;
zinc-64 (48.89%), zinc-66 (27.81%), zinc-68
(18.57%), zinc-67 (4.11%) and zinc-70
(0.62%). Based on this, what is the average
atomic mass of zinc?
Elements and the
Periodic Table
Arrangement of the table
• The periodic table is arranged in order
of increasing atomic number.
• They are in groups according to similar
chemical and physical properties.
• What gives them similar properties?
Arrangement of the table
• Elements in the same group have the
same number of valence electrons.
• This means they tend to lose or gain
electrons in bonding the same way and
make similar compounds.