Download atomic number - Z

Atoms & The Periodic Table
Chapters 4 & 5
• John Dalton developed an atomic theory in
1808. Dalton’s was the first atomic theory
with a scientific basis.
• Atom - the smallest part of an element that
still has the element’s properties.
• Atoms are the building blocks of molecules.
• Atoms are made of protons, neutrons, and
electrons.
• nucleus - small, dense with a positive
electric charge located at the center of
each atom.
• made of protons - a subatomic
particle that has a positive charge
• neutrons - a subatomic particle that
has no charge
• Electrons - subatomic particles with
negative charges.
• move around outside the nucleus
• Unreacted atoms have no overall charge.
– Although atoms are made of charged
particles, they do not have an overall charge
because they have an equal number of
protons and electrons whose charges exactly
cancel.
• Bohr’s model compares electrons to planets.
– In 1913, Niels Bohr suggested that electrons
in an atom move in set paths around the
nucleus much like the planets orbit the sun in
our solar system.
• In Bohr’s model, electrons can only be in
certain energy levels.
Bohr’s Model
• Electrons are found in orbitals
within energy levels.
• Orbital - a region in an atom
where there is a high probability of
finding electrons
• An s orbital is shaped like a sphere
• A p orbital is dumbbell shaped and
can be oriented three different
ways in space:
• Valence electrons - found in the outermost
shell of an atom and determine the atom’s
chemical properties.
• The electrons in an atom that participate in
chemical bonding
• Every atom has between one and eight
Periodic Table
• The periodic table groups similar elements
together.
– Makes it easier to predict the properties of an
element based on where it is in the periodic
table.
• Elements are listed in order of number of
protons
• Periodic law - states that when elements are
arranged this way, similarities in their
properties will occur in a regular pattern.
• The periodic table helps determine
electron arrangement.
• periods - horizontal rows in the periodic
table
– As the number of protons an atom has
increases as you move from left to right
across a period, so does its number of
electrons.
• Elements in the same group have similar
properties.
• Group - a vertical column of elements in
the periodic table.
• Atoms of elements in the same group have
the same number of valence electrons, so
these elements have similar properties.
• Ion - an atom or group of atoms that has
lost or gained one electrons and has a
negative or positive charge
• A lithium atom loses one electron to form a 1+
charged ion:
• Ion - an atom or group of atoms that has
lost or gained one electrons and has a
negative or positive charge
• A lithium atom loses one electron to form a 1+
charged ion:
•A fluorine atom gains one electron to form a 1
charged ion:
• atomic number - Z, of an atom equals the
number of protons in the nucleus.
• mass number - A, of an atom equals the
number of protons plus the number of
neutrons in the nucleus.
Atomic number
Mass Number
• isotope - an atom that has the same
number of protons as other atoms of the
same element do but that has a different
number of neutrons.
– Example: Hydrogen has three isotopes,
• If you know the atomic number and mass
number
of an atom, you can calculate the number
of
neutrons it has.
• Example: uranium-235 has a mass number of
235. Like all uranium atoms, it has an atomic
number of 92. The number of neutrons it has
is:
Mass number (A):
Atomic number (Z):
Number of neutrons:
235
–92
143
Isotopes
•average atomic mass - is a weighted average
of the masses of all naturally-occurring isotopes
of an element.
• The elements are classified into three groups.
• Metals - elements that are shiny and conduct
heat and electricity well
• most elements
• Nonmetals, all except hydrogen of which are
found on the right side of the periodic table,
may be solids, liquids, or gases at room
temperature.
• Metalloids / Semiconductors - elements that
can conduct electricity under certain
conditions
• between metals and nonmetals
• alkali metals- in Group 1
are very reactive.
• alkaline-earth metalsfound in Group 2 of the
are somewhat less reactive
than the alkali metals.
• The transition metals,
found in Groups
3–12
Nonmetals
• Carbon is found in three
different forms and can form
many compounds
• Nonmetals and their
compounds are plentiful on
Earth.
• Halogens - located in Group
17
• Noble gases, found in Group
18
– they are unreactive.
• Metalloids / Semiconductors intermediate conductors of heat
and electricity
• Silicon is the most familiar
semiconductor.
– Silicon is an important part of
computer chips, as well as
other semiconductor devices
such as transistors, LED
display screens, and solar
cells
A mole (mol) is the number of particles that is the same
as the number of atoms of carbon in 12 g of carbon12. useful for counting small particles
• Avogadro’s constant - the number of particles per
mole of a substance: 6.022 × 1023
• Moles and grams are related.
• molar mass - mass in grams of 1 mol of a substance
– For example, 1 mol of carbon-12 atoms has a molar mass of
12.00 g.
– The molar mass of an element = average atomic mass,
which is listed in the periodic table.
• To convert between moles and grams and
vice versa, you can use a conversion
factor: a ratio that is derived from the
equality of two different units.
• Let’s say that a shopkeeper knows that
exactly10 gumballs have a total mass of
21.4 g. This relationship can be written as
either one of two equivalent conversion
factors:
10 gumballs
21.4 g
21.4 g
10 gumballs
Math Skills
Conversion Factors What is the mass of
exactly 50 gumballs?
1. List the given and unknown values.
Given: mass of 10 gumballs = 21.4 g
Unknown: mass of 50 gumballs = ? g
2. Write down the conversion factor that
converts number of gumballs to mass.
The conversion factor you choose should
have the unit you are solving for (g) in the
numerator and the unit you want to cancel
(number of gumballs) in the denominator.
21.4 g
10 gumballs
3. Multiply the number of gumballs by
this conversion factor, and solve.
21.4 g
50 gumballs 

10 gumballs
107g
Converting Mass to Amount Determine the
amount of iron present in 352 g of iron.
1. List the given and unknown values.
Given: mass of iron = 352 g Fe
molar mass of iron = 55.85 g/mol Fe
Unknown: amount of iron = ? mol Fe
• 2. Write down the conversion factor that
converts grams to moles.
•
The conversion factor you choose should
have what you are trying to find (moles of Fe)
in the numerator and what you want to cancel
(grams of Fe) in the denominator.
1 mol Fe
55.85 g Fe
3. Multiply the mass of iron by this
conversion factor, and solve.
1 mol Fe
352 g Fe 

55.85 g Fe
6.30 mol Fe