Atoms and Elements
Goals:
1. Describe atomic structure and define atomic
number and mass number.
2. Understand the nature of isotopes and
calculate atomic weight from the isotopic
masses and abundances.
3. Explain the concept of the mole and use molar
mass in calculations.
4. Know the terminology of the periodic table.
The Atomic Structure
Experimental basis
• One of the pieces of evidence for the fact that
atoms are made of smaller particles came from
the work of Marie Curie (1876-1934).
• She discovered ___________, the
spontaneous disintegration of some elements
into smaller pieces.
- Rays emitted by polonium and radium – radioactive
atoms disintegrate and emit rays: alpha (a), beta (b),
gamma (g).
**Contradicted Dalton’s postulate: “atoms are indivisible”.
Experiment: Radiation
• Three rays:
Experiment: Cathode-Ray Tubes
• Electrons:
Same charge-to-mass ratio detected for different
elements: electrons are present in all atoms.
Millikan (charge of the electron), Goldstein (positive ions).
Rutherford’s Gold Foil Experiment
• Nucleus and the protons:
(+)
Chadwick (neutrons).
The Atomic Structure
• J.J. Thomson and E. Rutherford (England ~1900)
established a model of the atom – the basis of
modern atomic theory.
• Three subatomic particles:
– Electrically positive protons
– Mass = 1.672622 x 10-24 g
– Relative mass = 1.007 atomic mass units (u)
– Electrically neutral neutrons
– Relative mass = 1.009 u
– Electrically negative electrons
– Relative mass = 0.0005 u
– Which particle is heavier, which particle is lighter?
____________
_______________
Atom Composition
• The atom is mostly empty
space.
• protons and neutrons in the
nucleus.
• the number of electrons is
equal to the number of
protons.
• electrons in space around
the nucleus.
• extremely small. One
teaspoon of water has 3
times as many atoms as the
Atlantic Ocean has
teaspoons of water.
How big is an atom?
• The radius of the typical atom is
between 30 and 300 pm (3 x 10-11 m to 3
x 10-10 m). The radius of the nucleus is
about 0.001 pm.
• If an atom were a macroscopic object,
about the size of a football stadium,
what would be the radius of the
nucleus?
– __________________
What is Atomic Number?
Atomic number (Z) is the number of
_____________ in the nucleus of an
element.
13
Al
26.981
Atomic number
Atom symbol
Atomic weight
• All atoms of the same element have the same
number of __________ in the nucleus.
What is Mass Number?
The mass number (A) is the number of
____________ plus the number of
____________ in the nucleus of an
element.
C atom with 6 protons and 6 neutrons is the
mass standard = 12 atomic mass units (u)
1 u = 1.661 x 10-24 g
Mass Number (A) = # protons + # neutrons
A
12
C
X
6
Z
What is the mass number of an iron atom
with 30 neutrons?
• The periodic table gives the atomic number for
iron:
Atomic number
26
Atom symbol
Fe
Atomic weight
• Why the periodic table gives a WEIGHT of =
55.845 ?
Percent Abundance
50%/50% mixture
90%/10% mixture
What are isotopes? How do they
affect atomic weight?
• Isotopes: ___________________________
__________________________________.
• Because of the presence of atoms, the mass of
a collection of atoms is an average of values.
Average mass = ATOMIC WEIGHT
Boron is 19.9% 10B and 80.1% 11B. That is, 11B is
80.1 percent abundant on earth.
• What is the atomic weight of boron?
_________________
How many isotopes for H?
1
1H
1 proton and 0
neutrons, protium
2
1H
1 proton and 1
neutron, deuterium
3
1H
1 proton and 2
neutrons, tritium
(radioactive)
Gallium has two isotopes: 69Ga and 71Ga.
How many protons and neutrons are in the nuclei of each
of these isotopes? If the abundance of 69Ga is 60.1%,
what is the abundance of 71Ga?
Students should become familiar with the
use of atomic number and mass number,
especially in isotopes calculations.
How are the masses of isotopes and
their % abundances determined?
• They are experimentally determined by using
a ___________________________.
By changing the magnetic field, a beam of
charged particles of different mass can be
focused on the detector, and a spectrum
of masses is observed.
Copper exists as two isotopes: 63Cu (62.9298 u) and 65Cu
(64.9278 u). What is the approximate percentage of
63Cu in samples of this element?
Congratulations! You discovered a
new element…
you name wyzzlebium (Wz). The average atomic mass of Wz was
found to be 303.001 amu and its atomic number is 120.
A) If the masses of the two isotopes of Wz are 300.9326 amu
and 303.9303 amu, what is the relative abundance of each
isotope?
Congratulations! You discover a new
element…
B) What are the isotopic notations of the
A
two isotopes? (e.g. ZW).
C) How many neutrons are in one atom of
the more abundant isotope?
What is Atomic Weight?
• This tells us the mass of one atom of an
element relative to one atom of another
element.
• OR — the mass of 1000 atoms of one element
relative to 1000 atoms of another.
• Example: an O atom is approximately 1.333
times heavier than an C atom.
• Define one element as the standard against
which all others are measured
• Standard = carbon
Counting atoms
• In chemical reactions, how can we
predict the amount of product given the
mass of reactant?
Mg burns in air (O2) to produce
white magnesium oxide, MgO.
The Periodic Table
• Dmitri Mendeleev developed the modern
periodic table. Argued that element
properties are periodic functions of their
atomic weights.
• We now know that element properties are
periodic functions of their __________
__________ (G. J. Moseley) . – Law of
Chemical Periodicity
Students should become familiar with the
organization of elements in the periodic table.
The Periodic Table
Period
Family or
Group
Elements
• Metals
–
–
–
–
–
Solid (except for mercury)
Conduct electricity
Ductile (can be drawn into wires)
Malleable (can be rolled into sheets)
Alloys (solutions of one metal into another metal)
• Metalloids (semimetals)
– Physical characteristics of metals, but chemical
characteristics of nonmetals.
• Nonmetals
– Do not conduct electricity (except from C – graphite)
Hydrogen
• Non-metal.
• Most abundant element in
the universe.
• Lightest element.
Group IA: Alkali Metals
• React with water to give
alkaline solutions.
• Found in nature only
combined in compounds.
Group IIA: Alkaline Earth Metals
• React with water to give alkaline
solutions (except Be).
• Found in nature only combined in compounds.
Group IIIA
Al resists corrosion
(in HNO3).
Cu
Al
Gallium can be
a liquid at
room temp.
• All metals, but boron is a metalloid.
• Aluminum is the most abundant metal in the earth’s
crust by mass (8.2%).
Group IVA
Diamond
Quartz, SiO2
• C has large biological importance.
• Nonmetal: C, Metalloids: Si, Ge, Metals: Sn, Pb.
• All form analogous compounds such as their oxides: CO2,
SiO2, GeO2, SnO2, PbO2.
What are allotropes?
• Characteristic of the chemistry of nonmetals.
• Allotropes: _________________________
_______________________ each having its own
properties.
• C: graphite, diamond, buckminsterfullerene.
The periodic table
Practice
• Decide which represents more mass:
a) 10 atoms of Fe or 10 atoms of K
b) 9 g of Na or 0.5 mol of Na
Group VA
• N2: makes up about ______ of
earth’s atmosphere.
• N and P have biological importance.
• P has several allotropes.
• Nonmetals: N2 and P, Metalloids: As
and Sb, Metal: Bi.
White and red P • Analogous oxides: N2O5, P2O5, As2O5.
Group VIA
• O2: makes up ______of earth’s
atmosphere.
• S, Se, Te are the chalcogens (present
in cupper ores).
• O2 has an allotrope: O3 (ozone).
• Nonmetals: O2, S and Se, Metalloids:
Te, Metal: Po (radioactive).
• Analogous oxides: SO2, SeO2, TeO2.
Group VIIA: Halogens
• All nonmetals, named the halogens
(salt forming, they react violently
with metals of group IA.
• All exist as diatomic molecules.
• F2 and Cl2 are gases. Br2 is a liquid.
I2 is a solid.
Group VIIIA: Noble Gases
• All are gases.
• The least reactive elements (called inert gases).
• Helium is the second most abundant element in the universe.
But non is abundant on earth or earth’s atmosphere (also
called rare gases).
The Transition Elements
Lanthanides
Actinides
The Transition Elements
• Most occur naturally in compounds.
Ag, Au, and Pt are much less
reactive and can be found in nature
as pure elements.
• Some are of biological importance:
Fe, Zn, Cu, Co.
Essential Elements
• Found in hemoglobin (Fe), vitamin B12 (Co).
Practice
•
•
•
•
Element in the 2th period, group 3A
Alkali metal in the 4th period
Metalloid in group 6A
Noble gas in 3th period
Units for counting
1 dozen = 12 objects
Eggs are measured by the dozen
1 ream = 500 objects
Paper is measured by the ream, 500
sheets
Counting atoms
Chemistry is a quantitative
science—we need a “counting unit.”
MOLE
Definition: 1 MOLE is the amount of
substance that contains as many
particles (atoms, molecules) as there
are in __________________.
How many Particles in one Mole?
• Amedeo Avogadro (1776-1856).
• There is Avogadro’s number of particles in a
mole of any substance.
6.02214199 x
23
10
1 mole = 6.022 x 1023 items
What is Molar Mass?
1 mol of 12C
= 12.00 g of C
= 6.022 x 1023 atoms
of C
12.00 g of 12C is its MOLAR MASS
Taking into account all of the
isotopes of C, the molar mass of C
is 12.011 g/mol.
One Mole Amounts
Summary
• The atomic mass expressed in grams is the
molar mass of an element. It is different for
each element.
1 molar mass = atomic mass of an element in grams
• One mole of any element contains Avogadro’s
number of atoms.
1 mol of atoms = 6.022 x 1023 atoms
You can use these relationships to make
conversions between number of atoms, mass,
and moles.
Mass
Moles Conversion
It is essential to convert from moles to
mass and from mass to moles.
Moles x
grams
= grams
Grams x
1 mol
= moles
1 mol
grams
molar mass
1/molar mass
What amount of Mg in moles is represented by
0.200 g? How many atoms?
12
Mg
24.3050
Atomic number
Atom symbol
Atomic weight
Mg has a molar mass of 24.3050 g/mol.
Students should become familiar with calculations
using molar mass and avogadro’s number.
Remember
• Go over all the contents of your
textbook.
• Practice with examples and with
problems at the end of the chapter.
• Practice with OWL tutor.
• Practice with the quiz on CD of
Chemistry Now.
• Work on your assignment for Chapter 2.