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MR. SURRETTE
VAN NUYS HIGH SCHOOL
CHAPTER 3: ELEMENTS AND ATOMIC STRUCTURE
CLASSNOTES
CONCEPT OF THE ATOM
The ancient Greek Democritus is crediting with coining the phrase atomos. Atomos means “indivisible”
from which the word atom later derived. He considered atoms to be indestructible building blocks
from which all substances are created.
EARLY ATOMIC MODEL
In the early 1800’s John Dalton re-discovered the concept of atoms. He tried to create a model of how
atoms behave. Based on his research, he determined that all atoms of an element have the same mass
and behave the same way.
EARLY ATOMIC MODEL
He also concluded that atoms combine in whole number ratios to form compounds.
DISCOVERY OF THE ELECTRON
In 1897, Joseph John (“JJ”) Thomson discovered the electron. He then proposed a model about how an
atom might be built. He called his model the “plum pudding atom.”
DISCOVERY OF THE ELECTRON
It had negatively charged electrons (raisins) stuck into a lump of positively charged protons (the dough).
“PLUM PUDDING” MODEL
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DISCOVERY OF THE NUCLEUS
In 1909, Ernest Rutherford sent alpha particles (He-4 nuclei) as “missiles” toward a thin sheet of gold.
As expected, most of the alpha particles went right through the sheet.
DISCOVERY OF THE NUCLEUS
Occasionally, however, one of the alpha particles would deflect or even bounce back from the gold
sheet.
RUTHERFORD’S EXPERIMENT
THE NUCLEUS AND THE PROTON
Rutherford concluded that almost all the mass and positive charge of the atom is concentrated in a dense
center, which he called the nucleus. He also defined the proton as the smallest unit of positive charge
inside the nucleus.
SUBATOMIC PARTICLES
Today we know that the atom is composed of three types of particles: protons, neutrons, and electrons.
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
SUBATOMIC PARTICLES
Subatomic
Particle
Mass (kg)
Mass (amu)
Proton
1.67193 x 10-27
1.0073
Neutron
1.672 x 10-27
1.0087
Electron
9.1 x 10-31
5.5 x 10-4
ATOMIC STRUCTURE
An atom is mostly empty space between the electrons and the nucleus. This presents a conceptual
problem: How do atoms form solids with all this empty space?
ATOMIC STRUCTURE ANALOGY
Imagine a jungle gym on a children’s playground. If you are a bug up close, you would see a large
jungle gym with plenty of room to crawl between the bars.
ATOMIC STRUCTURE ANALOGY
As an adult a block away, however, you would see a small jungle gym that has all the bars right next to
each other.
ATOMIC STRUCTURE ANALOGY
In the same way, macroscopic particles are much bigger than atoms and “don’t see” the empty spaces.
ELEMENTS
Elements on the periodic table are defined by their number of protons. For example, hydrogen has 1
proton, helium has 2 protons, magnesium has 12 protons, etc.
ATOMIC NUMBER
The number of protons Z in the nucleus is called the atomic number.
ATOMIC MASS
The number of protons Z plus the number of neutrons N is the atomic mass A:
A=Z+N
THE PERIODIC TABLE
Dimitri Mendeleev (1843 – 1907) developed the periodic table. The horizontal rows of elements in the
periodic table are called periods.
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MR. SURRETTE
VAN NUYS HIGH SCHOOL
THE PERIODIC TABLE
Elements in the same vertical column are called groups. Elements in the same group share similar
physical and chemical properties.
METALS
About 85 percent of the known elements are metals, defined as those elements that are shiny, opaque,
and good conductors of electricity and heat.
THE PERIODIC TABLE
NONMETALS
The nonmetallic elements are grouped together on the right side of the periodic table. In contrast to
metals, nonmetals are very poor conductors of electricity and heat.
METALLOIDS
Six elements are classified as metalloids: boron, B; silicon, Si; germanium, Ge; arsenic, As; antimony,
Sb; and tellurium, Te.
METALLOIDS
Situated between the metals and the non-metals in the periodic table, the metalloids have both metallic
and nonmetallic characteristics.
ATOMIC GROUPS
Because of similar properties, elements in the same vertical column are called a group. In the modern
periodic table, the 18 known groups are numbered from left to right.
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MR. SURRETTE
VAN NUYS HIGH SCHOOL
THE ATOMIC GROUPS
ALKALI METALS (Group 1)
The word alkali means “soap forming.” Early in human history people discovered that ashes composed
of group 1 elements mixed with water produce soap.
ALKALI METALS (Group 1)
Alkali metals are highly reactive and form strong bases in water like potassium hydroxide (KOH) and
sodium hydroxide (NaOH).
ALKALINE METALS (Group 2)
The elements of group 2 also form slippery alkaline solutions when placed in water. Medieval
alchemists noted that certain minerals they called “earth” did not melt or change when put in fire.
ALKALINE METALS (Group 2)
As a result, group 2 elements are called the alkaline or alkali-earth metals.
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VAN NUYS HIGH SCHOOL
GROUPS OF THE PERIODIC TABLE
TRANSITION METALS (Group 3 – 12)
The elements of groups 3 through 12 are all metals that do not form alkaline solutions with water.
Collectively they are called the transition metals.
TRANSITION METALS (Group 3 – 12)
The transition metals include: iron, Fe; copper, Cu; nickel, Ni; chromium, Cr; silver, Ag; and gold, Au.
THE HALOGENS (Group 17)
Elements of group 17 are called the halogens (“salt-forming” in Swedish) because of their tendency to
form salts. Fluorine and chlorine gases are classic examples of halogens.
DIATOMIC ELEMENTS
The halogens are extremely reactive. They are always found in diatomic pairs: F2(g), Cl2(g), Br2(l),
I2(s), and At2(s). Other reactive elements like hydrogen H2(g) and oxygen O2(g) are diatomic as well.
NOBLE GASES (Group 18)
Group 18 elements are unreactive gases that rarely combine with other elements. They were dubbed the
noble gases because nobility did not interact with common folk.
IONS
In general, metal atoms lose electrons to become positively charged ions called cations. Nonmetal
atoms tend to gain electrons to become negatively charged ions called anions.
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VAN NUYS HIGH SCHOOL
IONS
For example, the metal sodium atom loses one electron to become the cation Na+. The nonmetal
chlorine atom tends to gain one electron to become the anion Cl -.
OXIDATION/REDUCTION
The loss of electrons is called oxidation. For example, a lithium atom (3 protons and 3 electrons) can
lose one electron and becomes a lithium cation Li+ (3 protons and 2 electrons).
OXIDATION/REDUCTION
The gain of electrons is called reduction. For example, an oxygen atom (8 protons and 8 electrons) can
gain two electrons and become an oxygen anion O-2 (8 protons and 10 electrons).
OXIDATION/REDUCTION
Some atoms can undergo both oxidation and reduction. For example, an iron atom (26 protons and 26
electrons) can lose three electrons and become an iron cation Fe+3 (26 protons and 23 electrons).
OXIDATION/REDUCTION
The Fe+3 cation can then gain back an electron to become a Fe+2 cation (26 protons and 24 electrons).
OXIDATION REACTIONS
Oxidation reactions are sometimes symbolized by [O] and an arrow:
[O]
Fe0 ----- > Fe+3
REDUCTION REACTIONS
Reduction reactions are sometimes symbolized by [H] and an arrow:
Fe+3
[H]
----- > Fe+2
ISOTOPES
Isotopes are atoms of the same element that possess different numbers of neutrons. For example,
neon has three isotopes.
ISOTOPES OF NEON
Ne-20 has 10 protons and 10 neutrons. It is 90.48% abundant in nature.
Ne-21 has 10 protons and 11 neutrons. It is 0.27 % abundant in nature.
Ne-22 has 10 protons and 12 neutrons. It is 9.25% abundant in nature.
ISOTOPES OF NEON (continued)
This means for every 10,000 neon atoms: 9048 are Ne-20, 27 are Ne-21, and 925 are
Ne-22.
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VAN NUYS HIGH SCHOOL
Example 1. Magnesium (atomic number 12) has three isotopes:
Mg-24: 23.99 amu 78.99% relative abundance
Mg-25: 24.99 amu 10.00% relative abundance
Mg-26: 25.98 amu 11.01% relative abundance
Compute the average atomic mass (amu) of magnesium.
1A.
(1) A (Mg) = (23.99 x 0.7899) +
(24.99 x 0.1000) + (25.98 x 0.1101)
(2) A (Mg) = 18.949701 + 2.499 + 2.85159
(3) A (Mg) = 24.30029 amu
(4) A (Mg) = 24.30 amu
Example 2. Lithium (atomic number 3) has two isotopes:
Li-6: 6.015123 amu 7.42% abundance
Li-7: 7.016005 amu
Compute the average atomic mass of lithium.
2A.
(1) Total mass of lithium must equal 100%.
(2) 100 – 7.42 = 92.58
(3) Relative abundance for Li-7 is 92.58%
(4) A (Li) = (6.015123 x 0.0742) + (7.016005 x 0.9258)
(5) A (Li) = 0.446322127 + 6.495417429
(6) A (Li) = 6.941739556 amu
(7) A (Li) = 6.94 amu
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