Download Atoms, Molecules and Ions In This Chapter

Atoms, Molecules and Ions
Chapter 2
In This Chapter
• History of atoms.
• Subatomic particles.
• Atomic numbers, mass numbers.
• Isotopes and Atomic weights.
• Compounds, Molecules and Ions.
• Nomenclature.
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Elements
• An element is a pure substance that can’t be
changed into a simpler form of matter by a
chemical reaction.
• Elements consist of atoms; the smallest structural
unit of an element that retains the chemical
properties of that element.
• Atoms are considered as the building blocks of
every chemical around us.
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The Origin of Elements
• After a very short time (matter of seconds) that the universe was
produced by the Big Bang the only elements present were hydrogen
and helium.
• After millions of years the cooling of the universe caused the
hydrogen and helium to collect together and form large clouds, due
to gravity.
• These clouds eventually contracted, became hotter and hotter and
burst to become stars.
• Intense heat within the stars made the hydrogen and helium atoms to
smash together and produce other heavier elements.
• This merging releases even more heat which is responsible for
starlight.
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Names of Elements
• Names come from various sources including early
Greek, Latin or German names describing some
property of the element, names of places, names of
people.
– Iodine from Greek word iodes – violet like.
– Bismuth from German weisse masse – white mass,
miners called it wismat.
– Germanium and Francium – discovered in
– Element number 110 was recently named
Darmstadtium to honor the German city in which it was
discovered.
– Einsteinium and Curium – to honor
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Symbols of Elements
• An universally accepted abbreviation for the name
of the element.
• Elemental symbols are unique.
• 14 elements have single letter symbols, while the
rest have 2.
• The following rules are typically followed:
– Symbols have either one or two letters.
– If only one letter is used it is capitalized, C, H, O.
– If 2 letters are used only the first letter is capitalized,
Na, Ca, Hg.
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Early Atomic Theories
• Democritus, a Greek philosopher (460 –370 BC)
proposed the idea of an “atom” as an indivisible
component of matter, using the word atomos
which means indivisible.
• John Dalton, in 1808, published A New System of
Chemical Philosophy in which he presented his
theory of atoms.
• Dalton’s theory was the first one to provide a
systematic description of the relationship between
atoms and matter.
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Dalton’s Atomic Theory
• Dalton’s basic postulates (ideas) were:
– All matter is composed of atoms which are indivisible
and indestructible. Atoms are considered as the
ultimate chemical particles.
– An element is composed of identical atoms with fixed,
identical properties and masses.
– Compounds are formed by the combination of atoms
of 2 or more different elements in a fixed whole
number ratio.
– A chemical reaction involves a combination,
separation or rearrangement of atoms. Atoms are
neither created nor destroyed.
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Problems with Dalton’s Theory
• Atoms can be divided further.
• Elements can have more than one mass.
• Here is what we know about the atom now:
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Regions of an Atom
• All atoms consist of 2 distinct regions.
• The nucleus: A small dense region at the center
which contains positively charged protons and
neutral (with no charge) neutrons.
• Surrounding the nucleus is a diffuse region which
contains the negatively charged electrons.
• Atoms of different elements have different # of
protons, electrons and neutrons.
• The # of protons decides the identity of the
element.
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Properties of Subatomic Particles
Proton (p)
Charge = 1 Mass = 1.6725 x 10-24 g
Neutron (n) Charge = 0 Mass = 1.6750 x 10-24 g
Electron(e-)
Charge =
-1
Mass = 9.1095 x 10-28 g
• Protons and neutrons have masses that are similar to
each other.
• The electron is about 1800 times lighter than the other
2.
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Historical Discoveries
• J. J. Thomson studied electrical beams in partially
evacuated cathode-ray tubes and proposed that
the cathode-ray consisted of negatively charged
particles, which we now know of as the electron.
• Thomson determined the charge to mass ratio for
the electron and proposed a model for the atom
called the plum pudding atom.
• Thomson won the Nobel Prize for Physics in
1906 for his work.
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