Introduction To Chemistry
Start with Atoms
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Key Concept
Atoms and Elements
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An element is a fundamental
substance made of one type of
atom.
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The atom is the smallest unit of an
element that still retains the
element’s properties.
its building blocks are protons,
electrons, and neutrons.

Isotopes are atoms of an element
that vary in the number of neutrons.
Start With Atoms
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An element is a fundamental
substance made up of only
ONE KIND of atom.
Atoms are built from three
subatomic particles:
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Protons
Neutrons
Electrons
Start With Atoms
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Protons
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Protons carry a positive
charge.
The symbol of protons
is p+ .
Protons will be in the
nucleus of an atom.
The atomic number of
each element indicates
the number of protons
present!
Atomic Number
This element, Neon, has
10 protons!
Start With Atoms
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Neutrons
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Neutrons don’t have ANY
charge.
Neutrons are located in the
nucleus of the atom.
Neutrons contribute to the
atomic mass of the atom!
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Atomic Number
To figure out the number of
neutrons, simply subtract the
atomic number from the
atomic mass.
When an atom has an unusual
amount of neutrons, it
becomes an isotope.
Atomic Mass
10-20.2 = 10.2 neutrons
Start With Atoms
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Electrons
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Have a negative charge.
The symbol for electrons is e- .
Electrons float around the
atom in several clouds or
shells.
The number of electrons
equals the number of protons
in the atom.
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The positive charge of protons
and the negative charge of
electrons balance each other
out in a BALANCED and
NORMAL atom.

When either protons or
electrons outnumber the other,
then the atom becomes an ion.
This element, Neon, has
10 protons AND 10
electrons.
Start With Atoms
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The number of protons,
electrons, and neutrons in an
element predicts their
behavior under a variety of
conditions inside and outside
the body.

In 1869, Dmitry Mendeleev
arranged the known elements
in a repeating pattern, based
on their chemical properties.

He used gaps in his periodic
table to correctly predict what
elements had not yet been
discovered!
Dmitry Mendeleev
only cut his hair
once a year!
Periodic Table

Elements fall into order
in the table according to
their atomic number.
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All elements in each
vertical column have the
same number of
electrons that are
available for interaction
with other atoms. As a
result, they behave in
similar ways.
Any elements after
atomic number 92 are
extremely unstable.
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Some have been formed
in exceedingly small
quantities in laboratories.
Putting Radioisotopes To Use
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Key Concept
Why Electrons Matter

Atoms acquire, share, and
give up electrons. Whether
one atom will bond with
others depends on the
number and arrangement of
its electrons.
Putting Radioisotopes To Use
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Most naturally occurring elements have
isotopes.
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Isotopes are one of two or more atoms of
the same element (same number of
protons) that differ in their number of
neutrons.
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Carbon has three isotopes, nitrogen has two,
and uranium has fifteen isotopes!
A superscript number to the left of an
element’s symbol is the isotopes mass
number.
 12C
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= carbon with 6 protons and 6 neutrons
13C = carbon with 6 protons and 7 neutrons
14C = carbon with 6 protons and 8 neutrons
Putting Radioisotopes To Use
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Some isotopes are unstable, or
radioactive. They are called
radioisotopes.
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A radioisotope will spontaneously emit
energy in the form of subatomic particles
and x-rays when its nucleus disintegrates.
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This process is called radioactive decay, and it
can transform one element into another.

Ex. 13C and 14C are radioisotopes of carbon.
Each predictably decays with a particular amount
of energy into a more stable product.
 After 5,700 years, about half of the atoms in a
sample of 14C will have turned into 13N
(nitrogen) atoms.
 Researchers use radioactive decay to
estimate the ages of rocks and biological
remains.
Putting Radioisotopes To Use
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Different isotopes of an element are still
the same element.
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For the most part, carbon is carbon,
regardless of how many neutrons it has and
living systems use 12C the same way as 14C.
Researchers and clinicians who want to track
a particular substance can use a tracer.
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Tracers are molecules in which a radioisotope has
been substituted for a more stable isotope.
They can be delivered into a cell or a multicelled
body and can be used to track the pathway or
destination of a substance of interest with the
help of radioactivity-detecting instruments.
Researchers had plants
take up radioactive gas
(carbon dioxide made
with 14C) and tracked
the radioisotope
through steps by which
plants produce simple
sugars and starches.
Putting Radioisotopes To Use
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PET (Positron-Emission
Tomography) uses radioisotopes
to study metabolism.
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Clinicians attach a radioisotope to
glucose or another sugar and
inject it into a patient who is
moved into a PET scanner.
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Cells in different parts of the body
absorb the tracer at different rates.
The scanner detects radiation caused
by energy from the decay of the
radioisotope and uses it to form an
image on a monitor.
Such images reveal variations and
abnormalities in metabolic activity.
What Happens When Atoms Bonds with
Atom?
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Key Concept
Atoms Bond
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The bonding behavior of
biological molecules starts
with the number and
arrangement of electrons in
each type of atom. Ionic,
covalent, and hydrogen
bonds are the main
categories of bonds between
atoms in biological
molecules.
What Happens When Atoms Bonds with
Atom?
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Electrons in an atom are arranged in orbitals,
or volumes of space around the atomic
nucleus. These orbitals are called electron
clouds, electron shells, or energy levels.
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Each shell has a specific amount of space that
electrons can occupy.
 The shell closest to the nucleus of the atom (the
first shell or 1s) can hold up to TWO electrons.
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This shell has the lowest energy.
The second shell (2s) can hold up to EIGHT
electrons.
The third shell (3s) can hold up to EIGHT
electrons.
From Atoms To Molecules
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Atoms with vacancies in their
outermost shell tend to give up,
acquire, or share electrons.
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Chemical bonds are atoms sharing
their electrons with each other.
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A molecule is simply two or more atoms
of the same or different elements joined
in chemical bond.
Compounds are molecules that consist of
two more different elements in
proportions that never vary.

Hydrogen has 1 e- in its
outermost shell. As the first shell
can have up to 2 e-, hydrogen can
bond with 1 other element.
Water is an example.

All water molecules have one oxygen bonded
to two hydrogen atoms—H2O
Carbon has 4 e- in its outermost
shell. As the third shell can have
up to 8 e-, carbon can bond with
FOUR other elements.
Ion Formation And Ionic Bonds
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When an atom contains as many
electrons as protons, these charges
balance each other, so the atom
has a net charge of zero.
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When an atom gains an extra
electron, it acquires a net negative
charge—It becomes a negative ion
When an atom loses an electron, it
acquires a net positive charge-it
becomes a positive ion
When a positively charged ion
attracts a negatively charged ion, the
two associate closely with one
another-- this is an ionic bond.
Cl has 7 p+ in its outermost shell
and tends to grab e- from
someplace else, making it
negative Cl-.
Na has 1 e- in its outermost
shell and tends to lose it,
making it positive Na+
Cl- and Na+ attract and form
NaCl or table salt.
Covalent Bonding
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If both atoms have room for an extra
electron, they can share one in a hybrid
orbital that spans both atomic nuclei.
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When atoms share an electron they are
joined in a single covalent bond.
Covalent bonds are stable and are much
stronger than ionic bonds.
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In a nonpolar covalent bond, two atoms are
sharing electrons equally, so the molecule
shows no difference in charge between the
two ends.
In a polar covalent bond, two atoms do not
share electrons equally which results in one
side being more negative than the other.
 This is called electronegativity.
Both H’s are
sharing the eequally
The H’s are
slightly
positive
The O is
slightly
negative
Hydrogen Bonding
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A hydrogen bond is a weak
attraction that has formed between
a covalently bound hydrogen atom
and an electronegative atom.
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Because H bonds are weak, they form
and break easily.
Collectively, however, many hydrogen
bonds contribute to the properties of
liquid water, and play important roles
in the structure and function of
biological molecules.
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Many of these bonds hold DNA’s two
nucleotide strands together.