Download Elements, Compounds, Bonds

Elements, Compounds, Bonds
Matter
How is it organized?
Elements, Atoms & Compounds
• Element: simplest type of matter with
unique chemical properties
– 92 elements occur naturally
• Atom: Smallest particle of an element
that has the chemical characteristics of
that element
• Compound: Substance containing two or
more elements in a fixed ratio
Atoms
• Composed of three
subatomic particles:
– protons (+) & neutrons
(0) make up the nucleus
– electrons (-, e-) in
orbitals (clouds)
Periodic Table
Atoms
• Mass of atom = summed
mass of its protons and
neutrons
– 1 proton = 1 atomic mass
unit or amu
– 1 neutron ~1 amu
• Mass = Atomic weight
Approximate atom
electron (-)
nucleus
proton (+)
neutron (0)
Pea = nucleus
Atoms
• Stable atoms always have same # protons.
# protons = atomic #
– Carbon [C] = 6
• Atoms can have differing #’s of neutrons.
# protons + # neutrons = Atomic
weight (mass number)
–
–
12C
has 6 protons and 6 neutrons
13C has 6 protons and 7 neutrons
Periodic Table
Importance (by weight)
Valence and reactivity
• Remember: electrons
are arranged in valence
shells or energy levels
that “orbit” the
nucleus.
• # of electrons in
atoms influence their
reactivity, relative to
other atoms.
2 electrons
2 electrons
8 electrons
Atoms with unfilled electron
shells are reactive…
• Atoms are “greedy” (want full e- orbitals)
and “lazy” (don’t want to work hard to
achieve them)
• Results in formation of chemical bonds
• Three types of bonds
– Ionic bonds
– Covalent bonds
– Hydrogen bonds
Ionic bonds
• Ionic bonds form between anions (atoms that
stripped an e- from another atom) and cations
(atoms that lost an e-)
• Salts form by ionic bonding
• Basically, a strong electrostatic attraction
Covalent bonds
• Covalent bonds
form when atoms
share e-.
– Single or double
• H-H (H2)
• O=C=O (CO2)
• # of bonds formed
= # of e- needed to
fill outer shell
Non-polar Covalent bonds
• Equal sharing of e– Nuclei of similar size
have similar
elecronegativity:
pull on shared
electrons in a
covalent bond
Polar Covalent bonds
• Form due to
unequal sharing of
e• Relative size and
electrical
attraction of
nuclei differ
• Partial + and
partial – sides of
molecule result
Hydrogen bonds
• Weak attraction between
opposite charged poles of
polar molecules
• IMPORTANT bond
– determines structure of
large, complex molecules
– Produces cohesion (water
droplets)
Hydrogen bonds give H20 it’s
amazing properties
Structural bonds
• Responsible for
secondary
structure of
proteins
• -helix
• -pleated sheet
Allow water transport
• H-bonding creates surface tension
between water molecules
• As H20 evaporates, it pulls “attached” H20
molecules along with it
Ionic & covalent bonds form:
• Molecules: atoms held together by
covalent bonds
– H20, O2, H2, CO2
• Compounds: chemical substance made of
atoms of 2 or more elements, regardless
of type of bond joining them.
– H20, NaCl, CO2.
Concept Check
•
The reactive properties or chemical behavior of an
atom mostly depend on the number of
1)
2)
3)
4)
the
the
the
the
electrons in each electron shell of the atom.
neutrons found in the nucleus.
filled electron shells.
electrons in the outer electron shell of the atom.
Answer
•
The reactive properties or chemical behavior of an
atom mostly depend on the number of
4) the electrons in the outer electron shell of the atom.
Concept Check
•
Water molecules form
hydrogen bonds because
1)
the water molecule is
polar.
2) the oxygen molecule is
positively charged.
3) the water molecule forms
a tetrahedron.
4) the hydrogen atoms are
negatively charged.
Answer
•
Water molecules form
hydrogen bonds because
4) the water molecule is
polar.