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Chapter 2
Atom
Element
= the smallest unit of matter that cannot be broken down
= a pure substance made of only one kind of atom
Atoms are composed of even smaller parts,
called subatomic particles.
Protons = positive electric charge (1+)
Neutrons = neutral charge (0)
Electrons = negative charge (1-)
Each element has a unique number of protons
= atomic number.
• Atoms have equal numbers of P and e• The atomic number = tells the number of protons and the
number of electrons
•Mass number = the sum of the number of protons
and neutrons in the nucleus of an atom.
# Neutrons = Mass # - Atomic #
Carbon =
# protons?
# electrons?
# neutrons?
• YouTube - The element song
Chemical bonds
• Atoms want to be “stable” = full valence shell.
• Share or transfer electrons.
• To become stable, atoms will hold together by chemical
bonds.
• The strongest chemical bonds are covalent bonds and ionic
bonds.
Single
Atoms
Chemical
Bond
• A covalent bond is the sharing of a pair of electrons by
two atoms.
H
H
Hydrogen
H2
Subscript (2 atoms)
• Ionic bond = one atom strips an electron completely
from the other.
Ions = charged atoms or molecule
NaCl
Top 4 Atoms found in Organisms?
O = 65%
C = 18.5%
H = 9.5%
N = 3.3%
~75% Earth
Life evolved in water
Cells 70-95% Water
Astronomers
H2O
Water is a “Polar Molecule”
Polar Covalent Bond = electrons not shared equally
+
• Hydrogen bonds = a weak chemical attraction between polar
molecules.
H+  O-
Cohesion = water molecules bonding to each other.
“Surface tension” prevents
stretch or break the surface
Adhesion = clinging of one substance to
another (polar  polar).
Water is the “solvent” of life
Solution = homogenous mixture.
Solute =
substance dissolved
Solvent =
dissolving agent
Forms hydrogen bonds with charged and polar covalent
molecules.
Solvent?
Solute?
“Dissolve”
pH
pH scale = concentration of H+ in a solution
0 - 14
7 = neutral (H+ = OH-)
0-6 = acidic (H+)
8-14 = basic (OH-)
Acid = any compound that forms
hydrogen ions (H+) ions in solution
Base = any compound that forms
hydroxide ions (OH-) in solution
Factor of 10
Cell pH = 6.5 – 7.5
“Homeostasis”
Buffers = weak acids or
bases that prevent
sudden changes in pH