Download Basic Chemistry - Biology with Radjewski

Basic Chemistry
Mini Lecture
AP Biology
Atom
Electron – negative charge
Neutron; no charge
Most atoms are neutral because the number
of electrons equals the number of protons.
Element—pure substance that contains only
one kind of atom
• Living things are mostly composed of 6
elements:
• Carbon (C) Hydrogen (H)
Nitrogen
(N)
• Oxygen (O) Phosphorus (P) Sulfur (S)
Basic Chemistry Concepts
• The number of protons identifies an element.
• Number of protons = atomic number
• See periodic table handout!
• For electrical neutrality, # protons = #
electrons.
• Mass number—total number of protons and
neutrons
Figure 2.1 Electron Shells
• Atoms with unfilled outer shells tend to
undergo chemical reactions to fill their outer
shells.
• They can attain stability by sharing electrons
with other atoms (covalent bond) or by losing
or gaining electrons (ionic bond)
• The atoms are then bonded together into
molecules.
• Octet rule—atoms with at least two electron
shells form stable molecules so they have
eight electrons in their outermost shells.
Ionic bonds
• Ions are charged particle that form when
an atom gains or loses one or more
electrons.
•
Cations—positively charged ions
•
Anions—negatively charged ions
• Ionic bonds result from the electrical
attraction between ions with opposite
charges.
•
The resulting molecules are called salts.
Covalent bonds
• Covalent bonds form when two atoms
share pairs of electrons.
• The atoms attain stability by having full
outer shells.
• Each atom contributes one member of the
electron pair.
• Carbon atoms have four electrons in the
outer shell—they can form covalent bonds
with four other atoms.
Strength and stability—covalent bonds are
very strong; it takes a lot of energy to break
them.
• Multiple bonds
• Single—sharing 1 pair of electronsC H
•
C C
Double—sharing 2 pairs of electrons
•
N N
Triple—sharing 3 pairs of electrons
Degree of sharing electrons is not always
equal.
• Electronegativity—the attractive force that
an atomic nucleus exerts on electrons
• It depends on the number of protons and
the distance between the nucleus and
electrons.
• If two atoms have similar
electronegativities, they share electrons
equally, in what is called a nonpolar
covalent bond.
• If atoms have different electronegativities,
electrons tend to be near the most
attractive atom, in what is called a polar
covalent bond