2. Chemical Composition of the Body
Atoms
Smallest units of matter that can undergo chemical change.
Nucleus (center) contains:
Protons (+ charge)
Neutrons (no charge)
Atomic mass:
Sum of protons and neutrons.
Atomic Number
Number of protons in an atom
Neutral atom
Number of protons = number of electrons
Isotopes
Vary in number of neutrons
Same in atomic number
Vary in atomic mass
Chemical element
Includes all of the isotopic forms of a given atom
Eg: Element Hydrogen: 3 isotopes
Most common: one proton
Deuterium: one proton, one neutron
Tritium: one proton, two neutrons
Commonly used in research
106 chemical elements
Elements
Four elements important to living organisms
Carbon (C)
Nitrogen (N)
Oxygen (O)
Hydrogen (H)
Electrons (outside the nucleus):
- charged
Occupy orbitals surrounding nucleus.
Valence electrons:
Electrons in the outer most orbital that participate in chemical reactions (if orbit incomplete).
Form chemical bonds.
Orbitals
Also called shells or energy levels
Electrons usually found within a given orbital
Levels (and max number of electrons)
First shell: 2 electrons
Second shell: 8 electrons
Third shell: usually 8 electrons
Chemical Bonds, Molecules, and Ionic Compounds
Chemical bonds:
Interaction of valence electrons between 2 or more atoms.
# bonds determined by # electrons needed to complete outer orbital.
Covalent Bonds
Atoms share their valence electrons.
Nonpolar bonds:
Electrons are equally distributed between the two identical atoms.
Strongest bond.
H2
Polar bonds:
Electrons are shared between two different atoms.
Electrons may be pulled more toward more atom.
Oxygen, nitrogen, phosphate pull electrons towards themselves.
Ionic Bonds
One or more valence electrons from an atom are completely transferred to a second atom.
First atom loses electrons, + charged (cation).
Second atom has more electrons,
- charged (anion).
Cation and anion attract, form ionic compound.
Weaker than polar bonds.
Dissociate easily when dissolved in H20.
NaCl Na+ and Cl-