Only a few elements are important in organisms.
Four elements make up 96% of living matter: carbon (C), hydrogen (H),
oxygen (O), and nitrogen (N).
Two electrons, which are negatively charged, move around the nucleus.
The atomic number of an element is the number of protons in the
The mass number (atomic mass) is the sum of protons plus neutrons.
Electrons exist only at fixed levels of energy, which are also called
The outermost shell is called the valence shell, and contains valence
Most chemical reactions involve valence electrons, since atoms are
most stable with complete valence shells.
Each hydrogen atom has a single electron (represented as a cloud) in
its valence shell - shell #1.
The two valence electrons be shared in a strong covalent bond, forming
an H molecule.
In the resulting molecule, the shared pair of electrons enable each
atom to complete its valence shell.
Two pairs of valence electrons can also be shared to form a double
A water molecule is polar because oxygen is more electronegative than
There is a partial negative charge on the oxygen and partial positive
charges on the hydrogen atoms.
The unequal sharing of electrons results in polar covalent bonds.
The sodium cations (Na^+) and chloride anions (Cl^-) in table salt
sodium chloride (NaCl) are each surrounded by 6 ions of the opposite
charge, forming a crystal.
A gecko toe is highly folded and contain tiny hairs that promote such
interactions that enable it to crawl on smooth surfaces.
The van der Waals interactions between the hair tip and a surface
allow a gecko to walk up a wall.