All organisms are made up of matter and all matter is made up
of elements.[35] Oxygen, carbon, hydrogen, and nitrogen are the four elements that account for
96% of all organisms, with calcium, phosphorus, sulfur, sodium, chlorine,
and magnesium constituting the remaining 3.7%.[35] Different elements can combine to
form compounds such as water, which is fundamental to life.[35] Life on Earth began from water
and remained there for about three billions years prior to migrating onto land.[36] Matter can exist
in different states as a solid, liquid, or gas.
The smallest unit of an element is an atom, which is composed of an atomic nucleus and one or
more electrons moving around the nucleus, as described by the Bohr model.[37] The nucleus is
made of one or more protons and a number of neutrons. Protons have a positive electric charge,
neutrons are electrically neutral, and electrons have a negative electric charge.[38] Atoms with
equal numbers of protons and electrons are electrically neutral. The atom of each specific
element contains a unique number of protons, which is known as its atomic number, and the sum
of its protons and neutrons is an atom's mass number. The masses of individual protons,
neutrons, and electrons can be measured in grams or Daltons (Da), with the mass of each proton
or neutron rounded to 1 Da.[38] Although all atoms of a specific element have the same number of
protons, they may differ in the number of neutrons, thereby existing as isotopes.[35] Carbon, for
example, can exist as a stable isotope (carbon-12 or carbon-13) or as a radioactive
isotope (carbon-14), the latter of which can be used in radiometric
dating (specifically radiocarbon dating) to determine the age of organic materials.[35]
Individual atoms can be held together by chemical bonds to form molecules and ionic
compounds.[35] Common types of chemical bonds include ionic bonds, covalent bonds,
and hydrogen bonds. Ionic bonding involves the electrostatic attraction between oppositely
charged ions, or between two atoms with sharply different electronegativities,[39] and is the
primary interaction occurring in ionic compounds. Ions are atoms (or groups of atoms) with an
electrostatic charge. Atoms that gain electrons make negatively charged ions (called anions)
whereas those that lose electrons make positively charged ions (called cations).
Unlike ionic bonds, a covalent bond involves the sharing of electron pairs between atoms. These
electron pairs and the stable balance of attractive and repulsive forces between atoms, when
they share electrons, is known as covalent bonding.[40]
A hydrogen bond is primarily an electrostatic force of attraction between a hydrogen atom which
is covalently bound to a more electronegative atom or group such as oxygen. A ubiquitous
example of a hydrogen bond is found between water molecules. In a discrete water molecule,
there are two hydrogen atoms and one oxygen atom. Two molecules of water can form a
hydrogen bond between them. When more molecules are present, as is the case with liquid
water, more bonds are possible because the oxygen of one water molecule has two lone pairs of
electrons, each of which can form a hydrogen bond with a hydrogen on another water molecule.