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All organisms are made up of matter and all matter is made up of elements. 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%. Different elements can combine to form compounds such as water, which is fundamental to life. Life on Earth began from water and remained there for about three billions years prior to migrating onto land. 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. 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. 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. 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. 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.
Individual atoms can be held together by chemical bonds to form molecules and ionic compounds. 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, 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.
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.Life arose from the Earth's first ocean, which was formed approximately 3.8 billion years ago. Since then, water continues to be the most abundant molecule in every organism. Water is important to life because it is an effective solvent, capable of dissolving solutes such as sodium and chloride ions or other small molecules to form an aqueous solution. Once dissolved in water, these solutes are more likely to come in contact with one another and therefore take part in chemical reactions that sustain life.
In terms of its molecular structure, water is a small polar molecule with a bent shape formed by the polar covalent bonds of two hydrogen (H) atoms to one oxygen (O) atom (H2O). Because the O–H bonds are polar, the oxygen atom has a slight negative charge and the two hydrogen atoms have a slight positive charge. This polar property of water allows it to attract other water molecules via hydrogen bonds, which makes water cohesive. Surface tension results from the cohesive force due to the attraction between molecules at the surface of the liquid. Water is also adhesive as it is able to adhere to the surface of any polar or charged non-water molecules.
Water is denser as a liquid than it is as a solid (or ice). This unique property of water allows ice to float above liquid water such as ponds, lakes, and oceans, thereby insulating the liquid below from the cold air above. The lower density of ice compared to liquid water is due to the lower number of water molecules that form the crystal lattice structure of ice, which leaves a large amount of space between water molecules. In contrast, there is no crystal lattice structure in liquid water, which allows more water molecules to occupy the same amount of volume.
Water also has the capacity to absorb energy, giving it a higher specific heat capacity than other solvents such as ethanol.Thus, a large amount of energy is needed to break the hydrogen bonds between water molecules to convert liquid water into gas (or water vapor).
As a molecule, water is not completely stable as each water molecule continuously dissociates into hydrogen and hydroxyl ions before reforming into a water molecule again. In pure water, the number of hydrogen ions balances (or equals) the number of hydroxyl ions, resulting in a pH that is neutral. If hydrogen ions were to exceed hydroxyl ions, then the pH of the solution would be acidic. Conversely, a solution's pH would turn basic if hydroxyl ions were to exceed hydrogen ions.