1 From atoms to molecules
The matter is anything that takes up space and has mass. It is helpful to remember that matter can exist as a solid, gas, liquid, or plasma. Not only are humans composed of matter, but so are the food we eat, the water we drink, and the air we breathe.
ELEMENTS
An element is one of the basic building blocks of matter; an element cannot be broken down by chemical means. Considering the world's variety of living and nonliving things, it's remarkable that there are only 92 naturally occurring elements. It is even more surprising that over 90% of the human body is composed of just four elements: carbon, nitrogen, oxygen, and hydrogen. Even so, other elements, such as iron, are important to our health. Iron-deficiency anemia results when the diet doesn't contain enough iron to make hemoglobin. Hemoglobin serves an essential function in the body because it transports oxygen, another element, to our cells.
Where do elements come from?
Normal chemical reactions do not produce elements. The majority of the heavier elements, such as iron, are produced only by the intense chemical reactions within stars. When these
stars reach the end of their lives, they explode, producing supernovas. Supernovas scatter heavier elements into space, where they are eventually involved in the formation of planets.
The explosions of stars formed the iron within your blood.
ATOMS
An atom is the smallest unit of an element that still retains the chemical and physical properties of the element. The same name is given to the element and the atoms of the element. Though it is possible to split an atom, it is the smallest unit to enter into chemical reactions. Physicists have identified a number of subatomic particles that make up atoms. The three best-known subatomic particles are positively charged protons, uncharged neutrons, and negatively charged electrons. Protons and neutrons are located within the nucleus of an atom, and electrons move around the nucleus.
ISOTOPES
Isotopes of the same type of atom have the same number of protons but different numbers of neutrons. Therefore, they have the same atomic number but their mass numbers are different.
COVALENT BONDING
Atoms share electrons in covalent bonds. Atoms are sharing electrons. Just as two hands participate in a handshake, each atom contributes one electron to the shared pair. These electrons spend part of their time in the valence shell of each atom. Therefore, they are counted as belonging to both bonded atoms.
Besides the single bond (covalent Bond), in which atoms share only a pair of electrons, a double, or a triple bond can be formed. In a double bond atoms share two pairs of electrons, whereas in a triple bond atoms share three pairs of electrons.
HYDROGEN BONDS
A hydrogen bond is the attraction of a slightly positive, covalently bonded hydrogen to a slightly negative atom in the vicinity. These usually occur between a hydrogen and either an oxygen or a nitrogen atom. A hydrogen bond is represented by a dotted line because it is relatively weak and can be broken rather easily.
ACIDS AND BASES
When water molecules dissociate (break up), they release an equal number of hydrogen ions (H+) and hydroxide ions (OH−). Only a few water molecules at a time dissociate, and the actual number of H+ or OH− is 10−7 moles/liter. A mole is a unit of scientific measurement for atoms, ions, and molecules.
ACIDS have a high H+ concentration.
BASES have a low H+ concentration.
Bases are substances that either take up hydrogen ions(H+) or release hydroxide ions (OH−). For example, an important base is sodium hydroxide (NaOH), which dissociates almost completely in this manner: NaOH---> Na+ + OH-
pH scale
The pH scale is used to indicate the acidity or basicity (alkalinity) of a solution. The pH scale ranges from 0 to 14. A pH of 7 represents a neutral state in which the hydrogen ion and hydroxide ion concentrations are equal. A pH below 7 is an acidic solution and a pH above 7 is a basic solution.
BUFFERS
From organisms to ecosystems, pH needs to be maintained within a narrow range to prevent negative consequences. Normally pH stability is possible because the body and the environment have buffers to prevent pH changes. At the ecosystem level, a problem arises when precipitation in the form of rain or snow becomes so acidic that the environment runs out of natural buffers in the soil and water.
Buffers help keep the pH within normal limits because they are chemicals or combinations of chemicals that take up excess hydrogen ions (H+) or hydroxide ions (OH−).
The pH of our blood when we are healthy is always about 7.4— just slightly basic (alkaline). Blood always contains a combination of some carbonic acid and some bicarbonate ions.
ORGANIC MOLECULES
There are four types of organic molecules: carbohydrates, lipids, proteins and nucleic acids. In biology, "organic" doesn't refer to how food is grown; it refers to a molecule that contains carbon (C) and hydrogen (H) and is usually associated with living organisms.
