Biology Chapter 2
In chemistry and physics, atomic theory is a theory of the nature of matter, which states that matter is organized into discrete units called atoms. The Greek word rendered “atom” meaning ‘indivisible’ was applied to the basic particle that constituted a chemical element, because the chemists of the era believed that these were the fundamental particles of matter. Not until around the turn of the 20th century, through various experiments with electromagnetism and
radioactivity, did physicists discover that the so-called “indivisible atom” was actually a conglomerate of various particles called subatomic particles- electrons, protons and neutrons- which can exist separately from each other. Since atoms were found to be actually divisible, physicists later invented the term “elementary particles” to describe subatomic particles. The electron is a subatomic particle with a negative electric charge. It is generally thought to be an elementary particle. The periodic table is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom’s atomic nucleus. Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation of the atom as a particular element. A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges between electrons and nuclei. Covalent bonding is a common type of bonding, in which the electronegativity difference between the bonded atoms is small or nonexistent. Bonds within most organic compounds are described as covalent. Ionic bonding is a type of electrostatic interaction between atoms which have a large electronegativity difference. A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond. A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. The substances initially involved in a chemical reaction are called reactants. Chemical reactions are usually characterized by a chemical change, and they produce one or more products, which usually have properties different from the reactants. Reactions often consist of a sequence of individual sub-steps, the so-called elementary reactions, and the information on the precise course of action is part of the reaction method. Chemical reactions are described with chemical equations, which graphically present the starting materials, end products, and sometimes intermediate products. Water is a chemical substance with the chemical formula H2O. Its molecule contains one oxygen and two hydrogen atoms connected by covalent bonds.