Atoms and Molecules

Atoms and Molecules Atoms, ions and molecules The atom is the smallest part of an element that retains its properties during a chemical reaction in which it can take part. The atom of an element consists of a central zone called the nucleus, in which there are protons, positively charged particles and neutrons, particles without charge. Around the nucleus there is a region occupied by negatively charged particles called electrons. Since each atom must be electrically neutral, the number of protons, atomic number, indicated by the letter Z (in a neutral atom is equal to the number of electrons). The sum of protons and neutrons is the mass number and is indicated by the letter A. There are cases in which atoms of the same element have the same atomic number (Z) but different mass number (A). These atoms are called isotopes. The particles, interacting with each other, unite and form molecules or transform into ions that are electrically charged atoms. The molecule is the smallest quantity of a substance able to preserve its chemical composition and to determine its properties and its chemical and chemical-physical behaviour. A negative ion, or anion, has more electrons than those expected by the atomic number while a positive ion, or cation, has fewer electrons than its atomic number. Covalent and lonic Bonds Atoms bind with: - the covalent bond: it is a chemical bond in which two atoms share pairs of electrons. This is because atoms tend to be stable with the least amount of energy; the ionic bond: given the electrostatic attraction that is established between the electrical charges of a cation and an anion, this type of bond takes shape. By forming bonds, the atoms become stable. Noble gases have complete electronic shells and are very stable: they do not bind together or with other atoms. In the formation of the covalent bond between identical atoms, the electrical charge of the electrons involved in the bond is evenly distributed around the two nuclei. In molecules formed by different atoms, instead, the shared electrons can be attracted with different strength by the two nuclei. An example of an lonic compound is table salt (NaCl). Sodium (Na+) is positively charged and chlorine negatively charged (Cl-). Properties of matter The properties of the material are divided into physical and chemical properties. Chemical properties can be observed by altering the chemical composition of matter (by means of chemical transformations or reactions). The physical properties (size, weight, etc.) are observable without any alteration of the body. Reagents' are defined as substances initially involved in the chemical reaction. New substances obtained during the process are 'produced'. Energy and thermodynamics Energy is the ability to do a job. Heat is a way of transferring energy from a warm body to a cold body. Kinetic energy is a form of energy associated with movement: an object that moves can do a job, transferring its movement to another matter. Heat is the kinetic energy associated with the movement of molecules within a body. The potential energy of a body is the energy it possesses because of its position or orientation relative to a force field. The first principle of thermodynamics or energy conservation law: 'energy can be converted from one form to another but it can neither be created nor destroyed'. The second principle of thermodynamics or Clausius' statement: 'heat cannot spontaneously flow from a cold body to a warmer one'. The third principle of Nernst's thermodynamics or theorem: 'it is not possible to reach absolute zero through a finite number of operations (thermodynamic transformations)'. Biological molecules In addition to water, the bodies of organisms are almost entirely made up of organic molecules: compounds that contain carbon atoms. Smaller molecules (monomers) bind together to become larger molecules (polymers). Macromolecules are divided into four main classes: carbohydrates, lipids, proteins and nucleic acids. Carbohydrates are compounds that contain carbon, oxygen and hydrogen and are the main sources of energy for cells. Lipids are mainly apolar molecules, capable of forming plasma membranes (phospholipids) or having cellular signalling functions (steroid hormones).