Nursing Entrance Exam

Substance A chemical substance is matter with a definite set of properties and composition. In other words, a mixture of elements, compounds, or a combination thereof is not a chemical substance, but it may be called a chemical. Most of the substances we encounter in our daily life are some kind of mixture, such as air, alloys, biomass, etc. Nomenclature, or the system used for naming chemical compounds, is a major part of the language of chemistry. Earlier in the history of chemistry, substances were given names by their discoverer, which often led to some confusion and difficulty. Today, the IUPAC system of chemical nomenclature allows chemists to identify by name specific compounds amongst the vast variety of possible chemicals. The International Union of Pure and Applied Chemistry (IUPAC) sets the standard nomenclature of chemical substances. Organic compounds are termed based on the organic nomenclature system, while inorganic compounds are named according to the inorganic nomenclature system. Additionally, the Chemical Abstracts Service has devised a method to index chemical substance. In this scheme, each chemical substance is identifiable by a number known as the CAS registry number. Molecule Molecules are two or more atoms joined through a chemical bond to form chemicals. Molecules differ from atoms in that molecules can be further broken down into smaller pieces and into elements. In fact, the 18 th -century definition of an atomwas “a recognizable structure that can no longer be broken down into smaller bits”. Atoms are joined into molecules in two main ways: through covalent bonds and ionic bonds. Covalent bonds are the primary type of chemical bond that forms molecules. They occur when atoms with only partially-filled valence electron shells, an atom’s outermost electron shell, come together to share electrons. Hydrogen atoms, for instance, each have only one electron, while their valence shell is capable of holding two. When two hydrogen atoms come together, each share the other’s electron, using it to occupy its valence shell’s free space, thus forming the H2 molecule, hydrogen gas. Not all covalent bonds are the same. Different atoms have different levels of positive charge coming from in their nuclei, and although under normal circumstances, the negative charge of the atom’s electrons balances that out (keeping the atom electrically neutral), the chemical bonding process has a way of exploiting this situation. If we look at the H2 molecule again, everyday experience tells us that it has a strong tendency to seek out and bond with oxygen (O) molecules forming H2O, or water. There are two main reasons for this. The first comes from the regular old covalent bonds that are already holding H2 together. If bonded to another atom, hydrogen gains the ability to form a new valence shell that can hold six electrons. Since oxygen is the only molecule naturally to have six electrons in its valence shell, it is the most eager to bond with hydrogen. However, oxygen also has eight protons in its nucleus compared to the total of two in the H2 molecule. This means that as the atoms come closer and prepare to bond, the electrons fromboth atoms are pulled closer to the oxygen

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