CHEM (PRELIM)

• Chemical bonds are the forces that hold atoms together in a compound and determine the chemical properties of the substance.
• Compound: A pure substance that is made up of two or more elements in a fixed ratio by mass.
• Ionization energy is a periodic property.
• When lithium loses one electron, it becomes a lithium ion; it still has three protons in its nucleus, but now only two electrons outside the nucleus, and therefore has a positive charge.
• Formula of a compound: Tells us the ratios of its constituent elements and identifies each element by its atomic symbol.
• NaCl: the ratio of sodium atoms to chlorine atoms in sodium chloride is 1:1.
• H 2 O: the ratio of hydrogen atoms to oxygen atoms in water is 2:1.
• Element: A substance (for example, carbon, hydrogen, and iron) that consists of identical atoms.
• There are 118 known elements.
• Of these, 98 occur in nature; the others have been made by chemists and physicists.
• Their symbols consist of one or two letters.
• Names are derived from a variety of sources: the English name of the element, people important in atomic science, geographic locations, planets, mythological sources, etc.
• Four representations of a water molecule.
• Mixture: A combination of two or more pure substances.
• The substances may be present in any mass ratio.
• Each substance has a different set of physical properties.
• Mixtures may be homogeneous or heterogeneous.
• If we know the physical properties of the individual components of the mixture, we can use appropriate physical means to separate the mixture into its component parts.
• John Dalton (1766 - 1844) proposed that all matter is composed of very tiny particles, which he called atoms.
• All atoms of the same element have the same chemical properties.
• Atoms of different elements have different chemical properties.
• Compounds are formed by the chemical combination of two or more of the same or different kinds of atoms.
• Molecules are a tightly bound combination of two or more atoms that acts as a single unit.
• Law of Conservation of Mass states that matter can be neither created nor destroyed.
• Monatomic elements consist of single atoms; for example, helium (He) and neon (Ne).
• A box in an orbital box diagram represents an orbital and an arrow represents an electron.
• Electron configuration and the Periodic Table were constructed on the basis of trends (periodicity) in chemical properties.
• The size (radius) of an atom is determined by the radius of its outermost occupied orbitals.
• Only after each 2 p orbital has one electron is a second added to any 2 p orbital.
• Orbitals have definite shapes and orientations in space.
• After the 1 s and 2 s orbitals are filled, a 5th electron is put into the 2 p x, a 6th into the 2 p y, and a 7th into the 2 p z.
• The radius of a chlorine atom (99 pm) is determined by the size of its three 3p orbitals, the radius of a carbon atom (77 pm) is determined by the size of its three 2 p orbitals.
• Carbon (atomic number 6) has the electron configuration 1s 2 2s 2 2p 1 2p 1.
• Noble gas notation indicates the electron configuration of all filled shells, represented by the symbol of the noble gas immediately preceding the particular atom.
• In a Lewis dot structure, the symbol of the element represents the nucleus and filled shells.
• Elements in the same column (group) have the same configuration in their outer shells.
• Electron Configuration Rule 3: When there is a set of orbitals of equal energy, each orbital becomes half filled before any of them becomes completely filled.
• The valence shell is the outermost incomplete shell and a valence electron is an electron in the valence shell.
• A pair of arrows with heads in opposite directions represents a pair of electrons with paired spins.
• Nonmetals, except for hydrogen, lie on the right side of the Periodic Table, do not conduct electricity, and in chemical reactions, tend to accept electrons.