Summary HESI A2 CHEMISTRY STUDY GUIDE

HESI A2 CHEMISTRY STUDY GUIDE Atom The atom is the smallest unit of matter. Every atom contains one nucleus at least one electron. The nucleus contains at least one positively charged proton and an equal number of neutrally charged neutrons. Every atom also has at least one negatively charged electron bound to the nucleus. When the number of protons (the atomic number) is equal to the number of electrons, the atom is neutral. When the number of protons and number of electrons differ, the result is an ion. The way electrons are distributed around an atom or molecule is known as an electron configuration. Electrons are distributed among “shells” which may be thought of as energy levels. The configuration that corresponds to the lowest energy level is called ground state. Every other configuration is considered an excited state. Electrons always fill the lower energy levels first. To write an electron configuration: • Locate the atomic number • If the atom being studied is charged, this should be taken into account. Cations (positively charged) will have fewer electrons in the magnitude of the charge. Anions (negatively charged) will have extra electrons in the magnitude of the charge. • Understand the order of orbitals: o S – 1 single orbital, holds 2 electrons o P – 3 orbitals, holds 6 electrons o D – 5 orbitals, holds 10 electrons o F – 7 orbitals, holds 14 electrons • Fill the orbitals in the following order, according to the number of electrons of the atom being studied. o 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, 8s. • The number of electrons is written in superscript to the right of the orbital letter. For example, the configuration for neutral neon would be written as 1s2 2s2 2p6. Periodic Table The periodic table contains the most important information about all known elements. An element is a substance that is composed purely from one type of atom. The periodic table is broken into four main groups. • Main group elements: active metals in columns (also called groups) 1 and 2 as well as the metals, semimetals and nonmetals in columns 13-18. • Transition metals: the elements that serve as a transition between the two main groups • Lanthanides and Actinides: known as the inner transition metals because their atomic numbers are actually located between the first and second elements in the last two rows (also called periods) of transition metals.