Chemistry Notes - September 5, 2006 (2.5-2.8)

Chemistry Notes - September 5, 2006 (2.5-2.8) - September...

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September 5, 2006 Chemistry Notes Sections 2.5-2.8 Section 2.5 Of all the elements, only the six noble gases in Group 8A exist in nature as single atoms. They are called monoatomic gases. Most matter is composed of molecules or ions formed by atoms. A molecule is an aggregate of at least two atoms in a definite arrangement held together by chemical forces (chemical bonds) A molecule is not necessarily a compound Like atoms, molecules are electrically neutral Diatomic molecule – contains only two atoms Most molecules contain more than two atoms. Molecules containing more than two atoms are called polyatomic molecules. Ion – an atom or a group of atoms that has a net positive or negative charge. Negatively charged electrons may be lost or gained during chemical reactions. The loss of one or more electrons from a neutral atom results in a cation – an ion with a net positive charge. Anion – an ion whose net charge is negative due to an increase in the number of electrons. Ionic compound – formed from cations and anions. Monoatomic ions – contain only one atom. Polyatomic ions – ions containing more than one atom. Section 2.6 Chemical formulas – express the composition of molecules and ionic compounds in terms of chemical symbols Molecular formula – shows the exact number of atoms of each element in the smallest unit of a substance. Allotrope – one of two or more distinct forms of an element.
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Chemistry Notes - September 5, 2006 (2.5-2.8) - September...

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