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Tiny nucleus: diameter of about cm. Electrons: move about the nucleus at an average distance of about from it. If the nucleus were the size of the ball bearing, a typical atom would be the size of the stadium.

Nucleus contains protons (positive charge) and neutrons (neutral). Electrons (negative charge) surround the nucleus.

Nucleus small size with extremely high density accounts for most of atom’s mass piece of nuclear material about the size of a pea would have a mass of 250 million tons

Electrons account for most of atom’s volume their number and arrangement account for an atom’s chemical behavior involved when different atoms bond

number of protons in the nucleus of an atom the atomic number identifies the element has the symbol, Z total number of protons and neutrons in the nucleus of an atom has the symbol, A mass number = number of protons + number of neutrons

atoms with the same number of protons but different number of neutrons

Isotopes are defined by two numbers, its atomic number and its mass number. Isotopes are represented by nuclear symbols or by using hyphen notation. Nuclear Symbol Mass number (protons + neutrons) Atomic number (number of protons) Element symbol Hyphen notation Element name-mass number

Example: symbol for one particular isotope of sodium Mass number Atomic number Element symbol or Sodium-23 Two isotopes of sodium. Both have 11 protons and 11 electrons, but differ in the number of neutrons.

Chemical bonds: forces that hold atoms together in compounds. Covalent bond: results from sharing of electrons between different atoms; the resulting collection of atoms is called a molecule. Simplest method to represent a molecule is a chemical formula. Chemical formula: the symbols for the elements are used to indicate the types of atoms present and subscripts are used to indicate the relative numbers of atoms. Chemical formula for methane: CH 4

Different representations of molecules. Structural formula for methane. Individual bonds are shown. May or may not indicate the actual structure of the compound. Space-filling model of methane. This type of model shows the relative sizes of atoms as well as their relative orientation in the molecule. Ball-and-stick model of methane.

A second type of bond results from attractions between ions. An ion is an atom or groups of atoms that has a net positive or negative charge resulting from a loss or gain of electrons. The best known ionic compound is common table salt, or sodium chloride, which forms when neutral sodium and chlorine react.

Consider the formation of the sodium ion below. When an ion forms, electrons are transferred from one atom to another (the number lost must equal the number gained). With one electron transferred, sodium has 11 protons and only 10 electrons so has a net 1+ charge – it has become a positive ion. A positive ion is called a cation. Represented in shorthand form: Na → Na + + e -

Consider the formation of the chlorine ion below. If an electron is added to chlorine, it has 17 protons and 18 electrons which produce a net 1- charge – it has become a negative ion. A negative ion is called an anion. Represented in shorthand form: Cl + e - → Cl -

Anions and cations = opposite charges = attraction Ionic bonding: force of attraction between oppositely charged ions. Solid composed of oppositely charged ions is called an ionic solid or a salt.

Polyatomic ion: an ion composed of two or more atoms. Can form ionic bonds with other simple ions or with other polyatomic ions. Ammonium nitrate

Periodic Table: a chart showing all the elements arranged in columns with similar chemical properties. Letters in the boxes are the symbols for the elements. The number above the symbol is the atomic number (number of protons).

Most of the elements are metals (found to the left of the staircase in the periodic table). Characteristic physical properties: efficient conduction of heat and electricity malleability (they can be hammered into thin sheets) ductility (they can be pulled into wires) (often) a lustrous appearance all are solids at room temperature except mercury Tend to lose electrons to form positive ions.

Found to the right of the staircase on the periodic table. Encountered mostly in the form of compounds or mixtures of compounds. Poor conductors of heat and electricity. Neither malleable nor ductile. Many are gases at room temperature and many are solids (Bromine is the only liquid). Usually exist as molecules in their elemental form. Tend to gain electrons to form negative ions.

Grouped along the staircase. Properties lie between those of metals and nonmetals. Tend to be semiconductors; they conduct electricity, but not nearly as well as metals. Generally, metalloids behave as nonmetals, both chemically and physically.

Periodic Table Horizontal rows are called periods. Vertical columns are called groups or families. Elements in same column have similar chemical properties. Group 1 elements are called the alkali metals. very active elements readily form ions with a 1+ charge

Group 2 elements are called the alkaline earth metals. reactive and form ions with a 2+ charge Groups 3 through 12 are called the transition metals. form colored ions variable oxidation states

Group 17 are called the halogens. very reactive nonmetals form ions with 1- charge exist as diatomic molecules Group 18 are called the noble gases. exist as monatomic (single-atom) gases little chemical reactivity

The elements in the two long rows below the main body of the periodic table are called the inner transition elements. Elements are called the lanthanide elements because they follow lanthanum (Z = 57). Elements are called the actinide elements because they follow actinium (Z = 89).