Elements combine to form compounds chemical bonds. Chemical Bonding

Transcription

1 Elements combine to form compounds chemical bonds Chemical Bonding

2 Review Valence electrons Using periodic table to determine them. General bonding rules: If an atom has 1 to 3 valence electrons, it will lose them. the givers If an atom has 5 to 7 valence electrons, it will gain more. the takers If an atom has 4 valence electrons, it depends on the element it s bonding with. Tendency is to be a giver the further down the column you go Positive ions (cation) versus negative ions (anion) (the givers) (the takers) A giver becomes like the noble gas that precedes it and a taker becomes like the noble gas that follows it. What about hydrogen?

3 Remember this? A compound is a substance made from two or more different atoms chemically bonded together. For example, water(h 2 O) and salt (NaCl) are compounds. A compound always contains two or more elements joined in fixed amounts.

4 Reactions that affect an Atom Nuclear reactions these type reactions change the composition of an atom s nucleus. Chemical reactions the most common one where atoms interact only through their valence electrons; the nuclei remain unchanged.

5 Compounds The properties of a compound depend on: Which atoms the compound contains How the atoms are arranged Atoms are not changed by chemical reactions, but merely rearranged into different compounds. Atoms are neither created nor destroyed by chemical reactions law of conservation of matter

6 Chemical Formulas A chemical formula represents the atoms of elements and their ratios (fixed amounts) in a chemical compound. Example: Carbon Dioxide is a compound of 1 atom of carbon bonded to 2 atoms of oxygen. To indicate the chemical formula we use C and O with a subscript of 2 beside oxygen to show there are 2 atoms. This is how we get CO 2

7 Chemical bonds Chemical bonds are the glue that holds the atoms of elements together in compounds. Chemical bonds form when the valence electrons in the electron cloud around the atoms interact. When atoms exchange their valence electrons to form compounds, the total number of electrons given must equal the total number taken. Therefore, all compounds are electrically neutral.

8 Chemical Bonding basic rules Elements tend to gain, lose, or share valence electrons in order to acquire the e - configuration of a noble gas. The transition metals do not consistently form bonds that follow this trend; however, since they are metals they are givers. Octet rule- atoms tend to gain, lose, or share electrons in order to acquire a full set of valence electrons eight! (2 for Helium)

9 Lewis Dot Diagrams G.N. Lewis used dots to represent the valence electrons around an atom in his teaching of chemical bonding. An electron dot diagram is a model of an atom in which each dot represents a valence electron. The chemical symbol in the middle represents the nucleus and all the other electrons in the atom. It is those valence electrons that are lost, shared, or added to in a chemical interaction between atoms of elements.

10 Dot Diagram Examples

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13 Types of Chemical Bonds We will learn about two types (there are more) of chemical bonds Ionic and Covalent bonds. Ionic bonds come from ions being formed. They are formed when atoms lose or gain electrons: Taking e - makes a negative ion or anion the takers Giving e - makes a positive ion or cation - the givers An element s location on the periodic table gives clues as to the type of ions the atoms of that element might form with another atom. Ionic bonds are formed when: Usually metals react with elements in Group 15, Group 16, Group 17 (the nonmetals) Metalloids reacting with metals

14 Ionic Bonds So what happens when an atom of an element from Group 1, like Na, meets an atom of an element from Group 17, like Cl? Na gives an electron and forms a positive ion and Cl takes that electron to form a negative ion. What happens to particles with opposite charges? They are drawn toward each other by electrical attraction. So with ionic bonds you ve got givers and takers forming that glue to make the compound. This force of attraction between positive and negative ions is called an ionic bond. Oxidation numbers refers to how many electrons are given or taken when chemical bonding occurs. It is equal to the charge an atom has when it ionizes. The overall charge on the compound formed must equal zero; the number of electrons lost by one atom must equal the number of electrons gained by the other atom. Also, the outer shells of each atom will be complete (8) they are noble!!

15 Ionic bonds chemical formula Mg (Magnesium) bonding with Cl (Chlorine) A Mg ion (Mg +2 ) gives away two electrons, but a Cl ion (Cl -1 ) needs only one more electron to complete its outermost energy level. Therefore, it takes two Cl ions to combine with one Mg ion in order to make a neutral compound. The chemical formula would be: Symbol of positive ion MgCl 2 Symbol of negative ion subscript of how many atoms

16 Naming Ionic Compounds The name of an ionic compound is based on the names of the ions that made it. The positive ion uses the name of the atom which formed it. The name of the negative ion is formed by dropping the last part of the name of the atom and adding the suffix ide. In our example the ionic compound is MgCl 2 Magnesium Chloride.

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19 Covalent Bonds Another way in which atoms can bond together is by sharing electrons. A pair of shared electrons between two atoms is called a covalent bond. co means partner and valent refers to valence. They share valence electrons. In a covalent bond, neither atom gains or loses an electron, therefore, no ions are formed. They share in pairs; so think of the Care Bears who like to do things together. Each atom in the compound formed has a share in the number of electrons required to complete the valence orbit. However, each atom still achieves nobility by sharing happy, happy, happy!!! Phil Robertson The covalent bonding is caused by the mutual electrical attraction between the two positive nuclei of the two atoms of the bond, and the electrons shared between them. Compounds that contain only covalent bonds are called molecular compounds.

20 Covalent Bonds Nonmetals reacting with another nonmetal will usually form a covalent bond. (Remember, hydrogen is a nonmetal.) A metalloid reacting with a nonmetal will usually form a covalent bond. H and Groups 14 thru 17 (non metals only) Naming simple covalent compounds ( see handout advanced classes )

21 Covalent Bonds Chemical Formula A molecular formula begins with the element that is closest to the lower left corner of the periodic table, except that hydrogen is almost never written first (H 2 O and acids like HCl, H 2 SO 4 are exceptions). Then the other element symbols are listed.

22 Summary Ionic vs. Covalent Ionic Bonds atoms transfer electrons to fill their outermost energy level. Usually metals reacting with nonmetals will form an ionic bond Usually a metalloid reacting with a metal will form an ionic bond Ionic bonds are very strong, good conductors of electricity Ionic bonds have high melting points, dissolve in water 1 3 valence electrons, atoms want to lose them in order to be noble. 5 7 valence electrons, atoms want to gain more in order to be noble. 4 valence electrons, atoms can t decide; so it depends on the element it is bonding with. Covalent Bonds atoms share electrons to fill their outermost energy level. Nonmetals reacting with another nonmetal will usually form a covalent bond A metalloid reacting with a nonmetal will usually form a covalent bond They tend to be gases or liquids at room temperature They are very poor conductors of electricity, low melting points and low boiling points.

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24 Polyatomic Compounds We ve seen compounds that look like this: Pb (NO 3 ) 2 Ba (OH) 2 They are called polyatomic compounds How are they formed and how do we bond these?

25 Polyatomic Ions Monoatomic ions - ions formed by one element. Polyatomic ions ions formed by more than one element. Polyatomic ions are formed by a group of atoms that have a charge. Since they are ions, they can have positive (+) or negative (-) charges; i.e. oxidation numbers

26 Polyatomic bonding Polyatomic ions are formed by the covalent bonding of two or more non-metals in such a way that there is still an electrical charge. Since they are ions, polyatomic bonding is ionic bonding and follow the same rules for formulas. Think of everything inside the parenthesis as one ion; so in Pb (NO 3 ) 2 This is the polyatomic ion (NO 3 ) The name of a polyatomic compound is: the name of the positive ion plus the name of the negative ion.

27 Common Polyatomic Ions (OH) -1 Hydroxide ion (CO 3 ) -2 Carbonate ion (SO 4 ) -2 Sulfate ion (PO 4 ) -3 Phosphate ion (NO 3 ) -1 Nitrate ion (HCO 3 ) -1 Bicarbonate ion (C 2 H 3 O 2 ) -1 Acetate ion