II. CHEMICAL REACTIONS
A. More about Electrons
1. Electrons orbit around the nucleus at different energy levels or shells the 1st shell (k shell) can hold no more than 2 electrons; the next shell (l shell) can hold 8 electrons, 2 electrons in each of 4 orbitals; the next shell (m shell) also holds 8 electrons. These will be the only shells that we will deal with in this class.
2. An atom is the most stable when all of its shells are completely filled (the k shell fills first, then the l shell, then the m shell, & so on).
3. The atoms of many elements have partially filled outer shells, therefore they are not very stable; these atoms tend to react with other atoms to completely fill their outer shells &, in doing so, they form chemical bonds; it is important to remember that it's the electrons of an atom that participate in the chemical bonds that form between atoms. Molecules are formed when 2 or more atoms are joined together by interactions between the electrons of their outer electron shells.
B. Chemical Bonds
1. Ionic bonds
a. definition - in ionic bonds electrons are transferred to other atoms to completely fill outer shells; atoms are electrically neutral, but when they gain or lose electrons in combining with other atoms, they are called ions (charged atoms) & they take on a positive or negative charge; in other words, the transfer of electrons upsets the balance of protons & electrons in an atom; atoms that lose electrons are positively charged, atoms that gain electrons are negatively charged; ionic bonds involve the attractions between these oppositely charged ions. So, before you can have an ionic bond, you have got to have oppositely charged ions, & to create ions you have got to transfer electrons.
b. example: NaCl (sodium chloride- table salt); Na (at. # 11) has one electron in its outer m shell - it needs 7 electrons to fill this shell - it is easier for Na just to give this electron away, & eliminate the m shell entirely. Cl (at. # 17) has 7 electrons in its outer m shell - it only needs 1 electron to fill its outer shell. Therefore, when Na & Cl atoms react, Na gives up its outer electron to Cl. Because Na gives up an electron, it now has 11 protons & 10 electrons, resulting in a positively charged atom. Cl now has 17 protons & 18 electrons, resulting in a negatively charged atom. Na+ & Cl- ions are attracted to each other because of their opposite charges & an ionic bond is formed. The attraction is the ionic bond! Only the electron # changes when ions are formed!
2. Covalent bonds - more common in the human body & are more stable.
a. definition - Electrons are not transferred, but are shared. The shared electrons spend part of their time around the nucleus of one atom & part of their time around the other. Each pair of electrons shared equals one covalent bond (if 2 pair of electrons are shared between 2 atoms, a double covalent bond is formed, a triple covalent bond occurs when 3 pr. of electrons are shared). We only discuss single covalent bonds.
b. example: methane (CH4); a carbon atom can form four covalent bonds - it has 4 electrons in its outer shell, therefore it needs 4 more electrons to fill its outer shell; hydrogen has one electron in its outer shell, therefore it needs one electron to fill its outer shell. Rather than give 4 electrons away or accept 4 electrons, carbon shares its 4 electrons in its outer shell with 4 Hydrogen atoms.
c. polar vs. nonpolar - covalent bonds can be polar or nonpolar; if both atoms exert the same pull on the shared electrons (equal sharing), the bond is nonpolar (example: methane); if there is unequal sharing of electrons, the bond is polar; in molecules with polar covalent bonds, there is an atom that has a much larger nucleus (more protons) than the other atoms in the molecule; the atom with the most protons is more attractive to the shared electrons, so the electrons spend most of their time around this atom's nucleus; all of these electrons spending most of their time around a particular nucleus gives this part of the molecule a partial negative charge; the other atom(s) in the molecule acquire a partial positive charge, because the shared electrons are not spending much time around them. Shared electrons in polar covalent bonds are not spending all of their time around a particular nucleus - if this were the case then we would be talking about electrons being transferred (as in ionic bonds).
4. Hydrogen bonds - These bonds result from polar covalent bonds; they form between molecules & occur between the slightly negative atom of one molecule & the slightly positive atom of another molecule. These bonds can occur between hydrogen & oxygen & between hydrogen & nitrogen. Hydrogen bonds are weaker than ionic & covalent bonds, because the charges on the molecules are "partial" or weak charges.
C. Chemical Reactions - A chemical reaction when atoms or molecules (called reactants) collide and are transformed into different combinations of the same atoms or molecules (called products). In this process, chemical bonds break and new ones form. In living systems special proteins called enzymes catalyze these chemical reactions (they make them “go”). We'll talk more about enzymes later.
A. More about Electrons
1. Electrons orbit around the nucleus at different energy levels or shells the 1st shell (k shell) can hold no more than 2 electrons; the next shell (l shell) can hold 8 electrons, 2 electrons in each of 4 orbitals; the next shell (m shell) also holds 8 electrons. These will be the only shells that we will deal with in this class.
2. An atom is the most stable when all of its shells are completely filled (the k shell fills first, then the l shell, then the m shell, & so on).
3. The atoms of many elements have partially filled outer shells, therefore they are not very stable; these atoms tend to react with other atoms to completely fill their outer shells &, in doing so, they form chemical bonds; it is important to remember that it's the electrons of an atom that participate in the chemical bonds that form between atoms. Molecules are formed when 2 or more atoms are joined together by interactions between the electrons of their outer electron shells.
B. Chemical Bonds
1. Ionic bonds
a. definition - in ionic bonds electrons are transferred to other atoms to completely fill outer shells; atoms are electrically neutral, but when they gain or lose electrons in combining with other atoms, they are called ions (charged atoms) & they take on a positive or negative charge; in other words, the transfer of electrons upsets the balance of protons & electrons in an atom; atoms that lose electrons are positively charged, atoms that gain electrons are negatively charged; ionic bonds involve the attractions between these oppositely charged ions. So, before you can have an ionic bond, you have got to have oppositely charged ions, & to create ions you have got to transfer electrons.
b. example: NaCl (sodium chloride- table salt); Na (at. # 11) has one electron in its outer m shell - it needs 7 electrons to fill this shell - it is easier for Na just to give this electron away, & eliminate the m shell entirely. Cl (at. # 17) has 7 electrons in its outer m shell - it only needs 1 electron to fill its outer shell. Therefore, when Na & Cl atoms react, Na gives up its outer electron to Cl. Because Na gives up an electron, it now has 11 protons & 10 electrons, resulting in a positively charged atom. Cl now has 17 protons & 18 electrons, resulting in a negatively charged atom. Na+ & Cl- ions are attracted to each other because of their opposite charges & an ionic bond is formed. The attraction is the ionic bond! Only the electron # changes when ions are formed!
2. Covalent bonds - more common in the human body & are more stable.
a. definition - Electrons are not transferred, but are shared. The shared electrons spend part of their time around the nucleus of one atom & part of their time around the other. Each pair of electrons shared equals one covalent bond (if 2 pair of electrons are shared between 2 atoms, a double covalent bond is formed, a triple covalent bond occurs when 3 pr. of electrons are shared). We only discuss single covalent bonds.
b. example: methane (CH4); a carbon atom can form four covalent bonds - it has 4 electrons in its outer shell, therefore it needs 4 more electrons to fill its outer shell; hydrogen has one electron in its outer shell, therefore it needs one electron to fill its outer shell. Rather than give 4 electrons away or accept 4 electrons, carbon shares its 4 electrons in its outer shell with 4 Hydrogen atoms.
c. polar vs. nonpolar - covalent bonds can be polar or nonpolar; if both atoms exert the same pull on the shared electrons (equal sharing), the bond is nonpolar (example: methane); if there is unequal sharing of electrons, the bond is polar; in molecules with polar covalent bonds, there is an atom that has a much larger nucleus (more protons) than the other atoms in the molecule; the atom with the most protons is more attractive to the shared electrons, so the electrons spend most of their time around this atom's nucleus; all of these electrons spending most of their time around a particular nucleus gives this part of the molecule a partial negative charge; the other atom(s) in the molecule acquire a partial positive charge, because the shared electrons are not spending much time around them. Shared electrons in polar covalent bonds are not spending all of their time around a particular nucleus - if this were the case then we would be talking about electrons being transferred (as in ionic bonds).
4. Hydrogen bonds - These bonds result from polar covalent bonds; they form between molecules & occur between the slightly negative atom of one molecule & the slightly positive atom of another molecule. These bonds can occur between hydrogen & oxygen & between hydrogen & nitrogen. Hydrogen bonds are weaker than ionic & covalent bonds, because the charges on the molecules are "partial" or weak charges.
C. Chemical Reactions - A chemical reaction when atoms or molecules (called reactants) collide and are transformed into different combinations of the same atoms or molecules (called products). In this process, chemical bonds break and new ones form. In living systems special proteins called enzymes catalyze these chemical reactions (they make them “go”). We'll talk more about enzymes later.