Introduction
Living things are made up of atoms, but in most cases, those atoms aren’t just floating around individually.
Instead, they’re usually interacting with other atoms (or groups of atoms).
For instance, atoms might be connected by strong bonds and organized into molecules or crystals. Or they
might form temporary, weak bonds with other atoms that they bump into or brush up against. Both the strong
bonds that hold molecules together and the weaker bonds that create temporary connections are essential to
the chemistry of our bodies, and to the existence of life itself.
Why form chemical bonds? The basic answer is that atoms are trying to reach the most stable (lowest-energy)
state that they can. Many atoms become stable when their valence shell is filled with electrons or when they
satisfy the octet rule (by having eight valence electrons). If atoms don’t have this arrangement, they’ll “want”
to reach it by gaining, losing, or sharing electrons via bonds.
Ions and ionic bonds
Some atoms become more stable by gaining or losing an entire electron (or several electrons). When they do
so, atoms form ions, or charged particles. Electron gain or loss can give an atom a filled outermost electron
shell and make it energetically more stable.
Forming ions
Ions come in two types. Cations are positive ions formed by losing electrons. For instance, a sodium atom loses
+
an electron to become a sodium cation, Na . Negative ions are formed by electron gain and are called anions.
Anions are named using the ending “-ide”: for example, the anion of chlorine Cl is called chloride.
When one atom loses an electron and another atom gains that electron, the process is called electron transfer.
Sodium and chlorine atoms provide a good example of electron transfer.
Sodium (Na) only has one electron in its outer electron shell, so it is easier (more energetically favorable) for
sodium to donate that one electron than to find seven more electrons to fill the outer shell. Because of this,
+
sodium tends to lose its one electron, forming Na .
Chlorine (Cl), on the other hand, has seven electrons in its outer shell. In this case, it is easier for chlorine to
gain one electron than to lose seven, so it tends to take on an electron and become Cl .
When sodium and chlorine are combined, sodium will donate its one electron to empty its shell, and chlorine
will accept that electron to fill its shell. Both ions now satisfy the octet rule and have complete outermost
shells. Because the number of electrons is no longer equal to the number of protons, each atom is now an ion
+
and has a +1 (Na ) or –1 (Cl ) charge.
In general, the loss of an electron by one atom and gain of an electron by another atom must happen at the
same time: in order for a sodium atom to lose an electron, it needs to have a suitable recipient like a chlorine
atom.
Making an ionic bond
Ionic bonds are bonds formed between ions with opposite charges. For instance, positively charged sodium
ions and negatively charged chloride ions attract each other to make sodium chloride, or table salt. Table salt,
like many ionic compounds, doesn't consist of just one sodium and one chloride ion; instead, it contains many
ions arranged in a repeating, predictable 3D pattern (a crystal).
Certain ions are referred to in physiology as electrolytes (including sodium, potassium, and calcium). These ions
are necessary for nerve impulse conduction, muscle contractions and water balance. Many sports drinks and
dietary supplements provide these ions to replace those lost from the body via sweating during exercise.
Covalent bonds
Another way atoms can become more stable is by sharing electrons (rather than fully gaining or losing them),
thus forming covalent bonds. Covalent bonds are more common than ionic bonds in the molecules of living
organisms.
For instance, covalent bonds are key to the structure of carbon-based organic molecules like our DNA and
proteins. Covalent bonds are also found in smaller inorganic molecules, such H2O, CO2, O2. One, two, or three
pairs of electrons may be shared between atoms, resulting in single, double, or triple bonds, respectively. The
more electrons that are shared between two atoms, the stronger their bond will be.
As an example of covalent bonding, let’s look at water. A single water molecule, H2O, consists of two hydrogen
atoms bonded to one oxygen atom. Each hydrogen shares an electron with oxygen, and oxygen shares one of
its electrons with each hydrogen. The shared electrons split their time between the valence shells of the
hydrogen and oxygen atoms, giving each atom something resembling a complete valence shell (two electrons
for H, eight for O). This makes a water molecule much more stable than its component atoms would have been
on their own.
Polar covalent bonds
There are two basic types of covalent bonds: polar and nonpolar. In a polar covalent bond, the electrons are
unequally shared by the atoms and spend more time close to one atom than the other. Because of the unequal
distribution of electrons between the atoms of different elements, slightly positive (δ+) and slightly negative
(δ–) charges develop in different parts of the molecule.
In a water molecule (above), the bond connecting the oxygen to each hydrogen is a polar bond. Oxygen is a
much more electronegative atom than hydrogen, meaning that it attracts shared electrons more strongly, so
the oxygen of water bears a partial negative charge (has high electron density), while the hydrogens bear
partial positive charges (have low electron density).
In general, the relative electronegativities of the two atoms in a bond – that is, their tendencies to "hog" shared
electrons – will determine whether a covalent bond is polar or nonpolar. Whenever one element is significantly
more electronegative than the other, the bond between them will be polar, meaning that one end of it will
have a slight positive charge and the other a slight negative charge.
Nonpolar covalent bonds
Nonpolar covalent bonds form between two atoms of the same element, or between atoms of different
elements that share electrons more or less equally. For example, molecular oxygen O2 is nonpolar because the
electrons are equally shared between the two oxygen atoms. Another example of a nonpolar covalent bond is
found in methane CH4. Carbon has four electrons in its outermost shell and needs four more to achieve a stable
octet. It gets these by sharing electrons with four hydrogen atoms, each of which provides a single electron.
Reciprocally, the hydrogen atoms each need one additional electron to fill their outermost shell, which they
receive in the form of shared electrons from carbon. Although carbon and hydrogen do not have exactly the
same electronegativity, they are quite similar, so carbon-hydrogen bonds are considered nonpolar.
1. Which of the following molecules would one expect to have a non-polar covalent bond? HCl, F2, HF, ClF?
2. How many electrons are being shared between the two carbon atoms in the compound ethyne? Ethyne
has the Lewis structure: H−C≡C−H. 1, 2, 3, 6?
3. If a covalent bond were to be formed between a nitrogen atom (electronegativity 3.0) and an oxygen atom
(electronegativity 3.5), which of the following statements would best describe such a bond? Non-polar
covalent; Polar covalent where the oxygen atom carried the partial negative charge; Polar covalent where the
nitrogen atom carried the partial negative charge; Polar covalent where the oxygen atom carried the partial
positive charge
4. The three dots in the Lewis dot diagram for boron indicate that it: can bond with three other atoms; can
only form triple covalent bonds; has three valence electrons; scores a 3 on Pauling's electronegativity scale?
5. A chemical bond is: Made up of electrons; An attractive force that holds atoms together; All of the above;
None of the above?
6. If an atom of nitrogen gains 3 electrons, what is its overall charge? Neutral - No charge; +3; -3; -1?
7. Which of the following is true of ionic bonds? Electrons are completely transferred during bonding;
Electrons are shared during bonding; Electrons are not involved with bonding; All of the above?
8. Which of the following describes covalent bonds? Bonds form because of opposite charges; Bonds form to
fill outer electron shells; Electrons are transferred between atoms; Covalent bonds are magical?
9. Carbon tetrachloride contains how many atoms of chlorine? 1, 2, 3, 4?
10. Covalent bonds are formed between two metallic elements? TRUE; FALSE?
11. Anion is another term for an atom that gains one or more electrons? TRUE; FALSE?
12. When a metal reacts with a nonmental, it makes: an ionic compound; a covalent compound; a low melting
point; water molecules?
13. Ionic or covalent: NaF? Ionic; Covalent?
14. Ionic or covalent: NaH? Ionic; Covalent?
15. In a covalent bond, one or more electrons are? Shared; lost; gained; transferred?
16. The correct Lewis-dot structure for Iodine ( I ) would have how many valence electrons? 2, 4, 7, 8?
17. What group of elements satisfies the octet rule without forming compounds? Halogen; noble gas; alkali
metal; alkaline-earth metal?
18. What two types of atoms make a covalent bond? 2 Nonmetals; 1 Nonmetal and 1 Metal; 2 Metals; 2 Noble
Gases?
19. Metals are good conductors of electricity because they: form crystal lattices; contain positive ions; contain
mobile valence electrons; form ionic bonds?
20. Ionic Bonding involves...: The transfer of protons; The transfer of neutrons; The transfer of electrons; None
of the above?
21. Predict the bond that will form between Se and Cl. Ionic; Covalent?
22. What do metals conduct? Heat; electricity; both; neither?
23. NaCl: Ionic; Covalent?
24. What type of chemical bond is formed when one atom donates an electron to another atom? Ionic;
Covalent?
25. Which of the following is true about covalent bonding? Up to 8 electrons can be donated from one
element to another; Electrons are donated from the outer shell; Electrons are shared in pairs; All of the
above; None of the above?
26. The bond formed when Mg combines with O2 is? ionic; covalent; metallic?
27. The process of becoming an ion from an atom is called as? Ionization; electrolysis; electron affinity?
28. Covalent bond is formed between atoms of? Metals; non-metals; metals and non-metals?
29. Hydrogen Fluoride has a (an) ________ bond and its chemical formula is ________? covalent, HF; ionic,
HF; covalent, H2F?
30. Calcium Oxide is a (an) _____ compound and the formula for Calcium Oxide is ________? covalent, CaO2;
ionic, CaO; ionic, CaO2?
31. Oxygen has 6 valence electrons and it is diatomic. How many covalent bonds are there in an O2
molecule? Single; double; triple?
32. The chemical formula of the product formed from the reaction between Mg and O 2 is _______? MgO2;
MgO; Mg2O?
33. Magnesium Bromide is a (an) ________ compound? Metallic; covalent; ionic?
34. How is the bond in F2 different from the bond in KCl? F2 is covalent and KCl is ionic; F2 is ionic and KCl is
ionic; F2 is ionic and KCl is covalent
35. Write the chemical formula for a compound that has one Calcium atom and 2 Chlorine atoms. Predict the
bond between them. CaCl2, ionic; CaCl2, covalent; Ca2Cl, ionic
36. The type of bond in which electrons are shared between two atoms? Ionic; Covalent; London; Hydrogen?
37. Water would be a good example of: Ionic bond; Covalent bond?
38. What is the charge on the ions formed by the alkaline earth metals? +1, +2, -1, -2?
39. What is the most correct name for the ionic compound formed by Fe²⁺ and Cl⁻? iron chloride; iron(I)
chloride; iron(II) chloride; iron(III) chloride?
40. What type of bonds are formed in N₂O₄ and what is the name of this compound? covalent, dinitrogen
tetroxide; covalent, nitrogen tetroxide; ionic, nitrogen oxide; ionic, dinitrogen oxide?
41. The bond between sulfur (electronegativity value 2.5) and chlorine (electronegativity value 3.0) would
be: not formed; ionic; polar covalent; nonpolar covalent?
42. Ionic compounds may contain polyatomic ions. For example, the formula of magnesium nitrate is: MgNO₃
Mg₂NO₃; Mg(NO₃)₂; Mg₂(NO₃)₃
43. What is the formula of phosphorus trichloride? KCl; KCl₃; P₃Cl; PCl₃?
44. How many electrons are gained/lost by magnesium and what is the charge on the ion that it forms? loses
2 electrons to form a magnesium ion with a 2- charge; gains 2 electrons to form a magnesium ion with a 2charge; loses 2 electrons to form a magnesium ion with a 2+ charge; gains 2 electrons to form a magnesium ion
with a 2+ charge?
45. The electron-dot structure of carbon has how many dots? 2, 4, 6, 8?
46. Covalent compounds generally have high melting points? True or false?
47. A covalent bond exists when the electronegativity difference between two atoms is small or zero. True or
false?
48. Classify the following formulas as representing ionic compounds, covalent molecules or incorrect: PCl3,
K3O, Li2O, Na2SO4, C6H12
49. Which of the following are ionic and which are molecular? NO, K2O, SrCO3, SiCl4, AsH3
50. Draw a suitable Lewis structure for Li2S.