How atoms bond, and how to tell which bond you are looking at
HESI A2 topic: Chemical bonding
- Atoms usually bond to reach eight valence electrons; hydrogen needs only two.
- Ionic bonds transfer electrons (metal + nonmetal); covalent bonds share them.
- Electronegativity rises left to right and falls down a group; fluorine is highest.
- Electronegativity gap, roughly: under 0.4 nonpolar, 0.4 to 1.8 polar, over 1.8 ionic.
- Hydrogen bonds are weak pulls between molecules, not bonds inside them.
Key facts
- The octet rule: main-group atoms tend to bond until they have eight valence electrons; hydrogen needs only two.
- Ionic bonds are attractions between oppositely charged ions, usually formed when a metal transfers electrons to a nonmetal.
- Covalent bonds are shared electron pairs; a double bond shares two pairs and a triple bond shares three.
Show all 7 key factsShow fewer
- Electronegativity increases from left to right across a period and decreases down a group; fluorine is the most electronegative element (4.0).
- Rough guide for electronegativity difference: below 0.4 nonpolar covalent, 0.4 to 1.8 polar covalent, above 1.8 ionic.
- Metallic bonding is metal nuclei held in a sea of delocalized electrons, which is why metals conduct and can be hammered into shape.
- A hydrogen bond is an attraction between molecules, formed when H is bonded to F, O or N; it is only about 5 to 10% as strong as a covalent bond.
Quick check1 question · 2 more at the end
How does electronegativity generally change as you move from left to right across a period of the periodic table?
Correct answer: C.
Electronegativity, an atom's pull on bonding electrons, increases from left to right across a period and decreases down a group. That is why fluorine, near the top right, is the most electronegative element and the group 1 metals on the far left are the least. Noble gases are usually left out of the trend.
A There is a clear trend across a period; the group 1 metals on the left have the lowest values and the halogens on the right have very high ones.
B Electronegativity decreases going down a group, not across a period; this choice mixes up the two directions.
C Correct: atoms farther right in a period pull more strongly on shared electrons, so electronegativity increases.
D The trend does not reverse partway across a period; it rises steadily toward the halogens.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Atoms bond to reach a stable outer shell, usually eight valence electrons. A metal and nonmetal form an ionic bond by transferring electrons; nonmetals form covalent bonds by sharing them. Unequal sharing makes a polar covalent bond. Electronegativity, which rises across a period and falls down a group, predicts the bond type.
1 Why do atoms form bonds at all?Atoms bond so that each one ends up with a stable set of outer electrons. 1 min
Atoms bond so that each one ends up with a stable set of outer electrons. Those outer electrons are the valence electrons, and most main-group atoms are most stable with eight of them. This tendency is the octet rule. Hydrogen is the exception: it is satisfied with two.
A Lewis symbol draws an element's valence electrons as dots around its symbol. When atoms bond, a shared pair can be drawn as a dash. Pairs that sit on one atom and are not shared are called lone pairs.
2 What is electronegativity, and how does it predict the bond?Electronegativity is how strongly an atom pulls on the electrons in a bond. 1 min
Electronegativity is how strongly an atom pulls on the electrons in a bond. It rises from left to right across a period and falls going down a group. Fluorine is the strongest puller at 4.0; the group 1 metals are the weakest.
Subtract the two atoms' values to predict the bond. These cutoffs are rough guides, not hard laws.
| Electronegativity difference | Bond type | Example |
|---|---|---|
| Less than 0.4 | Nonpolar covalent | H–H (difference 0) |
| 0.4 to 1.8 | Polar covalent | H–Cl (0.9), O–H (1.4) |
| More than 1.8 | Ionic | Na–Cl (2.1) |
3 How do ionic bonds work?An ionic bond is the pull between a positive ion and a negative ion. 1 min
An ionic bond is the pull between a positive ion and a negative ion. Metals lose electrons easily and become cations; nonmetals gain electrons and become anions. Each usually ends up with the electron arrangement of a noble gas, so calcium becomes Ca²⁺ and oxygen becomes O²⁻.
Ionic compounds such as table salt (NaCl) form a rigid crystal of alternating ions, not separate molecules. They are brittle, have high melting points, and conduct electricity only when melted or dissolved, because only then can the ions move.
4 What makes a covalent bond polar or nonpolar?A covalent bond is a pair of electrons shared between two atoms, usually two nonmetals. 1 min
A covalent bond is a pair of electrons shared between two atoms, usually two nonmetals. Sharing one pair makes a single bond, two pairs a double bond (the C=C in ethylene), and three pairs a triple bond (carbon monoxide).
If the two atoms are identical, as in H₂ or Cl₂, they share equally and the bond is nonpolar. If one atom is more electronegative, it pulls the shared pair closer and gets a partial negative charge (δ−), while the other gets a partial positive charge (δ+). That is a polar covalent bond. In water, oxygen is the δ− end and each hydrogen is δ+.
5 What is a metallic bond?In a metal, the atoms' nuclei sit in a "sea" of electrons that are free to move. 1 min
In a metal, the atoms' nuclei sit in a "sea" of electrons that are free to move. This metallic bonding explains why metals are shiny, conduct heat and electricity well, and can be hammered or drawn into wire without shattering.
6 Are hydrogen bonds real bonds?A hydrogen bond is an attraction between molecules, not a bond inside one. 1 min
A hydrogen bond is an attraction between molecules, not a bond inside one. It forms when hydrogen is covalently bonded to fluorine, oxygen or nitrogen. That hydrogen carries a strong δ+ charge and is attracted to a lone pair on a nearby F, O or N.
Hydrogen bonds are only about 5 to 10% as strong as a covalent bond, but they add up. Based on similar molecules, water would be expected to boil near −80 °C; it actually boils near 100 °C because hydrogen bonds hold its molecules together. They also hold the two strands of DNA together, weakly enough that the strands can separate.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Hydrogen bonds are attractions between molecules, not the covalent O–H bonds inside a water molecule.
Watch out: Electronegativity decreases going down a group, not increases; the top-right elements (except noble gases) attract electrons most.
Watch out: In a polar bond, the more electronegative atom gets the partial negative charge, not the partial positive one.
Watch out: Solid ionic compounds do not conduct electricity; they conduct only when melted or dissolved, because then the ions can move.
Watch out: Two identical atoms, like the two in Cl₂, always share electrons equally, so that bond is nonpolar.
How to remember it
Memory hook: "Ionic gives, covalent shares, metals pool, hydrogen bonds only hold hands." The first three are true chemical bonds; a hydrogen bond is only a light grip between neighboring molecules.
Self-check:
- Rank C, N, O and F by electronegativity, and say which corner of the table has the highest values.
- Classify a bond with an electronegativity difference of 2.5, then one of 0.2.
- Explain why boiling water does not break its O–H bonds.