Chemistry 2e · Atoms, Molecules, and Ions
The Periodic Table
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In 30 seconds
The periodic table is chemistry's master map: a chart that organizes every known element so the behavior of one helps predict the behavior of its neighbors. All 118 confirmed elements appear in order of increasing atomic number, arranged into 7 horizontal rows called periods and 18 vertical columns called groups (or families). The layout encodes the periodic law Properties repeat at regular intervals when elements are arranged by increasing atomic number. Full entry →: when elements are ordered by atomic number, their properties repeat at regular intervals. The modern table descends from Dmitri Mendeleev's 1869 version, which left blank spaces for elements not yet discovered and predicted their properties in advance.
Why this matters
The table is a prediction engine and a safety tool. Mendeleev's blank spaces were filled by gallium, scandium, and germanium, whose measured properties matched his predictions — proof that the arrangement captures real, repeatable chemistry. At a glance the table tells you whether an element is a metal that forms positive ions or a nonmetal that forms negative ions, the first step toward writing correct formulas and names in the next two topics. Iodine (group 17) is essential for thyroid hormones; technetium (group 7) is used in diagnostic imaging. For exams, position-based reasoning — period, group, metal versus nonmetal, predicted ion charge — is one of the most frequently tested skills in general chemistry.
The college version
Core Concepts
Periods, groups, and the periodic law
Read the table like a grid. Each row is a period; there are seven, numbered 1–7. Each column is a group; there are 18, numbered 1–18 (older texts use A/B labels). The periodic law states that properties are periodic functions of atomic number: elements in the same group have the same number of valence electrons and therefore similar chemistry. Mendeleev ordered his table by atomic mass, but Henry Moseley showed in 1913 that atomic number is the correct principle — it fixed places where mass ordering put similar elements in different columns (e.g., tellurium and iodine).
Metals, nonmetals, and metalloids
About three-quarters of the elements are metals, on the left and across the middle: shiny, malleable, ductile, and good conductors. The nonmetals occupy the upper-right corner (hydrogen is the exception, sitting alone above group 1) and are dull, brittle, and poor conductors. Between them lies a staircase of metalloids — boron, silicon, germanium, arsenic, antimony, tellurium (polonium and astatine are often included) — with intermediate properties; silicon and germanium are semiconductors. At room temperature most elements are solids; eleven are gases (H₂, N₂, O₂, F₂, Cl₂, and the noble gases); exactly two are liquids: bromine and mercury.
Families with shared chemistry
Group 1, the alkali metals (Li, Na, K, Rb, Cs, Fr), are soft, shiny metals so reactive they combine vigorously with water. Group 2, the alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra), are also reactive. The transition metals (groups 3–12) include iron, copper, zinc, and gold, and form colorful compounds with variable ion charges. Group 17, the halogens (F, Cl, Br, I, At, Ts), are reactive nonmetals forming −1 ions. Group 18, the noble gases (He, Ne, Ar, Kr, Xe, Rn, Og), are famously unreactive because their valence shells are full. The lanthanides and actinides, displayed below the main table, belong to periods 6 and 7.
Reading the table: valence electrons and ion charges
For main-group elements the group number gives the valence-electron count: group 1 has 1, group 2 has 2, group 13 has 3, and so on through group 18 (helium excepted, with 2). That count drives ion formation: metals on the left lose electrons to become cations; nonmetals on the right gain electrons to become anions, both approaching a noble-gas configuration. The pattern gives predictable charges: group 1 → 1+, group 2 → 2+, group 13 → 3+ (typically), group 15 → 3−, group 16 → 2−, group 17 → 1−. Transition metals are less predictable (Fe²⁺/Fe³⁺, Cu⁺/Cu²⁺), which is why their names carry Roman numerals in the next topic. Finer trends (radius, ionization energy, electronegativity) come in Chapter 6, but the charge pattern alone already lets you write many formulas correctly.
How It Works / Step-by-Step Process
- Locate the element by name, symbol, or period + group coordinates.
- Classify it as metal, nonmetal, or metalloid by position relative to the staircase.
- Identify its family if it belongs to a named group.
- For main-group elements, read valence electrons from the group number and predict the ion charge.
- Use that charge to anticipate formulas and reactions; check predictions against known behavior of group neighbors.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| period | group | A period is a row; a group is a column. Similar chemistry repeats down groups, not across periods. |
| ordering by atomic mass | ordering by atomic number | The modern table is sorted by atomic number; mass ordering misplaced some elements (Te/I). |
| hydrogen as an alkali metal | hydrogen as a nonmetal | H sits in group 1 but is a nonmetal — one electron, but no metallic behavior. |
| metalloid | metal or nonmetal | Metalloids are in between and are semiconductors (Si, Ge). |
| alkali metals | alkaline earth metals | Group 1 vs group 2: 1+ vs 2+ ions, and different reactivity. |
| "whole group behaves identically" | trends within a group | Reactivity changes smoothly down a group — alkali metals get more reactive downward. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
The periodic table is a giant chart where every known element has its own box, arranged so elements that act alike stand in the same column — like sorting a huge toy collection by type. If you know a box's row and column, you can guess a lot about the element before you ever see it: whether it is a metal, how it will react, even what charge its ion will have. Mendeleev used empty boxes to predict elements nobody had found yet, and he was right.
Worked example
Example 1: Locating and classifying an element
An element sits in period 3, group 17. Identify it, classify it, and predict the charge of its ion.
Step 1 — locate: period 3 is the third row; group 17 is the halogen column, so the element is chlorine (Cl). Step 2 — classify: it lies right of the staircase, so it is a nonmetal. Step 3 — predict the ion: a group 17 nonmetal gains one electron to reach a noble-gas configuration, forming Cl⁻.
Example 2: Predicting a formula from group positions
An element in period 2, group 16 combines with an element in period 3, group 1. Identify the elements and write the compound's formula.
Group 16, period 2 is oxygen (O); group 1, period 3 is sodium (Na). Oxygen needs two electrons, forming O²⁻; sodium has one valence electron, forming Na⁺. Two Na⁺ ions balance one O²⁻, so the formula is Na₂O — charge balance in action, developed fully in the next topic.
Example 3: Mendeleev's prediction in action
In 1869 Mendeleev left a gap below aluminum and named the missing element eka-aluminum ("one below aluminum"), predicting its properties — atomic mass near 68 and density near 5.9 g/cm³ — from trends within the group. Gallium, discovered in 1875, has atomic mass 69.7 and density 5.91 g/cm³. The close match convinced chemists the arrangement was a law of nature.
Key takeaways
- 7 periods, 18 groups; for main-group elements, group number = valence-electron count (He excepted).
- Periodic law: properties repeat at regular intervals when elements are arranged by increasing atomic number.
- Metals left/center, nonmetals upper right, metalloids along the staircase (B, Si, Ge, As, Sb, Te).
- Family names: alkali metals (1), alkaline earth metals (2), transition metals (3–12), halogens (17), noble gases (18).
- Main-group ion charges: 1+ / 2+ / 3+ on the left; 3− / 2− / 1− on the right.
- Hydrogen is a nonmetal despite sitting in group 1; only Br and Hg are liquid at room temperature.
- Safety: alkali metals react violently with water and halogens are toxic — never taste or touch chemicals; follow your instructor's lab safety rules.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
How many periods and groups does the periodic table have?
Show answer
Seven periods and eighteen groups.
Why is the modern table ordered by atomic number rather than atomic mass?
Show answer
Moseley showed atomic number (nuclear charge) determines element identity and periodicity; mass ordering placed similar elements in the wrong columns (e.g., Te and I).
Name the six common metalloids along the staircase.
Show answer
Boron, silicon, germanium, arsenic, antimony, tellurium (B, Si, Ge, As, Sb, Te).
An element is in period 4, group 2. Name it, classify it, and predict its ion.
Show answer
Calcium (Ca); a metal (alkaline earth); it forms Ca²⁺.
Which two elements are liquids at room temperature?
Show answer
Bromine (Br) and mercury (Hg).
What did Mendeleev's blank spaces represent, and what happened when the missing elements were found?
Show answer
Placeholders for undiscovered elements; when gallium, scandium, and germanium were found, their properties closely matched Mendeleev's predictions.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- periodic law
- Properties repeat at regular intervals when elements are arranged by increasing atomic number.
- period / group
- A row (period) or column (group) of the table.
- metal / nonmetal / metalloid
- Shiny conductors (metals), dull insulators (nonmetals), or in-between elements (metalloids).
- alkali metal
- Group 1 element (Li–Fr).
- halogen
- Group 17 element (F–Ts).
- noble gas
- Group 18 element with a full valence shell.
- valence electron
- An electron in the outermost shell that participates in bonding.
Sources & references
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