Period — Science definition

Defined in 2 courses:

A horizontal row ordered by atomic number; in the introductory shell model, the period relates to the highest occupied principal shell. (Physical Science: Matter, Forces & Energy)
A horizontal row of the periodic table; its elements have their outermost electrons in the same highest energy level (same n). (Physical Science: Chemistry)

Where it is taught

The Periodic Table — Properties of Matter, Physical Science: Matter, Forces & Energy, Grade 8
The Periodic Table — Atoms, Electrons & Periodicity, Physical Science: Chemistry, Grade 11

In use

Each horizontal row is a period, and each vertical column is a group (also called a family).
The Periodic Table

Outer n=3 gives period 3; five valence electrons give main-group 15; this is phosphorus.
The Periodic Table

Questions using period

Problem 1

As you move left to right across a period, atomic size generally…

increases
decreases
stays the same
becomes zero

Answer and explanation

decreases

Atomic size generally decreases across a period because added protons pull the electron cloud inward.

Problem 2

Supplied records describe element A with atomic number 11 and outer-shell electron count 1, and B with atomic number 12 and outer-shell count 2. Both are main-group elements in period 3. Place the records in order by atomic number and explain which properties their position can suggest. State a limit of a shell drawing and of group rules.

Answer and explanation

A precedes B because atomic number counts protons and 11 is less than 12. In these main-group examples, the outer electrons help explain characteristic bonding patterns. Period and group organize patterns, not a guarantee that all neighboring elements behave identically. Shell circles are a simplified electron-energy representation; transition elements and helium need qualifications to simple valence rules.

Response criteria

Use the supplied conditions and relevant evidence.
Explain the scientific mechanism or calculation.
State a meaningful limitation or testable next step.

Problem 3

Moving left to right across a period, the atomic number

decreases
stays the same
increases by one each time
doubles

Answer and explanation

increases by one each time

The table is ordered by increasing atomic number.

Problem 4

Explain the difference between a period and a group, including what each one tells you about an atom's electrons.

Answer and explanation

A period is a horizontal row; every element in it has its outermost electrons in the same highest energy level (same n). A group is a vertical column; its elements all have the same number of valence electrons, which is why they share similar chemistry.

Taught alongside

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