Learning focus
Develop precise chemical language, connect equations and practical observations, interpret periodic trends and apply salt-preparation rules to unfamiliar examples.
Increasing proton number
The Periodic Table is arranged in order of increasing proton number, also called atomic number. Proton number uniquely identifies the element and normally increases by one from one element to the next.
Periods
Periods are horizontal rows. For the first 20 elements, period number equals the number of occupied electron shells. Properties change across a period as outer-shell electron number increases.
Groups
Groups are vertical columns. Elements in the same group have the same number of outer-shell electrons for Groups I-VII and therefore show similar chemical properties. Group VIII elements have full outer shells.
Periodic repetition
Chemical properties repeat in a pattern because outer-shell electronic structures repeat. The table is therefore periodic rather than merely a list of elements.
Practical or data skill
Use proton numbers and electronic configurations to place unknown elements in simplified Periodic Table positions.
Examination tip
The ordering principle is increasing proton number, not relative atomic mass.
Review questions and suggested answers
Question 1
What is a period?
Suggested answer
A horizontal row.
Question 2
What is a group?
Suggested answer
A vertical column.
Question 3
What quantity orders the elements?
Suggested answer
Proton number or atomic number.
