The periodic table is an arrangement of elements according to their?

Step 1: Analyze the question The question asks what property is used to arrange elements in the modern periodic table.

Step 2: Explain the concept

• Mendeleev's early periodic table (1869) arranged elements by increasing atomic mass. This caused some problems, for example tellurium and iodine appeared in the wrong order.
• In 1913, Henry Moseley showed that elements are better arranged by atomic number, which is the number of protons in the nucleus.
• The Modern Periodic Law states: the properties of elements are a periodic function of their atomic numbers.

Step 3: Check each option

• A. Oxidation number: This varies with the compound an element is in, so it can't be used to order elements. Wrong.
• B. Atomic number: Elements are arranged in order of increasing atomic number (number of protons). Correct.
• C. Mass number: This is protons plus neutrons and differs between isotopes of the same element, so it is not used. Wrong.
• D. Number of neutrons: Neutron number varies between isotopes of an element. Wrong.

Step 4: Answer The periodic table is arranged according to atomic number, which is option B.

Exam tip: Remember "Moseley = More protons": the modern table goes by proton count (atomic number), while Mendeleev's older version used atomic mass. WAEC often tests this difference, so keep it in mind.