A scientist placed element P before element Q in an early periodic table and left a gap for element E.

Later, the atomic numbers of P and Q were discovered and element E was found.

Decision	Information available for the early table	Later evidence
Order of P and Q	Atomic weight of P = 39.95; atomic weight of Q = 39.10	Atomic number of P = 18; atomic number of Q = 19
Gap for E	Predicted relative atomic mass = 68.0; predicted density = 6.0 g/cm³	Measured relative atomic mass = 69.7; measured density = 5.9 g/cm³

Use the table to explain why P should come before Q, even though P has the greater atomic weight.

Explain how the later evidence about element E supports the scientist's decision to leave a gap.

A scientist placed element P before element Q in an early periodic table and left a gap for element E.

Later, the atomic numbers of P and Q were discovered and element E was found.

Decision	Information available for the early table	Later evidence
Order of P and Q	Atomic weight of P = 39.95; atomic weight of Q = 39.10	Atomic number of P = 18; atomic number of Q = 19
Gap for E	Predicted relative atomic mass = 68.0; predicted density = 6.0 g/cm³	Measured relative atomic mass = 69.7; measured density = 5.9 g/cm³

Use the table to explain why P should come before Q, even though P has the greater atomic weight.

Explain how the later evidence about element E supports the scientist's decision to leave a gap.

1.2 The periodic table

1.2 The periodic table Questions