Video Transcript
A student is learning about periodicity trends of period three elements. She draws a sketch graph to show a trend of a property across the period but forgets to label the 𝑦-axis. What label best fits the 𝑦-axis? (A) Boiling point, (B) atomic radii, (C) electrical conductivity, (D) melting point, or (E) ionic radii.
The question asks about periodicity trends. Periodicity trends refers to trends or patterns or recurring variations in the properties of elements with increasing atomic number. The student is looking specifically at the trends in the period three elements. And these are listed on the 𝑥-axis of the graph in order of increasing atomic number from left to right. The 𝑦-axis label was mistakenly left off. Nevertheless, we can deduce the label for this axis by examining what is happening to the curve in the graph.
For sodium with lowest atomic number of this series or period, a property is plotted here. As we move to magnesium with a higher atomic number than sodium, we notice that the 𝑦-axis value is slightly higher and even higher for aluminum, which happens to have the highest value for this whole period. Notice that sodium, magnesium, and aluminum, which are all metals, have high 𝑦-axis values relative to the other elements. Silicon, a metalloid, has a lower value than the three metals. And the nonmetals phosphorus, sulfur, chlorine, and argon all have the same 𝑦-axis value, which is close to zero.
Which properties have high values for metals, medium or intermediate values for metalloids, and relatively low, almost zero, value for nonmetals? The answer must be electrical conductivity. Metals in general conduct electricity very well and so have high electrical conductivity values. Metalloids are semiconductors and so have intermediate values. And in general, nonmetals are nonconductors and so have very low, approximating zero, value. So, (C) electrical conductivity must be our answer, but let’s check this by ruling out the other answer options.
We can rule out (A) boiling point and (D) melting point since it cannot be true that phosphorus, sulfur, chlorine, and argon have exactly the same boiling point or exactly the same melting point. We can also rule out ionic radii immediately since argon is a noble gas, and noble gases do not ionize or form ions under normal conditions. And so we cannot plot a data point for ionic radius for a noble gas. Only under very rare and extreme conditions would a noble gas form an ion. We can also rule out (B) atomic radii. This is because as we move from left to right across period three and as the atomic number increases, so the atomic radius decreases. The graph would look something like this. With increasing atomic number, so there is an increase in nuclear charge, causing the atom size or radius to decrease.
Finally, what label best fits the 𝑦-axis of this graph? The answer is (C) electrical conductivity.
