Trends in a group

1. A trend is a gradual change in a property, in one overall direction, as you move from element to element down or up a group.
2. Elements in a group have the same number of outer-shell electrons, but each element has one more occupied shell than the element above it.
3. Because the change from one element to the next is always the same kind of change, properties tend to change steadily down a group rather than randomly.
4. Properties that often show a trend include melting point, boiling point, density, atomic radius and reactivity.
5. A property may increase down a group, decrease down a group, or show no clear trend.
6. A property that increases down a group must decrease up the same group, so the direction you read in always matters.
7. Questions often give you data for a group you have not studied, such as Group II or Group VI, and expect you to find the trend from that information alone.

Reading trends from data

1. Put the elements in order: list them from the top of the group to the bottom, which is the order of increasing proton number.
2. Read one property at a time: follow the values for a single property in that order.
3. Compare the ends: compare the value for the first element with the value for the last element to find the overall direction.
4. Check the middle: confirm that the values in between move in the same direction.
5. Treat each property separately: one property can increase down a group while another property decreases down the same group.
6. Use descriptions as well as numbers: words such as "reacts slowly", "reacts quickly" and "reacts violently" form an ordered scale, so they show a trend in reactivity just as numbers do.
7. Read charts in the same way: in a bar chart, the elements are usually placed along the horizontal axis in group order, so you read the heights of the bars from left to right.

Writing a complete trend statement

1. A complete statement names the property, the direction of change and the direction in the group.
2. "The melting point increases down the group" is complete, because it gives all three parts.
3. "It increases" is incomplete, because it does not say which property changes or in which direction you are reading.
4. Use down the group when reading from top to bottom and up the group when reading from bottom to top.
5. When a question asks you to use the data, quote values with their units as evidence, such as "from $115{}^{\circ }C$ for sulfur to $450{}^{\circ }C$ for tellurium".
6. Comparative words such as higher, lower, denser and more reactive are the clearest way to compare two elements in the same group.
7. A change along a period is not a group trend, so only describe the elements in the column you have been given.

Anomalies and unclear patterns

1. An anomalous value is a value that does not fit the pattern shown by the rest of the data.
2. If one value breaks an otherwise steady trend, describe the overall trend and name the element whose value does not fit.
3. Do not ignore an anomalous value, and do not let one odd value hide the trend shown by all the others.
4. If the values rise and fall with no overall direction, state that the property shows no clear trend.
5. A trend does not need equal steps between neighbouring elements, so values of $5$, $9$, $20$ and $22$ still show a steady increase.

Estimating a missing value

1. When a trend is steady, a missing value lies between the values of the elements directly above and below it.
2. For an element at the bottom of the group, continue the pattern in the same direction.
3. Keep the size of an estimated step similar to the steps already shown in the data.
4. An estimate does not need to match the real value exactly, as long as it fits the trend shown by the data.