Reading Atomic Masses from the Periodic Table

Introduction

You do not need to memorise the relative atomic mass of every element, because the periodic table gives them to you. But each box on the table holds several numbers, and mixing them up is one of the most common mistakes in chemistry. This page shows how to read a periodic table box, how to pick out the relative atomic mass, and which rounded values are worth knowing by heart.

Core explanation

What is in a box? A typical periodic table box shows the element's symbol , its name , and two numbers. One is a whole number, the atomic number , which counts up by one across the table: H 1, He 2, Li 3 and so on. The other, usually larger and often with decimals, is the relative atomic mass , Ar.

How to tell which is which. Different tables put the numbers in different places, so always check the key printed on the table. If there is no key, use two clues: the relative atomic mass is (except for hydrogen) the larger number, and it often has a decimal point . For carbon you might see 6 and 12.011; the 12.011 is Ar.

Common values. Many school tables round Ar values. These are worth knowing:

Element Symbol Ar (rounded) --- --- --- Hydrogen H 1 Carbon C 12 Nitrogen N 14 Oxygen O 16 Sodium Na 23 Magnesium Mg 24 Aluminium Al 27 Sulfur S 32 Chlorine Cl 35.5 Potassium K 39 Calcium Ca 40 Iron Fe 56 Copper Cu 63.5

Use the values you are given. Exams provide a data sheet. If it says Cl = 35.5 and Cu = 63.5, use those, not a more precise value you have seen elsewhere. Rounding differently from the mark scheme can lose marks in longer calculations.

The trend. Relative atomic mass generally increases as atomic number increases, so heavier atoms appear lower down and further right. There are a few famous exceptions, such as argon (Ar 39.9) coming before potassium (Ar 39.1), which you will meet when studying the periodic table in more depth.

Step-by-step reasoning

To read Ar for an element:

1. Find the element's symbol on the table. 2. Read the key to see which number is the relative atomic mass. 3. If no key, choose the larger number, usually the one with decimals. 4. Round only as the question or data sheet instructs.

Visual explanation

Picture the box for oxygen: a large "O" in the centre, "oxygen" beneath it, a small 8 in one corner, and 16.00 in another. The 8 is the element's place in the table; the 16.00 tells you its atoms are sixteen times the atomic mass unit.

Real-world analogy

A supermarket label may show the price, the price per kilogram and a product code. All are numbers, but only one tells you what you pay. You must know where to look, just as you must know which number in a periodic table box is Ar.

Real-world example

Pharmacists and industrial chemists read relative atomic masses from reference tables every day to work out how much of each ingredient is needed. A mistake in reading the table, such as using the atomic number for sodium (11) instead of its Ar (23), would give a mass less than half the correct value.

Why?

Why are some Ar values printed to one decimal place, like chlorine 35.5, while others are rounded to whole numbers? For most light elements the natural average is very close to a whole number, so rounding causes little error. Chlorine's average lies almost exactly halfway between two whole numbers, so rounding it to 35 or 36 would introduce a large error.

Common misconception

"The atomic number is the mass of the atom." The atomic number is a whole-number position in the table; it is roughly half the Ar for many light elements. Oxygen has atomic number 8 but Ar 16.

Worked example

Question: A periodic table box for an element shows the numbers 12 and 24.3 with no key. Which is the relative atomic mass, and which element is it?

Reasoning: The larger number with a decimal, 24.3, is Ar. The atomic number is 12. Element 12 with Ar 24.3 is magnesium.

Answer: Ar = 24.3; magnesium.

Quick check

1. What is the rounded relative atomic mass of nitrogen? Answer: 14.

Exam focus

Use only the Ar values from the data sheet supplied. Check the key before reading numbers. A frequent examiner comment is that candidates used atomic numbers instead of relative atomic masses; a quick check that Ar is the larger number avoids this.

Advanced insight

Since 2009 IUPAC has given some elements, such as hydrogen, carbon and chlorine, an interval rather than a single value, because their isotope mix varies slightly depending on where the sample comes from. For school work a single conventional value, such as Cl 35.45 or 35.5, is used.

Summary

Each periodic table box shows a symbol, a name, the atomic number and the relative atomic mass. Ar is usually the larger number, often with decimals; check the key. Useful rounded values include H 1, C 12, N 14, O 16, Na 23, Cl 35.5 and Ca 40. Always use the values provided in the question or data sheet.

Practice questions

1. Give the rounded relative atomic masses of hydrogen, carbon and oxygen. Answer: H = 1, C = 12, O = 16. 2. A box shows the numbers 20 and 40.1. Which number is Ar, and which element is this? Answer: 40.1 is Ar, and the element with atomic number 20 is calcium. 3. A student uses 11 for the mass of sodium in a calculation. What mistake have they made? Answer: They used the atomic number of sodium instead of its relative atomic mass, which is 23. 4. Why is chlorine usually given as 35.5 rather than a whole number? Answer: Its natural average atomic mass lies about halfway between 35 and 36, so rounding to a whole number would introduce a large error.