Scientific Understanding

**What sections should I know before attempting to learn this section?**

**How do I find the neutrons of an element?**

Depending on the information you are given there can be many different ways to find the neutrons of an element. We are going to start off by finding the neutrons of an element using the periodic table. Other sections that may help you find he neutrons in a different way (starting with different information) including the sections labeled ISOTOPES or ATOMIC NOTATION.

If you want to find the amount of neutrons an atom of an element has using the periodic table we should first explain where you find that information in each box on the periodic table. The atomic mass is at the bottom of each individual box on the periodic table. It is usually a number with digits that go past the decimal. For the case of Helium, the atomic mass is about 4.002. For most instances of calculation you can just round the atomic mass to the nearest whole number. So 4.002 becomes 4. The word atomic mass can also be know by the words atomic weight, molar mass, or molar weight. All of them mean the same thing as far as you are concerned. Since the vast majority of the atomic mass comes from the protons and neutrons, you can just think of the atomic mass as the protons plus the neutrons. Another way to write that is:

atomic mass = protons + neutrons.

If we know that the protons each contribute 1 mass and the neutrons each contribute 1 mass, then we can come up with a way to determine the amount of neutrons in each element. We simply take the atomic mass minus the atomic number (protons) and we will get the neutrons. Another way to write that is:

neutrons = atomic mass – atomic number.

When you do this remember to round the mass to eliminate the decimal. Like Iron has a mass of 55.85 on the periodic table but you need to round that to 56 when you want to calculate the number of neutrons.

**Examples:** How many neutrons do each of these elements have according to the periodic table? These are answered with information from this periodic table.** VIDEO Determining Neutrons from periodic table Examples 1.**

Sulfur |
16 |

Bromine |
45 |

Barium |
81 |

**VIDEO Determining Neutrons from the Periodic Table Demonstrated Example 1****:** How many neutrons does sodium have according to the periodic table? These are answered with information from this periodic table.

What is the symbol for sodium?

Answer: Na

What is the mass for sodium?

Answer: 23 (rounded to the nearest whole number)

What is the atomic number (proton number)?

Answer: 11

What is the formula to solve for neutrons?

Answer:

neutrons = mass – protons

? = 23 – 11

What is the complete answer?

COMPLETE ANSWER: sodium has 12 neutrons according to the periodic table.

**PRACTICE PROBLEMS**: How many neutrons do each of these elements have according to the periodic table? Make sure you have this periodic table link open when answering these questions.

Oxygen |
8 |

Chromium |
32 |

Potassium |
20 |

Mercury |
121 |

Chlorine |
18 |

Manganese |
31 |