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# Mass Number

## Numerical relationship between protons and neutrons.

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Part of the Family

### Part of the Family

Credit: Selena N. B. H.
Source: http://www.flickr.com/photos/moonlightbulb/6425796905/
License: CC BY-NC 3.0

Thanksgiving dinner with the family can be an enjoyable occasion. Sitting down at the table with siblings, parents, and grandparents provides an opportunity to catch up on each other’s lives, to renew those family ties, and to see how everyone has changed.

#### Why It Matters

• A family shares many things, among them a common name. Whether it is Smith or Jones (or something much longer and more complicated), the family name ties the family together as a single entity. Each family shares a unique history and set of behaviors. The family name could be considered the “atomic number” for that group, providing a reference for that common identity.
• Conversation around the dinner table at Thanksgiving often deals with the properties of the individual members. One granddaughter is quite tall while her sister is somewhat shorter. Uncle Harry could stand to lose several pounds and should skip the dessert (but you know he won’t). Grandma has always been slim and has still kept her youthful figure. The individual members of the family can be distinguished by their size and weight.

Credit: Jason D'Great
Source: http://www.flickr.com/photos/jason_weemin/758092116/
License: CC BY-NC 3.0

Family members may share the same name and many of the same genes, but each member is unique [Figure2]

• Each element has a unique name and a unique nuclear structure in terms of protons. All atoms of an element have the same number of protons. The mass number of the atoms might be different because of the differences in the number of neutrons in each individual atom. The number of neutrons does not alter the identity of the atom, but does serve to give an idea of its size.
• Watch a video about atomic numbers and mas numbers at the link below:

#### Show What You Know

Use the links below to learn more about atomic numbers and mass numbers. Then answer the following questions.

1. What was Henry Moseley studying?
2. What did he discover?
3. What is the IUPAC rule about naming elements with atomic numbers greater than 100 that indicates the atomic number?
4. Why was the atomic mass of oxygen chosen to be sixteen?
5. When were other isotopes of oxygen discovered?

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