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# Protons

## Defining characteristics of positively charged sub atomic particles and their role in atomic structure

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What's in a Number?

### What's in a Number?

License: CC BY-NC 3.0

The “open” sign on the left is filled with neon gas. The explosion on the right occurred when sodium was added to water. Neon and sodium have atomic numbers that are just one digit apart. But the two elements could hardly be more different in their properties. Why are they so different?

#### The Back Story

• It all begins with their atomic numbers. The atomic number of an atom is the number of protons in its nucleus. Neon has an atomic number of 10, and sodium has an atomic number of 11.
• Review atomic number by watching this short video:

#### Explore More

At the links below, learn more about atomic number, valence electrons, and the elements neon and sodium. Then answer the questions that follow.

1. True or false: Atomic number is always the same for all atoms of a given element.
2. True or false: Atoms of different elements may have the same or different atomic numbers.
3. How is atomic number related to mass number?
4. What determines the number of electrons an atom normally has? Why?
5. What are valence electrons? Why do they determine how an element behaves in chemical reactions?
6. Contrast the reactivity of neon and sodium. Explain the difference in reactivity. In your explanation, include these concepts: atomic number, protons, electrons, and valence electrons.
7. When sodium reacts with chlorine, it forms the compound sodium chloride, or table salt. Chlorine has 7 valence electrons. Why is it a “perfect match” for sodium in chemical reactions?

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