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## Atomic Number and Energy Levels The number of protons in an atom is known as the **atomic number**. Hydrogen, for example, has an atomic number of 1, meaning it has 1 proton and 1 electron. Electrons orbit the nucleus at a high speed, forming a spherical electron cloud. These electrons occupy...

## Atomic Number and Energy Levels The number of protons in an atom is known as the **atomic number**. Hydrogen, for example, has an atomic number of 1, meaning it has 1 proton and 1 electron. Electrons orbit the nucleus at a high speed, forming a spherical electron cloud. These electrons occupy a circular **electron shell**. Atoms contain equal numbers of protons and electrons and are therefore **electrically neutral**. For example, if an atom has 7 negative electrons, it would also have 7 positive protons. Each increase in atomic number is accompanied by a similar increase in the number of electrons orbiting the nucleus. These orbiting electrons exist within an **electron cloud**, organized into specific **energy levels**. **The first electron shell (the one closest to the nucleus) corresponds to the lowest energy level.** Each energy level has a maximum number of electrons it can hold: * The first energy level holds a maximum of **2** electrons * The second and third energy levels can hold a maximum of **8** electrons Electrons occupy these shells in a specific order. The first level fills completely before any electrons enter the second, and the second fills before any electrons enter the third. The **outermost electron shell** determines an atom's **chemical properties**. **Unstable Atoms** Atoms with **unfilled outermost energy levels** are **unstable** and tend to react with other atoms, usually in ways that give them full outer electron shells to become stable. **Stable Atoms** In contrast, atoms with **filled outermost energy levels** are stable and do not easily react with other atoms. Let us consider the following atoms: * **Hydrogen** * **Helium** * **Lithium** * **Neon**