**Elements with a Single Electron in their Outermost Shell**

In the periodic table, elements are arranged based on their atomic number, which corresponds to the number of protons in the nucleus of an atom. The arrangement also reveals the electronic configuration of elements, which describes the distribution of electrons in different energy levels or shells.

There are three elements that have a single electron in their outermost shell, also known as the valence shell. These elements are hydrogen (H), lithium (Li), and sodium (Na).

**1. Hydrogen (H):**
Hydrogen, the lightest and simplest element, has an atomic number of 1. Its electronic configuration is 1s1, indicating that it has one electron in its only shell, which is the first energy level. The valence electron of hydrogen occupies the 1s orbital.

**2. Lithium (Li):**
Lithium, with an atomic number of 3, has an electronic configuration of 1s2 2s1. This means that it has two electrons in the first energy level (1s orbital) and one electron in the second energy level (2s orbital). The valence electron of lithium is located in the 2s orbital.

**3. Sodium (Na):**
Sodium, having an atomic number of 11, has an electronic configuration of 1s2 2s2 2p6 3s1. This signifies that it has two electrons in the first energy level, eight electrons in the second energy level, and one electron in the third energy level. The valence electron of sodium is present in the 3s orbital.

**Significance of the Single Valence Electron:**
The presence of a single electron in the outermost shell of these elements makes them highly reactive. This is because atoms tend to gain, lose, or share electrons in order to achieve a stable electron configuration, typically with a full outer shell.

Hydrogen, with one electron in its valence shell, can either gain one electron to achieve a helium-like configuration or lose its single electron to become a proton. Lithium and sodium, with one valence electron each, readily lose their outer electrons to form positive ions with a stable electron configuration. This behavior is due to the tendency of elements to achieve a noble gas configuration, which is highly stable.

Overall, elements with a single electron in their outermost shell exhibit distinct chemical properties and reactivity due to the presence and behavior of their valence electrons.