Atoms consist of a nucleus surrounded by electrons that occupy specific regions known as electron orbitals. These orbitals can be visualized in two dimensions as energy shells. Each energy shell represents a layer where electrons can be found, with the innermost shell being closest to the nucleus and having the lowest energy. As you move outward, the energy of the shells increases, making the outer shells more reactive. This is particularly important when considering chemical bonds, as the electrons in the outermost shell, known as valence electrons, play a crucial role in an atom's reactivity.

For example, in a carbon atom, there are two energy shells. The first shell can hold a maximum of 2 electrons, while the second shell can accommodate up to 8 electrons. In carbon, the outermost shell contains 4 valence electrons, which are higher in energy and more reactive compared to the electrons in the inner shell. Understanding the distribution of electrons across these shells is essential for predicting how atoms will interact with one another.

Each element has a unique atomic structure characterized by its atomic number, which indicates the number of protons in the nucleus, and its mass number, which is the total number of protons and neutrons. For instance, hydrogen has an atomic number of 1, meaning it has 1 proton and 1 electron, filling its first energy shell. In contrast, nitrogen has 7 protons and 7 electrons, filling its first shell with 2 electrons and placing the remaining 5 in the second shell.

As you explore other elements, such as oxygen and phosphorus, you will notice that the first shell always fills with 2 electrons, while the second shell can hold up to 8 electrons. This pattern continues with additional shells, which can hold varying numbers of electrons. The understanding of these energy shells and the concept of valence electrons is fundamental in chemistry, particularly when studying how atoms bond to form molecules.