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Atoms are the basic building blocks of matter. They consist of three main parts: protons, neutrons, and electrons.
Protons and neutrons form the atomic nucleus, which is at the center of the atom. Protons have a positive charge, and neutrons carry no charge. Around the nucleus is a cloud of electrons. These electrons have a negative charge.

Electrons move in specific regions called orbitals or electron shells. Each shell can hold a certain number of electrons. For example, the first shell can hold up to 2 electrons, while the second can hold up to 8.

• Electrons are arranged in shells around the nucleus.
• Each shell can hold a maximum number of electrons.
• Electrons in the outermost shell determine the chemical properties of the element.

Understanding the atomic structure is crucial to grasp how elements interact and how they form compounds. Atomic structure forms the foundation for the study of Electron Configuration.

The periodic table is a comprehensive chart organizing all known elements by increasing atomic number. It is a valuable tool for chemists because it displays patterns that help predict the behavior of elements.
The table is arranged in rows called periods and columns called groups. Each element has a unique position based on its atomic number, which is the number of protons in its nucleus.

Elements in the same group have similar properties because they have the same number of electrons in their outermost electron shell. For instance, Carbon and Silicon are in the same group because they both have four valence electrons.

• Elements are organized by increasing atomic number.
• Groups contain elements with similar chemical properties.
• The position on the table is linked to the electron configuration.

The periodic table not only provides information about the elemental properties but also helps in predicting how different elements will react with each other.

Electron shells are layered zones around an atom's nucleus where electrons are likely to be found. Each shell corresponds to a principal quantum number (n), and each can hold a different number of electrons.
For example, the maximum number of electrons in a shell is determined by the formula: \[ 2n^2 \]where \( n \) is the shell number. The first shell (n=1) can hold up to 2 electrons, the second (n=2) holds up to 8, the third (n=3) can hold up to 18, and so forth.

Electrons fill orbitals starting with the lowest energy level, which forms the basis of the electron configuration of an atom. This was observed in the given solution:

• Sodium's first shell is filled with 2 electrons.
• The second shell fills with 8 electrons.
• The remaining single electron goes into the third shell.

This pattern of filling is critical to understanding the chemical bonding and reactivity of elements.