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Atoms are the fundamental building blocks of matter. Each atom consists of three main types of particles: protons, neutrons, and electrons. At the center of the atom is the nucleus, which contains protons (positively charged) and neutrons (neutral). Surrounding the nucleus are electrons (negatively charged), which orbit in various energy levels or shells.
The atomic number, found on the periodic table, tells us the number of protons in an atom's nucleus. Importantly, in a neutral atom, the number of electrons equals the number of protons. This balance of charges keeps the atom stable. Understanding atomic structure is essential for further exploring how atoms interact with each other, including how they share or exchange electrons.

The periodic table is a powerful tool for understanding elements and their behaviors. It organizes all known elements in rows (periods) and columns (groups) based on their atomic number and similar properties.

• Groups: Elements in the same column have the same number of valence electrons. Group numbers can help predict how many valence electrons an element has. For example, Group 1 elements have 1 valence electron, while Group 17 elements have 7.
• Periods: Elements in the same row share the same number of electron shells. Period 1 elements have one shell, Period 2 has two, and so on.

These patterns help predict how elements will react chemically and form bonds.

The valence level of an element refers to its outermost shell in its atomic structure. The electrons in this shell are known as valence electrons and play a key role in forming chemical bonds.
Elements aim to have a full valence level, which often means having eight valence electrons, a concept known as the "octet rule." This is what drives chemical reactions, as atoms will share, gain, or lose electrons to achieve a full valence level.
Understanding valence levels helps us predict the properties and reactivities of elements. For example, strontium's valence level is the 5th energy level with 2 electrons; gallium's valence level is the 4th with 3 electrons; and iodine's is the 5th with 7 electrons. These distinctions help explain why each element behaves the way it does chemically.