State whether each of the following elements is a gas, a liquid, or a solid under atmospheric conditions. Also state whether it exists in the elemental form as atoms, as molecules, or as a three-dimensional network: \(\mathrm{Mg}, \mathrm{Cl}, \mathrm{Si}, \mathrm{Kr}, \mathrm{O}, \mathrm{I}, \mathrm{Hg}, \mathrm{Br}\)

Chemical elements are the building blocks of matter, and understanding what they are and how they exist in nature is fundamental in the study of chemistry. An element is a pure substance that cannot be broken down into simpler substances by chemical means. Each element is defined by the number of protons it has in its nucleus, a property referred to as the atomic number.

Under atmospheric conditions, elements can be found in different physical states: solids, liquids, or gases. This depends on their individual properties and the ambient temperature and pressure. For instance, mercury (Hg) and bromine (Br) are the only elements that are liquids at room temperature, while others like magnesium (Mg) and silicon (Si) appear as solids.

Additionally, elements exist in different forms. Some, like krypton (Kr) and mercury (Hg), exist as individual atoms. Others, such as chlorine (Cl) and oxygen (O), form molecules with two atoms, known as diatomic molecules. Silicon (Si), on the other hand, forms a three-dimensional network structure, bonding with other silicon atoms in a crystalline lattice.

The states of matter describe the physical forms that different phases of matter take on. The most familiar states are solid, liquid, and gas. Solids have a fixed shape and volume, liquids have a definite volume but take the shape of their container, and gases have neither fixed volume nor shape, expanding to fill their container.

Solids, such as magnesium (Mg) and iodine (I), maintain their shape because their molecules are tightly packed together in a fixed arrangement. Liquids, like mercury (Hg) and bromine (Br), are more fluid because their molecules are less tightly bound and can move past one another. Gases, such as oxygen (O) and krypton (Kr), are even more mobile, with molecules that are far apart and move freely.

The state of a chemical element under atmospheric conditions is influenced by its melting and boiling points. For instance, nitrogen exists as a gas because its boiling point is below room temperature, while gold exists as a solid because its melting point is much higher than room temperature.

Molecular structure refers to the arrangement of atoms within a molecule. Understanding the molecular structure is crucial because it determines an element's or compound's properties and behavior. Atoms can be bonded in various ways, forming different molecular structures that influence an element's state of matter at a given temperature and pressure.

Some elements exist as individual atoms, known as monatomic elements, such as noble gases like krypton (Kr). Others, like chlorine (Cl) and oxygen (O), form diatomic molecules, where two atoms of the element share electrons and bond together. There are also elements that can bond with themselves to form complex structures, such as the three-dimensional network of silicon (Si).

Different types of bonds—covalent, ionic, and metallic—affect the stability and reactivity of the molecular structures. For example, the covalent bonds in oxygen molecules are strong, resulting in a stable diatomic gas under normal conditions, while the metallic bonds in magnesium create a solid structure with characteristic metal properties.