An atom is the smallest unit of matter that retains the properties of an element. Atoms are incredibly tiny, measuring about 0.1 nanometers across. Everything we see and touch is made up of atoms. These tiny building blocks can combine in different ways to form molecules like water, or arrange in patterns to create crystals like salt. To understand their size, if an atom were the size of a marble, a marble would be the size of Earth.
Atoms are made of three fundamental particles. Protons carry a positive electric charge and are located in the atom's center. Neutrons have no electric charge and are also in the center. Electrons carry a negative charge and move around the outside. Protons and neutrons have nearly identical masses, both about 1836 times heavier than electrons. Despite their tiny mass, electrons play a crucial role in chemical bonding and determining an atom's size.
The atom has a specific structure. At its center is the nucleus, containing protons and neutrons packed tightly together. The nucleus is incredibly dense and contains nearly all of the atom's mass. Surrounding the nucleus are electron shells, where electrons orbit at different energy levels. The first shell is closest to the nucleus, followed by the second shell, and so on. Most of an atom is actually empty space. To put this in perspective, if the nucleus were the size of a marble, the entire atom would be as large as a football stadium.
The atomic number of an element equals the number of protons in its nucleus. This number determines what element it is. Hydrogen has 1 proton, helium has 2, carbon has 6, and oxygen has 8. If you add or remove protons, you completely change the element. However, you can change the number of neutrons without changing the element - these are called isotopes. For example, carbon-12 has 6 neutrons while carbon-14 has 8 neutrons, but both are still carbon because they have 6 protons. In neutral atoms, the number of electrons equals the number of protons.
Atoms bond together by sharing or transferring electrons to form compounds and molecules. In ionic bonding, one atom transfers electrons to another, creating charged ions that attract each other. For example, sodium gives an electron to chlorine, forming sodium chloride or table salt. In covalent bonding, atoms share electrons to form molecules. Two hydrogen atoms share their electrons to form hydrogen gas. The octet rule explains why atoms bond - they want eight electrons in their outer shell to be stable, except hydrogen which only needs two electrons.