Nucleus: The Center of an Atom
The nucleus is located at the center of an atom. It is very small and has a positive electrical charge due to the protons. The nucleus contains particles called protons and neutrons, which make up nearly all of the atom's mass.
Electron Cloud: The External Area of an Atom
The electron cloud surrounds the nucleus and is longer than the nucleus. It has an equal but negative electrical charge compared to the protons. The electron cloud contains particles called electrons, and their mass is negligible compared to the protons and neutrons.
Historical Development of Atomic Structure
In the 5th century, ancient Greeks proposed the concept of the atom. In 1808, Dalton discovered the existence of protons and electrons. In 1904, Thomson further studied protons and electrons. In 1911, Rutherford proposed the structure of the atom. In 1913, Bohr introduced the concept of atomic orbits.
Atomic Number, Mass Number, and Valence
The atomic number represents the number of protons in an atom. The mass number is an approximation of the atom's mass. Valence refers to the exact number of electrons an atom has, which determines its ability to combine with other atoms by capturing, losing, or sharing electrons.
Classification of Elements
The beige area represents metals, which tend to lose electrons. The green area represents semimetals, which can exhibit both metallic and non-metallic properties depending on their location. The brown area includes hydrogen, which can gain electrons. The blue area represents noble gases, which do not easily combine with other elements due to their low reactivity.
Chemical Bonds and Stability
Atoms tend to join together to become more stable. The majority of atoms, except noble gases, form chemical bonds with other atoms.
Physical and Chemical Changes
A physical change does not alter the chemical composition of a substance, only its shape or position. For example, when a bicycle moves, its position changes but its nature remains the same. In contrast, a chemical change involves a change in the nature of the substance and the formation of new substances. For instance, frying an egg alters its composition, color, and flavor.
A chemical reaction is a process in which one or more substances change into different substances. The initial substances are called reagents, and the substances obtained after the reaction are called products.
Conservation of Mass in Chemical Reactions
The French chemist Lavoisier discovered that in all chemical reactions, the mass of the reagents is equal to the mass of the products. For example, when carbon reacts with oxygen, carbon dioxide is produced.
Environmental Impact of Chemical Reactions
Chemical reactions can have environmental consequences. For instance, burning fossil fuels releases sulfur dioxide, which can lead to the formation of sulfuric acid in the atmosphere. This can result in acid rain and other negative effects. Additionally, the greenhouse effect caused by human activities, such as the emission of CO2 and CH4, contributes to global warming, melting ice, and rising sea levels.