Chemical Bonds and Atomic Structure: Elements and Compounds
Matter: Mixtures, Pure Substances, Elements, and Compounds
Matter, when used as a composition, can be found in the form of mixtures or pure substances. A pure compound cannot be decomposed into simpler substances by physical changes. Compounds can be decomposed into other pure substances, called elements. Elements are simple substances and are not broken down into others. The units of matter are atoms. Elements are formed by the same type of atoms, and compounds are formed by combining atoms of different elements.
- Elements: Oxygen, calcium, hydrogen
- Compounds: Water, hydrochloric acid, sodium chloride
- Mixtures: Air, gas, seawater
Atomic Structure: Nucleus and Electron Shell
An atom is the smallest unit of a chemical element that maintains its properties and cannot be divided by chemical processes. The structure of an atom consists of two parts:
- The nucleus is the center of the atom and is formed by two types of particles: protons (p+), with a positive electric charge, and neutrons (n), which have no charge.
- The electron shell is around the nucleus and is formed by electrons (e-), with a negative electric charge.
The charge of the electron and proton is called the elementary charge.
Atomic Number, Mass Number, and Isotopes
The atomic number of an element indicates the number of protons in the atom’s nucleus. Z represents the number of protons. The mass number of an atom is the sum of protons and neutrons that make up the nucleus. A represents the number of protons + neutrons in the nucleus (Z + n). Isotopes are atoms that have the same atomic number but different mass numbers.
Electron Configuration and Valence Shell
The electrons in an atom are ordered into levels and sublevels, corresponding to the electron configuration. The last level or layer is called the valence shell. Spectra demonstrate evidence of the electron shell.
- The emission spectrum is when a protective plate is recorded in stripes of different colors.
- The discrete spectrum contains four lines.
Evolution of the Periodic Table
Johann Döbereiner’s Law of Triads noted the relationship between the atomic mass of elements and their properties. John Newlands’ Law of Octaves sorted the known elements according to the order of their atomic mass. Mendeleev’s Table: Dmitri Mendeleev published the periodic table with 63 known elements according to their atomic mass. In 1913, Henry Moseley identified the atomic number of each element, reorganized the table, placing 89 elements by increasing atomic number, and established the periodic law of Moseley.
Structure of the Periodic Table
The periodic table has 111 elements and is distributed into 7 periods and 18 rows or columns, also known as groups. The atomic radius is useful for calculating the volume of atoms. A cation is formed when an atom loses an electron, and an anion is formed when it gains an electron.
Classification of Elements
Elements are divided into metals and non-metals. Noble gases are formed by gases. Halogens are formed by non-metallic elements. Alkali metals have one electron in the valence shell. Transition metals are located in the center of the table.
Chemical Bonds: Ionic, Covalent, and Metallic
A chemical bond is the force that holds atoms together. The bond distance is when all atoms are located at a specific distance. Elements are formed by isolated atoms, molecules, and crystalline networks. Compounds are composed of molecules and crystalline networks.
- Ionic bond: The force of attraction between metallic and non-metallic atoms.
- Covalent bond: Occurs between two non-metallic atoms.
- Polar covalent bond: When atoms attract with different intensities.
- Metallic bond: The force that holds atoms together, forming a crystal structure built of cations and a cloud of electrons.