Chemical Elements, Bonds, and Structures: A Comprehensive Study
Chemical Elements and the Periodic System
Chemical elements are pure substances that cannot be decomposed into simpler substances. There are about a hundred chemical elements. Of these elements, an impressive variety of compounds is created, such as minerals, biochemical substances that produce chemical products, and technology. All known compounds are composed of atoms of elements combined together. To understand how to combine elements, it is essential to know their properties.
The Periodic System
The modern periodic system contains 115 elements in 18 columns called groups or families, grouped in ascending order of atomic number (Z), starting with hydrogen, Z = 1. The atomic number (Z) determines the properties of the elements. Therefore, these properties change regularly in the periodic system. Elements of the same group have a similar electronic structure in their last layer. At the same time, the system has a recurring period or a horizontal line of 7. Elements of the same period have the same number of layers in their electron distribution.
Recurring Properties of Elements
The properties of a chemical element are defined by the electrons in the last layer of its atoms. The tendency to gain or lose electrons depends on the position of the element in the periodic system. The tendency to gain or lose electrons increases as the atomic number increases within a group and as the atomic number increases across a period. The tendency of one atom to take over electrons increases across a group and to the right across a period. The metallic nature of elements is another property that changes periodically or regularly. Metals have a tendency to lose electrons.
Atomic Structure
According to the nuclear model, the atom consists of a nucleus and a surface. In the nucleus, neutrons and protons, which carry a positive electric charge, are concentrated, and this is where most of the atom’s mass resides. On the surface, electrons spin around the nucleus, distributed in layers or circular orbits. Electrons have a negative charge.
Lewis Diagrams of Atoms and Ions
Lewis diagrams of neutral atoms or ions represent the electrons in the last layer in a simple way. Individual electrons are indicated by points, while electron pairs are represented by a dash or two points. This last layer is involved in the formation of bonds. When atoms combine with another atom, they tend to acquire the structure of the nearest noble gas, which is said to make up an octet. The hydride ion is an exception, as it adopts the structure of helium, with two electrons in its last layer (H:). Elements of the same family have similar Lewis structures.
Chemical Bonds
The combination between atoms is made through the external layer of electrons, the valence electrons. Atoms are grouped together to achieve maximum stability and, therefore, to obtain a structure of minimum energy. Noble gases have a very stable electronic configuration, with eight electrons in their last layer. Atoms of other elements, when joined, tend to adopt the structure of noble gases, which is called the octet rule.
Types of Bonds
Partnerships between different types of atoms can be:
- Ionic Bond: A valence electron is transferred from one atom to another. These ions are attracted to each other because they have opposite charges.
- Covalent Bond: The combination involves the sharing of valence electrons.
- Metallic Bond: Valence electrons are shared among atoms.
Molecules and Crystals
Elements in the gas phase are diatomic molecules. Molecules of a compound are the smallest units that retain the properties of the compound and are composed of a limited number of atoms. Atoms are not always defined by forming groups of molecules. Sometimes they form crystals or glass-forming networks with a specific geometric structure.
Ionic Bonds
An ionic bond is formed when metal atoms transfer electrons to non-metal atoms. Therefore, this type of bond occurs between elements that are far apart in the periodic system. This bond generates compounds composed of ions with multiple electric charges. These ions are attracted to each other by means of electrical forces.
Metal/Non-Metal Ionic Bonds
In ionic compounds, stability is greater when the atoms are separated, and they achieve an octet, that is, the structure of noble gases.
Ionic Crystals
Ions with opposite charges attract each other, forming glass-like networks and ionic groups. Crystals are formed by the union between a metal ion and a non-metal ion. Ions occupy the knots of the electronic network. Ionic crystals can be of various types, forming three-dimensional networks, such as cubic and hexagonal networks. In the cesium chloride network, each cesium ion is surrounded by eight chloride ions, and vice versa. In the sodium chloride network, each sodium ion is surrounded by six chloride ions, and vice versa.
Properties of Ionic Compounds
Ionic compounds are very stable, and a lot of energy is required to completely dissolve their crystalline structure. They are solid, do not conduct electricity, have high melting and boiling points, and are hard. Ions can be separated from ionic compounds either by melting the compound or by dissolving it in water. In these cases, they can conduct electricity.
Covalent Bonds
A covalent bond is formed when two atoms share one or more pairs of electrons to achieve the structure of a noble gas. Covalent elements are in a gaseous state. Covalent bonds are created when non-metal atoms share valence electrons.
Covalent Bond Lewis Diagrams
Single, double, or triple bonds can be formed by sharing one, two, or three pairs of electrons, respectively. Lewis diagrams can be expressed by representing electrons with points or by using a dash to represent a pair of electrons in a covalent bond.
Covalent Crystals and Molecular Substances
There are two types of covalent compounds: covalent crystals and molecular substances. Molecular substances are composed of molecules, and the atoms within these molecules are linked by covalent bonds. Covalent molecular crystals are shown as solids. In molecular substances, the forces that hold atoms and molecules together are much weaker, known as van der Waals forces. Covalent crystals are huge molecules, as big as the crystal itself, and their atoms cannot be easily separated.
Properties of Covalent Compounds
Covalent bonds between atoms are strong, and molecular substances are poor conductors of electricity. However, they have low melting points due to the weak bonds between molecules. Covalent crystals are solid at room temperature. As a general rule, they are strong and have high melting points. They do not dissolve in water and hardly conduct electricity or heat.
Metallic Bonds
In metals, atoms are not defined as separate molecules.
The Electron Gas Model of the Metallic Bond
In a metal, valence electrons move freely, forming a kind of cloud of a large number of electrons. This electronic cloud surrounds all the atoms, forming what is called the electron gas model.
Metallic Crystal Lattice
Metal atoms tend to fill all the gaps, forming a much more compact crystal structure, with atoms packed more tightly.
Properties of Metals
Most metals are solid at room temperature, except for mercury, which is liquid. Metals conduct electric current because the electric charges in the electron cloud can move freely. They are ductile, easily extendable, and can be stretched. They are also malleable, meaning they can be formed into plates or sheets. This malleability is due to the ability of layers of atoms to shift without breaking. Metals have relatively high melting points and are not very soluble.
Alloys
Alloys are formed when more than one metal, or metals and other elements, are heated to high temperatures, melted, and dissolved into one another. Upon cooling, an alloy is obtained. An alloy is a solid solution organized in a metallic network formed by various elements. At least one of the elements must be a metal. Alloys have better properties than pure metals.
The Mole
The mole is the unit of the amount of substance. It is the amount of substance that contains the same number of particles as there are atoms in 12g of carbon-12, that is, 6.02 x 10²³ particles. One mole of an element, in grams, is equivalent to its atomic mass. The molar mass of any molecular compound (in grams) is the same as its molecular mass.
Molecular Mass
The molecular mass of a compound is the sum of the atomic masses of the elements that make up the compound. It is expressed in atomic mass units (u). The molecular mass of a covalent compound is obtained by calculating the mass of a molecule. For example, the molecular mass of hydrochloric acid (HCl) is the sum of the atomic mass of chlorine (35.5u) and the atomic mass of hydrogen (1u), which is 36.5u. In ionic compounds, the relative molecular mass is the sum of the atomic masses of the elements in the compound’s formula. For example, in the case of potassium iodide (KI), the molecular mass is the sum of the atomic mass of potassium (39u) and the atomic mass of iodine (127u), which is 166u.
Percentage Composition
The percentage composition of a compound is the amount (percentage) of each element per 100g of the compound.
Molar Volume of a Gas
The molar volume of any gas at standard temperature and pressure is 22.4 L/mol. Standard conditions are 273 K (0°C) and 1.013 x 10⁵ Pa (1 atm).
Molarity of a Solution
The concentration of a solution can be expressed in two ways: as a percentage of mass (for example, a 4% sodium hydroxide solution contains 4g of NaOH in 100g of solution) or as a percentage of volume (for example, 96% alcohol means that there are 96 ml of pure alcohol in 100 ml of solution). Molarity or molar concentration can also be used, which is the number of moles of solute per liter of solution. For example, if a solution contains 0.01 moles of solute per liter of solution, it is a 0.01 M or 0.01 mol/L solution.