Atomic Structure and Chemical Bonds: Key Concepts

Radioactivity

Radioactivity is the phenomenon whereby some substances emit radiation that can impress photographic plates, ionize gases, produce fluorescence, and cross through opaque to ordinary light. These elements are called radioactive.

Radiation can be:

• Alpha: Positively charged helium atoms, which are easily absorbed by matter.
• Beta: Electrons, which are more penetrating.
• Gamma: Electromagnetic in nature and even more penetrating.

Rutherford Model

The Rutherford model consists of:

• Nucleus: Composed of Z protons and N neutrons, responsible for the atom's mass and positive charge. The mass number A is equal to the sum of protons and neutrons (A = Z + N).
• Electron Cloud: Composed of electrons, responsible for the atom's negative electric charge. Since the atom is electrically neutral, and the proton and electron charges are equal, the number of electrons in the electron cloud is equal to the number of protons in the nucleus.

Chemical Element

A chemical element is a substance composed of atoms that all have the same atomic number.

Bohr's Postulates

Bohr's postulates are:

1. Electrons can only rotate in stationary orbits, and while in them, they do not emit or absorb energy. The energy for each orbit is called the energy level.
2. Allowed stationary orbits are determined by different integer values of n = 1, 2, 3, 4..., called the principal quantum number.
3. If an electron jumps from a higher orbit to a lower one, the energy difference E is emitted as radiation. However, if the jump occurs in the opposite direction, it absorbs E.

The study of spectra leads to the existence of different energy levels within the atom, corresponding to the possible states of electron distribution in the electron cloud.

Hund's Rule

Hund's Rule states that when filling orbitals of the same energy, electrons are arranged so that they are unpaired to the maximum extent, and their spins remain parallel.

Modern Atomic Theory

In modern atomic theory, the electron is blurred; its position and velocity cannot be fixed. We can only determine the probability that it is in a region of space. We no longer speak of orbits but of orbitals.

Atomic size increases down a group but decreases as you move through a period because the last layer remains as busy while the positively charged nucleus enhances the attraction of electrons, decreasing the atomic size.

Salt Bridge

A salt bridge occurs between atoms of elements that have very different electronegativities. It is the union of electrostatic forces exerted between ions of opposite signs.

Properties of salt bridges:

• High melting and boiling points.
• Small expansion coefficients.
• Relatively high hardness.

Ionization Energy

Ionization energy is the energy absorbed by an atom in the gaseous state to remove an electron.

Electron Affinity

Electron affinity is the energy released when an unstressed gaseous atom incorporates an electron.

Covalent Bond

In a covalent bond, atoms share pairs of electrons to achieve a noble gas structure, with eight electrons in the last layer. These shared electrons have opposite spins.

Features of covalent bonds:

• Form molecules.
• Often in liquid or gaseous states.
• Generally, they are not soluble in water and do not carry current.

According to Lewis, atoms can obtain a stable electron configuration by sharing electrons. An atom obtains a stable noble gas configuration when its valence shell holds eight electrons.

Electrons are not transferred from one atom to another but are shared in forming a molecule. Each pair of shared electrons constitutes a covalent bond.

Difference Between Molecule and Crystal Lattice

A molecule is formed by a specific number of atoms, and its formula indicates the class and number of atoms that compose it. In a crystal lattice, the number of ions is indefinite, and therefore it has no formula. The formulas of ionic compositions do not indicate the composition of the network but the relative amount of ions of each sign that must exist to maintain electrical neutrality.

Enthalpy

Enthalpy is a physical quantity that matches the amount of heat involved in a chemical reaction when it takes place at constant pressure.

Hess's Law

Hess's Law states that the enthalpy of a reaction is equal to the enthalpy of formation of the products minus the enthalpy of formation of the reactants.