Atomic Structure and Chemical Formulas

Isotopes and Atomic Structure

Isotopes

Atoms of the same element can have different masses but identical chemical properties. These are called isotopes.

The Neutron

Arranging elements by increasing atomic number resolves anomalies in the periodic table that arise when ordering by atomic mass. Isotopic masses, determined by mass spectrometry, are inconsistent with the mass of protons in the atomic number. Therefore, atomic mass must be explained by other factors.

Atomic Number (Z)

This number indicates the number of protons in the nucleus and defines the element. It is represented by the letter Z.

Mass Number (A)

This number indicates the total number of protons and neutrons (nucleons) in the nucleus and defines the isotope of an element. It is represented by the letter A.

Bohr Model (1913)

• Electron energy within an atom is quantized. Electrons occupy specific stationary states around the nucleus with defined energy values.
• Electrons move in circular orbits around the nucleus. Each orbit corresponds to an allowed energy level and is associated with a specific number.
• Allowed energy levels are those where the electron's angular momentum, mvr (mass × velocity × orbital radius), is a multiple of h/2π, where h is Planck's constant.
• Energy is absorbed or emitted only when an electron transitions between energy levels. The energy change (ΔE) and its frequency (ν) are related by: ΔE = E_f - E_i
  |ΔE| = hν
  ν = |ΔE| / h = |E_f - E_i| / h

Electronic Configuration and the Periodic Table

Electronic Configuration

This describes the distribution of electrons within an atom across different energy levels and orbitals around the nucleus.

Electronic Structure and the Periodic Table

• Elements in the same period have the same number of electron shells (complete or incomplete). This number corresponds to the period number.
• Elements in the same group have the same electronic structure in their outermost (valence) shell.

Ionization Energy and Electron Affinity

Ionization Energy (Ei)

The energy required to remove an electron from a neutral gaseous atom (X) to form a monocation (X+): X(g) + Ei → X+(g) + e-

Electron Affinity (Ea)

The energy exchanged when a neutral gaseous atom (X) gains an electron to form a monoanion (X-): X(g) + e- + Ea → X-(g)

Electronegativity

The ability of an atom in a molecule to attract electrons towards itself.

Chemical Formulas

A formula symbolically expresses the composition and structure of a chemical:

• Empirical Formula: Shows the elements and their simplest whole-number ratio in a compound. Used for compounds forming crystal lattices.
• Molecular Formula: Shows the elements and the actual number of atoms of each element in a molecule.
• Structural Formula: Shows how atoms are bonded within a molecule.
• Stereo-chemical Formula: Represents the three-dimensional arrangement of atoms and bonds in space.

The oxidation number of an element in a compound is the hypothetical charge an atom would have if all bonds were completely ionic.

Br Cl I S F O R H N P As Sb Se Te C Si B