Comprehensive Study Notes on Atomic Structure, Electronic Configurations, and Relative Atomic Mass

Atomic Structure & Sub-Atomic Particles

• An atom consists of a small, positively charged nucleus with negatively charged electrons moving around it in specific shells or energy levels.
• Most of an atom is empty space.
• The fundamental components and properties of sub-atomic particles are:
  • Proton:
  • Location: Located in the nucleus
  • Relative Mass: $1$
  • Relative Charge: $+1$
  • Neutron:
  • Location: Located in the nucleus
  • Relative Mass: $1$
  • Relative Charge: $0$
  • Electron:
  • Location: Located in the 'shells' (energy levels) surrounding the nucleus
  • Relative Mass: $\\frac{1}{1840}$ (or effectively $0$)
  • Relative Charge: $-1$

Atomic Identifiers: Atomic Number and Mass Number

• The atom of each element is defined by two fundamental numbers:

  • Atomic Number / Proton Number ($Z$):
  • Represents the total number of protons in the nucleus of an atom.
  • Indicates the total number of electrons in a neutral atom.
  • Determines the position of the element in the Periodic Table.
  • Defines the unique identity of the element.
  • Mass Number / Nucleon Number ($A$):
  • Represents the sum of the number of protons and neutrons in the nucleus of an atom.

• Atomic Symbol and Periodic Table Notation:

  • Elements are arranged in the Periodic Table in order of increasing atomic number ($Z$).
  • Element Name: Usually derived from a Greek or Latin word for the element or a substance containing the element.
  • Symbol: Short-hand abbreviation for the element name (e.g., $\text{C}$ for Carbon).
  • Distinction on Periodic Tables: Most Periodic Tables display two numbers against each symbol; the atomic number ($Z$) is always the smaller number of the two.

Arrangement of Electrons in Atoms

• Electrons move around the nucleus of an atom in special regions known as "shells" that indicate their specific energy level.

• Only electrons with the appropriate energy can occupy a given shell.

• Shells exist at fixed distances from the nucleus, and the shell with the lowest energy is closest to the nucleus.

• Maximum Capacity of Electron Shells:

  • Shell 1 (closest to nucleus / lowest energy): Maximum $2$ electrons
  • Shell 2: Maximum $8$ electrons
  • Shell 3: Maximum $18$ electrons ($8$, then $10$)
  • Shell 4: Maximum $32$ electrons ($8$, then $10$, then $14$)

• Rules for Filling Electron Shells (Electronic Configuration / E.C.):

  • Electrons fill up the shells in strict sequential order, beginning with the lowest energy level ($1^{\\text{st}}$ shell closest to the nucleus).
  • Electrons generally fill one shell completely before entering the next shell.
  • Electrons are placed in shells unpaired first in a clockwise direction, and are paired up only after each position in that shell is occupied.

• Chemical Reactivity and Valence Electrons:

  • Protons and neutrons reside inside the nucleus and have no part in chemical reactions.
  • Electrons ($e^-$) are exclusively responsible for the chemical properties of elements.
  • Valence Shell: The outermost shell of an atom.
  • Valence Electrons: The electrons located in the outermost shell that take part in chemical bonding.

First 20 Elements of the Periodic Table

• Below is the complete reference dataset for the first $20$ elements:
  • Hydrogen ($\text{H}$):
  • Atomic Number ($Z$): $1$
  • Mass Number ($A$): $1$
  • Number of Protons: $1$
  • Number of Electrons: $1$
  • Number of Neutrons: $0$
  • Electronic Configuration (E.C.): $1$
  • Helium ($\text{He}$):
  • Atomic Number ($Z$): $2$
  • Mass Number ($A$): $4$
  • Number of Protons: $2$
  • Number of Electrons: $2$
  • Number of Neutrons: $2$
  • Electronic Configuration (E.C.): $2$
  • Lithium ($\text{Li}$):
  • Atomic Number ($Z$): $3$
  • Mass Number ($A$): $7$
  • Number of Protons: $3$
  • Number of Electrons: $3$
  • Number of Neutrons: $4$
  • Electronic Configuration (E.C.): $2,1$
  • Beryllium ($\text{Be}$):
  • Atomic Number ($Z$): $4$
  • Mass Number ($A$): $9$
  • Number of Protons: $4$
  • Number of Electrons: $4$
  • Number of Neutrons: $5$
  • Electronic Configuration (E.C.): $2,2$
  • Boron ($\text{B}$):
  • Atomic Number ($Z$): $5$
  • Mass Number ($A$): $11$
  • Number of Protons: $5$
  • Number of Electrons: $5$
  • Number of Neutrons: $6$
  • Electronic Configuration (E.C.): $2,3$
  • Carbon ($\text{C}$):
  • Atomic Number ($Z$): $6$
  • Mass Number ($A$): $12$
  • Number of Protons: $6$
  • Number of Electrons: $6$
  • Number of Neutrons: $6$
  • Electronic Configuration (E.C.): $2,4$
  • Nitrogen ($\text{N}$):
  • Atomic Number ($Z$): $7$
  • Mass Number ($A$): $14$
  • Number of Protons: $7$
  • Number of Electrons: $7$
  • Number of Neutrons: $7$
  • Electronic Configuration (E.C.): $2,5$
  • Oxygen ($\text{O}$):
  • Atomic Number ($Z$): $8$
  • Mass Number ($A$): $16$
  • Number of Protons: $8$
  • Number of Electrons: $8$
  • Number of Neutrons: $8$
  • Electronic Configuration (E.C.): $2,6$
  • Fluorine ($\text{F}$):
  • Atomic Number ($Z$): $9$
  • Mass Number ($A$): $19$
  • Number of Protons: $9$
  • Number of Electrons: $9$
  • Number of Neutrons: $10$
  • Electronic Configuration (E.C.): $2,7$
  • Neon ($\text{Ne}$):
  • Atomic Number ($Z$): $10$
  • Mass Number ($A$): $20$
  • Number of Protons: $10$
  • Number of Electrons: $10$
  • Number of Neutrons: $10$
  • Electronic Configuration (E.C.): $2,8$
  • Sodium ($\text{Na}$):
  • Atomic Number ($Z$): $11$
  • Mass Number ($A$): $23$
  • Number of Protons: $11$
  • Number of Electrons: $11$
  • Number of Neutrons: $12$
  • Electronic Configuration (E.C.): $2,8,1$
  • Magnesium ($\text{Mg}$):
  • Atomic Number ($Z$): $12$
  • Mass Number ($A$): $24$
  • Number of Protons: $12$
  • Number of Electrons: $12$
  • Number of Neutrons: $12$
  • Electronic Configuration (E.C.): $2,8,2$
  • Aluminum ($\text{Al}$):
  • Atomic Number ($Z$): $13$
  • Mass Number ($A$): $27$
  • Number of Protons: $13$
  • Number of Electrons: $13$
  • Number of Neutrons: $14$
  • Electronic Configuration (E.C.): $2,8,3$
  • Silicon ($\text{Si}$):
  • Atomic Number ($Z$): $14$
  • Mass Number ($A$): $28$
  • Number of Protons: $14$
  • Number of Electrons: $14$
  • Number of Neutrons: $14$
  • Electronic Configuration (E.C.): $2,8,4$
  • Phosphorus ($\text{P}$):
  • Atomic Number ($Z$): $15$
  • Mass Number ($A$): $31$
  • Number of Protons: $15$
  • Number of Electrons: $15$
  • Number of Neutrons: $16$
  • Electronic Configuration (E.C.): $2,8,5$
  • Sulfur ($\text{S}$):
  • Atomic Number ($Z$): $16$
  • Mass Number ($A$): $32$
  • Number of Protons: $16$
  • Number of Electrons: $16$
  • Number of Neutrons: $16$
  • Electronic Configuration (E.C.): $2,8,6$
  • Chlorine ($\text{Cl}$):
  • Atomic Number ($Z$): $17$
  • Mass Number ($A$): $35$
  • Number of Protons: $17$
  • Number of Electrons: $17$
  • Number of Neutrons: $18$
  • Electronic Configuration (E.C.): $2,8,7$
  • Argon ($\text{Ar}$):
  • Atomic Number ($Z$): $18$
  • Mass Number ($A$): $40$
  • Number of Protons: $18$
  • Number of Electrons: $18$
  • Number of Neutrons: $22$
  • Electronic Configuration (E.C.): $2,8,8$
  • Potassium ($\text{K}$):
  • Atomic Number ($Z$): $19$
  • Mass Number ($A$): $39$
  • Number of Protons: $19$
  • Number of Electrons: $19$
  • Number of Neutrons: $20$
  • Electronic Configuration (E.C.): $2,8,8,1$
  • Calcium ($\text{Ca}$):
  • Atomic Number ($Z$): $20$
  • Mass Number ($A$): $40$
  • Number of Protons: $20$
  • Number of Electrons: $20$
  • Number of Neutrons: $20$
  • Electronic Configuration (E.C.): $2,8,8,2$

Isotopes

• Definition: Isotopes are atoms of the same element that have the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers).

• Isotope Examples & Deductions:

  • Chromium-58 vs Chromium-63:
  • Chromium-58 ($^{58}\\text{Cr}$): $24$ protons, $34$ neutrons, $24$ electrons
  • Chromium-63 ($^{63}\\text{Cr}$): $24$ protons, $39$ neutrons, $24$ electrons
  • Nitrogen-15 vs Nitrogen-20:
  • Nitrogen-15 ($^{15}\\text{N}$): $7$ protons, $8$ neutrons, $7$ electrons
  • Nitrogen-20 ($^{20}\\text{N}$): $7$ protons, $13$ neutrons, $7$ electrons

Relative Atomic Mass ($A_r$)

• Need for a Relative Mass Scale:

  • Atoms are extremely small and light, making conventional mass units (grams) impractical for measuring atomic masses.
  • Example: A single hydrogen atom has a mass of approximately $1.67 \\times 10^{-27}\\text{ kg}$.

• The Carbon-12 Standard:

  • Chemists established a relative scale in which the mass of an atom is calculated by comparison with the mass of an atom of the carbon-12 ($\\text{C-}12$) isotope.
  • Carbon-12 is assigned a mass of exactly $12.00\\text{ atomic mass units (a.m.u)}$.
  • Historical Development: Initially, hydrogen was used as the basis for the relative mass scale. Carbon-12 was ultimately chosen because it possesses greater stability and is abundant in nature.

• Relative Atomic Mass Comparisons:

  • A carbon atom is $12$ times as heavy as a hydrogen atom, so carbon has a relative atomic mass of $12$ (meaning $\\frac{1}{12}$ of the mass of carbon has the same mass as one atom of hydrogen).
  • The relative atomic mass of sodium is $23$, because one atom of sodium is $23$ times as heavy as one hydrogen atom.

• Mass Number vs Relative Atomic Mass ($A_r$):

  • Mass Number ($A$):
  • A count of the total number of protons and neutrons in one single atom.
  • Always a whole number.
  • Relative Atomic Mass ($A_r$):
  • Also known as atomic weight or atomic mass.
  • The weighted average mass of an atom of an element based on the relative natural abundance of all naturally-occurring isotopes of that element.
  • Represented as a decimal number on the Periodic Table.
  • Key Difference: Both figures reflect the number of protons and neutrons in a sample. However, the atomic mass on the Periodic Table is the average mass across a naturally-occurring sample of an element containing various isotopes, whereas the mass number applies to a single specific atom.

• Case Study: Chlorine Isotopes and Atomic Mass:

  • Chlorine has $9$ total isotopes with mass numbers ranging from $32$ to $40$.
  • Only three isotopes occur in nature:
  • Stable $^{35}\\text{Cl}$: $17$ protons, $18$ neutrons, mass number $35$, natural abundance of $75.77\\%$
  • Stable $^{37}\\text{Cl}$: $17$ protons, $20$ neutrons, mass number $37$, natural abundance of $24.23\\%$
  • Radioactive $^{36}\\text{Cl}$
  • Periodic Table Entry for Chlorine:
  • Name of element: Chlorine
  • Atomic number: $17$
  • Chemical symbol: $\text{Cl}$
  • Atomic mass ($A_r$): $35.453$