Elements and the Periodic Table: Concise Notes

Elements

  • Pure substance that cannot be broken down by chemical means.
  • Represented by one or two-letter symbols.
  • Arranged in the periodic table according to chemical properties.

Common Elements and Symbols

  • Bromine: Br
  • Calcium: Ca
  • Carbon: C
  • Chlorine: Cl
  • Chromium: Cr
  • Cobalt: Co
  • Copper: Cu
  • Fluorine: F
  • Hydrogen: H
  • Iodine: I
  • Lead: Pb
  • Magnesium: Mg
  • Manganese: Mn
  • Molybdenum: Mo
  • Nitrogen: N
  • Oxygen: O
  • Phosphorus: P
  • Potassium: K
  • Sodium: Na
  • Sulfur: S
  • Zinc: Zn

Periodic Table

  • Elements are divided into metals, nonmetals, and metalloids.
  • Metals:
    • Located on the left side of the table.
    • Good conductors of heat and electricity.
    • Shiny solids at room temperature (except mercury).
  • Nonmetals:
    • Located on the right side of the table.
    • Dull appearance.
    • Poor conductors of heat and electricity.
    • Can be solids, liquids, or gases at room temperature.
  • Metalloids:
    • Located along the solid line from Boron (B) to Astatine (At).
    • Have intermediate properties between metals and nonmetals.
    • Examples: Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium, Astatine.

Elements in the Human Body

  • Building-Block Elements (96% of body mass):
    • Oxygen (O), Carbon (C), Hydrogen (H), Nitrogen (N).
    • Found in proteins, carbohydrates, lipids, and nucleic acids.
  • Major Minerals:
    • Needed in amounts of at least 100 mg daily.
    • Magnesium (Mg) and Sulfur (S): Found in proteins.
    • Iodine (I): Needed for thyroid function.
  • Trace Elements:
    • Required in small quantities (less than 15 mg daily).

Compounds

  • Pure substance formed by chemically combining two or more elements.
  • Chemical formula:
    • Element symbols indicate elements in the compound.
    • Subscripts show the ratio of atoms (e.g., H_2O).

Structure of the Atom

  • Atoms are composed of subatomic particles:
    • Protons: Positive charge, mass of 1.6726 × 10^{-24} g.
    • Neutrons: No charge, mass of 1.6749 × 10^{-24} g.
    • Electrons: Negative charge, mass of 9.1094 × 10^{-28} g.
  • Nucleus:
    • Contains protons and neutrons.
    • Dense core of the atom.
  • Electron cloud:
    • Contains electrons.
    • Comprises most of the atom’s volume; mostly empty space.

Atomic Number and Mass Number

  • Atomic Number (Z):
    • Number of protons in the nucleus.
    • Identifies the element.
  • Mass Number (A):
    • Number of protons and neutrons in the nucleus.
    • A = Z + N
  • Neutron Number (N):
    • Number of neutrons in the nucleus.
      -N = A - Z

Isotopes

  • Atoms of the same element with different numbers of neutrons.
  • Isotope symbol: ^{A}_{Z}X, where A is the mass number, Z is the atomic number, and X is the element symbol.

Atomic Weight

  • Weighted average of the masses of naturally occurring isotopes.
  • Calculated using the formula:
  • Atomic Weight = \sum(Isotope Mass \times Relative Abundance)
  • 1 amu = 1.6606 \times 10^{-24} g

Periodic Table Features

  • Period: A row in the periodic table.
  • Group: A column in the periodic table.
  • Main Group Elements: Groups 1A (IA) to 8A (VIIIA).
  • Transition Metal Elements: Groups 1B to 8B.

Group Characteristics

  • Group 1A (Alkali Metals) and 2A (Alkaline Earth Metals):
    • Soft, shiny metals.
      -Good conductors of heat and electricity.
  • Group 7A (Halogens):
    • Very reactive.
    • Exist as diatomic molecules.
  • Group 8A (Noble Gases):
    • Very stable / Inert.

Electronic Structure

  • Electrons occupy specific energy levels or shells (n = 1, 2, 3, …).
  • Maximum number of electrons in a shell: 2n^2
  • Shells are divided into subshells (s, p, d, f).
  • Orbitals: Regions of space where electrons are likely to be found; each orbital holds two electrons.

Electron Configuration

  • Shows the arrangement of electrons in an atom’s orbitals.
  • Rules:
    • Fill orbitals in order of increasing energy (1s, 2s, 2p, 3s, 3p, 4s, 3d, … ).
    • Each orbital holds a maximum of 2 electrons.
    • For orbitals of equal energy, add one electron to each before pairing.

Valence Electrons

  • Electrons in the outermost shell (highest n value).
  • Group number equals the number of valence electrons for main group elements (except He).

Periodic Trends

  • Atomic Size:
    • Increases down a column.
    • Decreases across a row.
  • Ionization Energy:
    • Energy needed to remove an electron from a neutral atom.
    • Decreases down a column.
    • Increases across a row.