Elements, Atoms, and the Periodic Table

Chapter 2: Elements, Atoms, and the Periodic Table

Elements

• Definition: Pure substances from which all other things are built.
• Listing: Elements are organized in the periodic table.
  • Periodic Table shows various elements and their symbols.

Abundance of Elements

• Hydrogen is the most common element in the universe (about 90% of atoms).
• Following hydrogen is helium, which constitutes most of the remaining 10%.
• All other elements exist in relatively minimal amounts.

Composition of Earth and Human Body

Table 2.1: Elemental Composition of Earth

• Oxygen: 61% (Earth's Crust), 46.1% (Overall)
• Silicon: 28.2%
• Iron: 34.6%
• Aluminum: 8.23%
• Calcium: 1.4%
• Magnesium: 12.7%
• Sulfur: 0.20%
• All Others: 3.7%

Table 2.2: Elemental Composition of a Human Body

• Oxygen: 29.5%
• Carbon: 23%
• Hydrogen: 10%
• Nitrogen: 2.6%
• Calcium: 4.15%
• Phosphorus: 1.1%
• Sodium: 0.14%
• All Others: 0.174%

Naming of Elements

• Element names can originate from various sources: planets, mythological figures, minerals, colors, geographic locations, and famous personalities.

Examples

Chemical Symbols

• Chemical symbols represent the names of elements. They consist of one or two letters and start with a capital letter.

One-Letter Symbols

Two-Letter Symbols

List of Common Elements and Symbols

The Atom

• Definition: An atom is the smallest particle of an element that retains the characteristics of that element.
• Size: Atoms are extremely small, with sizes such as 5.4 × 10^{-10} m across.
• Dalton's Atomic Theory: Proposed about 1803 by John Dalton, it states:
  1. Each element consists of extremely small particles called atoms.
  2. All atoms of a given element are identical; atoms of different elements are different.
  3. Atoms of one element cannot be changed into atoms of another element by chemical reactions.
  4. Compounds form when atoms of more than one element combine in fixed ratios.

Structure of the Atom

• Subatomic Particles:
  • Protons: Positive charge (+)
  • Neutrons: No charge (neutral)
  • Electrons: Negative charge (-)
• Protons and neutrons are found in the nucleus, which holds the majority of an atom's mass, whereas electrons surround the nucleus and occupy most of the atom's volume.
• Protons and neutrons have significant mass, while electrons are approximately 1800 times smaller than protons and neutrons.

Mass of Subatomic Particles

Atomic Number

• Each element is assigned an atomic number, which equals the number of protons in an atom.
• This number appears above the symbol of the element in the periodic table.

Examples of Atomic Number

• Hydrogen (H): Atomic number is 1.
• Carbon (C): Atomic number is 6.
• Copper (Cu): Atomic number is 29.

Atoms are Neutral

• In a neutral atom, the number of protons equals the number of electrons, resulting in a net charge of zero.
• For example, Aluminum has 13 protons and 13 electrons: 13+ + 13- = 0.

Mass Number

• The mass number is the sum of protons and neutrons in the nucleus.
• It is not displayed in the periodic table but can be calculated using:
  • Mass number = Number of protons + Number of neutrons.

Isotopes

• Definition: Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different mass numbers.
• Example: Hydrogen has isotopes:
  • Normal Hydrogen (1 proton, 0 neutrons)
  • Deuterium (1 proton, 1 neutron)
  • Tritium (1 proton, 2 neutrons).

Atomic Mass

• The atomic mass is the weighted average of all naturally occurring isotopes of that element, represented on the periodic table below the element's symbol.

Quantum Mechanics and Electron Behavior

• Quantum mechanics describes electron behavior within an atom.
• Electrons occupy specific energy levels described by principal quantum numbers (n = 1, 2, …), with energy increasing further from the nucleus.

Electron Configurations

• The arrangement of electrons determines an element's chemical properties.
• The maximum number of electrons in different sublevels:
  • s: 2 electrons
  • p: 6 electrons
  • d: 10 electrons
  • f: 14 electrons

Electron Filling Order in Atoms

• Electrons fill energy levels and sublevels in increasing energy order.

Example: Electron Configuration for Carbon

• Carbon has 6 electrons: 1s² 2s² 2p²

Final Notes

• The periodic table organizes elements by increasing atomic number and demonstrates electron configurations in relation to chemical properties. Understanding elements, atoms, their structure, and behavior are foundational for further studies in chemistry.