Atomic Structure and Chemical Bonding

Atomic Structure and Chemical Bonding

Introduction to Atoms and Subatomic Particles

Atoms are the fundamental units of matter, composed of subatomic particles: protons, neutrons, and electrons. Understanding atomic structure is essential for studying chemical bonding and biological molecules.

• Atom: The smallest unit of an element that retains its chemical properties.

• Subatomic Particles:

  • Protons: Positively charged particles located in the nucleus.

  • Neutrons: Neutral particles also found in the nucleus.

  • Electrons: Negatively charged particles that orbit the nucleus in electron shells.

• Atomic Number (Z): The number of protons in the nucleus; determines the element.

• Mass Number (A): The sum of protons and neutrons in the nucleus.

Example: A carbon atom has 6 protons, 6 neutrons, and 6 electrons.

Electron Shells and Energy Levels

Electrons are arranged in shells or energy levels around the nucleus. The arrangement of electrons determines how atoms interact and bond with each other.

• First Shell: Holds up to 2 electrons.

• Second Shell: Holds up to 8 electrons.

• Third Shell: Holds up to 8 electrons (for main group elements).

• Valence Electrons: Electrons in the outermost shell; important for chemical bonding.

Example: Oxygen (atomic number 8) has 2 electrons in the first shell and 6 in the second shell.

Bohr Model of the Atom

The Bohr model represents atoms with electrons orbiting the nucleus in defined shells. This model helps visualize electron arrangement and predict chemical behavior.

• Shells are drawn as concentric circles around the nucleus.

• Each shell has a maximum number of electrons:

  • First shell: 2 electrons

  • Second shell: 8 electrons

  • Third shell: 8 electrons (for main group elements)

Example: Sodium (Na, atomic number 11) has 2 electrons in the first shell, 8 in the second, and 1 in the third.

Chemical Bonding

Atoms form chemical bonds to achieve stable electron configurations, often by filling their outermost electron shells.

• Ionic Bonds: Formed when electrons are transferred from one atom to another, resulting in oppositely charged ions that attract each other.

• Covalent Bonds: Formed when two atoms share one or more pairs of electrons.

• Octet Rule: Atoms tend to gain, lose, or share electrons to have eight electrons in their valence shell (except for hydrogen and helium, which aim for two).

Example: Sodium (Na) donates one electron to chlorine (Cl) to form NaCl (table salt), an ionic compound.

Summary Table: Subatomic Particles

Particle	Charge	Location	Relative Mass
Proton	+1	Nucleus	1
Neutron	0	Nucleus	1
Electron	-1	Electron shells	~0 (1/1836 of a proton)

Key Equations

• Atomic number: $Z = \text{number of protons}$

• Mass number: $A = \text{number of protons} + \text{number of neutrons}$

Additional info: Some content, such as the Bohr model diagrams and specific examples, was inferred from standard biology and chemistry curricula to provide a complete and coherent study guide.