Atomic Structure and Electron Shells

Atomic Structure

• Atoms are the fundamental building blocks of matter, consisting of a dense nucleus surrounded by electrons
• The nucleus contains positively charged protons and neutrally charged neutrons
• Electrons have a negligible mass and a negative charge
• The number of protons defines the element (atomic number)
• The mass number is the total number of protons and neutrons
• Atoms are electrically neutral, meaning the number of electrons equals the number of protons

Electron Shells

• Electrons orbit the nucleus in specific energy levels called electron shells
• The first shell can hold up to 2 electrons
• Subsequent shells can hold up to 8 electrons
• Atoms with unfilled outer shells tend to react to fill those shells, either by sharing electrons or gaining/losing them

Octet Rule

• Atoms tend to gain, lose, or share electrons to achieve a stable outer shell with 8 electrons (octet rule)
• This rule is important for chemical bonding and reactivity

Elements in Living Organisms

• Six elements comprise approximately 98% of living organisms: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur
• These elements are vital for biological processes and are found in various molecules like proteins, carbohydrates, and nucleic acids

Atomic Number and Mass Number

• Atomic number identifies an element based on the number of protons
• Mass number is the sum of protons and neutrons in the nucleus

Chemical Reactivity

• Reactivity of atoms depends on the number of electrons in their outermost shell