Atoms, Elements, and Chemical Bonds: Foundations of Life

Life, the Universe, and Everything: The Chemical Basis of Life

Introduction to Matter and Atoms

All living and non-living things are composed of matter, which is made up of chemicals. The fundamental unit of matter is the atom. Understanding atomic structure and chemical bonds is essential for studying biological molecules and life processes.

• Chemicals are constructed from atoms.

• Different types of matter are made from different elements, each consisting of different atoms.

The Atom

Atomic Structure

An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of three types of subatomic particles:

• Neutrons (n0): Mass = 1 atomic mass unit (amu), no electrical charge (neutral).

• Protons (p+): Mass = 1 amu, positive electrical charge.

• Electrons (e-): Mass is negligible (trivial), negative electrical charge.

Protons and neutrons are located in the nucleus, while electrons orbit the nucleus in electron shells.

Atomic Number and Mass

• Atomic Number: The number of protons in the nucleus; determines the element's identity.

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

Example: An atom with 6 protons is always carbon, regardless of the number of neutrons or electrons.

Partial Periodic Table and Elements in the Body

The periodic table organizes elements by atomic number. In biological systems, only a few elements make up most of the body mass:

• Oxygen (O): 65%

• Carbon (C): 18%

• Hydrogen (H): 10%

• Nitrogen (N): 3%

These four elements account for approximately 96% of the human body's mass.

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different atomic masses.

• Stable isotopes do not change over time.

• Radioisotopes are unstable and emit radiation as they decay.

Example: Hydrogen has three isotopes—protium (1 proton), deuterium (1 proton, 1 neutron), and tritium (1 proton, 2 neutrons).

Electron Shells and Valence Electrons

Electrons occupy energy levels called electron shells. The outermost shell is called the valence shell and determines the atom's chemical reactivity.

• Only valence shell electrons participate in chemical reactions.

• Atoms are most stable when their valence shell is full.

Ions and Isotopes

Ions: Formation and Types

An ion is an atom or molecule with a net electrical charge due to the loss or gain of electrons.

• Cation: Positively charged ion (loss of electrons).

• Anion: Negatively charged ion (gain of electrons).

Example: Sodium (Na) loses an electron to become Na+ (cation), and chlorine (Cl) gains an electron to become Cl- (anion).

Electron Shell Diagrams and Ion Formation

Electron shell diagrams illustrate how atoms achieve stable electron configurations by gaining or losing electrons, resulting in ion formation.

Chemical Bonds

Types of Chemical Bonds

Atoms bond together to form molecules through different types of chemical bonds:

• Covalent Bonds

• Ionic Bonds

• Hydrogen Bonds

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms to fill their valence shells.

• Each shared pair of electrons forms one covalent bond.

• Atoms can share more than one pair of electrons, forming double or triple bonds.

Example: In water (H2O), each hydrogen atom shares one electron with oxygen, forming two single covalent bonds.

• Double bonds: O=O, C=C

• Triple bonds: N≡N

Ionic Bonds

Ionic bonds are formed by the electrostatic attraction between oppositely charged ions (cations and anions).

• One atom donates an electron, becoming a cation.

• Another atom accepts the electron, becoming an anion.

• The resulting opposite charges attract, forming an ionic bond.

Example: Sodium chloride (NaCl) is formed when Na+ and Cl- ions bond together.

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom covalently bonded to an electronegative atom (like oxygen or nitrogen) and another electronegative atom. These bonds are crucial in stabilizing the structures of biological molecules such as DNA and proteins.

Molecules and Compounds

Molecules and Compounds

A molecule consists of two or more atoms bonded together. A compound is a molecule composed of two or more different elements.

• Elemental molecule: Contains only one type of element (e.g., O2, H2).

• Compound molecule: Contains different elements (e.g., H2O).

Representation of Molecules

Molecules can be represented in various ways, including:

• Electron shell diagrams

• Structural formulas

• Ball-and-stick models

• Space-filling models

Summary Table: Subatomic Particles

Particle	Symbol	Charge	Location	Relative Mass
Proton	p+	+1	Nucleus	1 amu
Neutron	n0	0	Nucleus	1 amu
Electron	e-	-1	Electron shell	~0 (1/1836 amu)

Key Equations

• Atomic Mass Number: $\text{Atomic Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}$

• Atomic Number: $\text{Atomic Number} = \text{Number of Protons}$

Additional info:

• Hydrogen bonds, while weak individually, are collectively strong and play a major role in the properties of water and the structure of macromolecules.

• Radioisotopes are used in medical imaging and dating of biological materials.