Chemical Foundation of Life: Atoms and Elements

Atoms and Elements

Atoms: The Smallest Building Blocks of Life

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

• Matter is anything that occupies space and has mass.

• Atoms are composed of sub-atomic particles: protons, neutrons, and electrons.

Elements: Unique Forms of Matter

• Element: A substance that cannot be broken down into smaller substances by chemical means.

• Each element has specific chemical and physical properties.

• Elements are designated by chemical symbols (e.g., Sulfur = S, Calcium = Ca).

Elements Essential for Life

Although the elements that make up living organisms are not unique, they occur in unique proportions compared to the non-living world.

Comparing the elements of living organisms to the non-living world

Elements in the Human Body

• About 25% of the 92 natural elements are essential for life.

• Trace elements (less than 0.01% of mass) are also required in small quantities (e.g., Boron, Chromium, Iron, Zinc).

Structure of the Atom

Sub-Atomic Particles

• Proton: Positively charged particle located in the nucleus.

• Neutron: Neutral particle located in the nucleus.

• Electron: Negatively charged particle located in orbitals around the nucleus.

Atomic Structure

• The nucleus contains protons and neutrons.

• Electrons are found in orbitals surrounding the nucleus, forming an electron cloud.

• Atoms are electrically neutral when the number of protons equals the number of electrons.

Example: Helium (He) atom has 2 protons, 2 neutrons, and 2 electrons.

Atomic Number and Atomic Mass

Definitions and Calculations

• Atomic Number: The number of protons in the nucleus of an atom. It defines the element.

• Atomic Mass: The total mass of an atom, approximately equal to the sum of protons and neutrons.

• Electrons have negligible mass and are not included in atomic mass calculations.

• Atomic mass is expressed in atomic mass units (amu).

• Number of neutrons can be calculated as:

Example: Carbon (C) has atomic number 6. Carbon-12 has 6 protons and 6 neutrons; Carbon-13 has 6 protons and 7 neutrons.

Isotopes

Definition and Applications

• Isotopes: Atoms of the same element with different numbers of neutrons, resulting in different mass numbers.

• All isotopes of an element have the same number of protons but may differ in neutrons.

• Radioactive isotopes decay spontaneously, emitting particles and energy.

• Applications include dating fossils, tracing atoms in metabolic processes, and diagnosing medical disorders.

Example: Hydrogen has three isotopes: Protium (), Deuterium (), and Tritium ().

Electron Shells and Chemical Properties

Electron Arrangement

• Electrons occupy energy levels or shells around the nucleus.

• The valence shell is the outermost electron shell.

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

• Elements in Group 18 (noble gases) have full valence shells and are chemically inert.

Sub-Atomic Particle Properties Table

Additional info:

• Quantum mechanics describes electron orbitals as regions of space where electrons are likely to be found, rather than fixed paths.

• Chemical reactions involve changes in the distribution of electrons between atoms, forming new substances.

• Molecules are formed when atoms share or transfer electrons through chemical bonds (covalent, ionic, hydrogen, van der Waals).