Atoms, Elements, and the Chemical Basis of Life

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Introduction to Matter and Life

What is Matter?

All living organisms and non-living things are composed of matter, which is anything that has mass and occupies space. The universe, including all forms of life, originated from the Big Bang approximately 13.8 billion years ago.

Matter: Anything that takes up space and has mass.
All visible matter is made up of atoms.
Atoms are the fundamental building blocks of matter.

Levels of Biological Organization

Biological systems are organized in a hierarchy from the smallest chemical units to the entire biosphere.

Atom → Molecule → Cell organelles → Cell → Tissue → Organ → Organism → Population → Community (Biocenosis) → Ecosystem → Biome → Biosphere
Each level represents increasing complexity and organization.

Atoms: Structure and Properties

Atomic Structure

Atoms consist of a central nucleus containing protons and neutrons, surrounded by a cloud of electrons.

Proton: Positively charged particle (+1 charge), located in the nucleus.
Neutron: Neutral particle (0 charge), located in the nucleus.
Electron: Negatively charged particle (-1 charge), found in electron clouds around the nucleus.
The number of protons defines the element.

Forces in the Atom

Electromagnetic force holds electrons in orbit around the nucleus due to attraction between opposite charges.
Protons and neutrons are held together in the nucleus by the strong nuclear force.
Electrons do not fall into the nucleus because they possess too much energy to be confined and are governed by quantum mechanical principles.

Scale and Structure of the Atom

Atoms are mostly empty space.
If the nucleus were the size of a small ball, the electron cloud would extend several football fields away.
This explains why matter is not as dense as it could be, and why we cannot walk through walls: the electron clouds repel each other due to electromagnetic forces.

Elements and the Periodic Table

Elements

An element is a substance that cannot be broken down into simpler substances by chemical means. Each element is defined by its atomic number (number of protons).

There are 92 naturally occurring elements.
The Periodic Table of Elements organizes elements by increasing atomic number and similar chemical properties.

Atomic Number and Atomic Mass

Atomic number (Z): Number of protons in the nucleus.
Atomic mass: Sum of protons and neutrons in the nucleus.
Example: Helium atom has 2 protons and 2 neutrons, so its atomic mass is 4.
Formula:

Isotopes and Their Applications

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

Example: Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon.
Some isotopes are stable, while others are radioactive and decay over time, releasing energy.

Table: Comparison of Carbon Isotopes

Isotope	Protons	Neutrons	Electrons	Stability
Carbon-12	6	6	6	Stable
Carbon-13	6	7	6	Stable
Carbon-14	6	8	6	Radioactive

Applications of Isotopes

Radioactive tracers are used in medicine for diagnostic imaging by injecting substances labeled with radioactive isotopes.
Radiometric dating uses the predictable decay of radioactive isotopes to estimate the age of rocks and fossils.
The half-life of an isotope is the time it takes for half of a sample to decay.

The Elements of Life

Major Elements in Living Organisms

Although there are 92 naturally occurring elements, only a small subset are essential for life. Four elements make up the majority of living matter.

Oxygen (O)
Carbon (C)
Hydrogen (H)
Nitrogen (N)
These four elements constitute about 96% of the human body’s mass.

Table: Elements in the Human Body

Element	Symbol	Percentage of Body Mass
Oxygen	O	65.0%
Carbon	C	18.5%
Hydrogen	H	9.5%
Nitrogen	N	3.3%
Sulfur	S	0.3%
Sodium	Na	0.2%
Chlorine	Cl	0.2%
Magnesium	Mg	0.1%
Trace elements	-	<0.01%

Atomic Structure: Electron Shells and Orbitals

Electron Shells

Electrons in an atom are arranged in shells or energy levels around the nucleus. Each shell has a characteristic energy and can hold a specific number of electrons.

The first shell can hold up to 2 electrons.
The second shell can hold up to 8 electrons.
Electrons fill the lowest available energy levels first (the shell closest to the nucleus).
Electron shells are often depicted as concentric circles, but in reality, electrons occupy regions of space called orbitals.

Electron Orbitals

Each shell is composed of one or more orbitals, which are regions where electrons are likely to be found.
Each orbital can hold up to 2 electrons with opposite spins.
The first shell has one s orbital (1s), the second shell has one s orbital (2s) and three p orbitals (2p).
Filling order: Electrons fill orbitals in a specific sequence based on energy levels.

Octet Rule

Atoms are most stable when their outermost shell is full, typically with 8 electrons (the octet rule).
This drives chemical bonding as atoms seek to achieve a full outer shell.

Example: Hydrogen and Helium

Hydrogen has 1 electron; its shell is full with 2 electrons.
Helium has 2 electrons; its shell is full and stable.

Additional info: The arrangement of electrons in shells and orbitals determines the chemical reactivity and bonding behavior of each element.