Atoms, Elements, and the Chemical Basis of Life

Study Guide - Smart Notes

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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 takes up space and has mass. The universe, including all forms of life, originated from the Big Bang approximately 13.8 billion years ago.

Matter: Anything that occupies space and has mass.
Organisms: Living systems composed of matter, organized in increasing complexity from atoms to the biosphere.
Example: A cell is made of molecules, which are made of atoms.

Levels of Biological Organization

Hierarchy from Atoms to Biosphere

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
Example: Oxygen atoms combine to form O2 molecules, which are essential for cellular respiration in organisms.

Atoms: The Building Blocks of Matter

Structure of the Atom

All visible matter is made of atoms. 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 a cloud around the nucleus.
Electrostatic force: Opposite charges attract (protons and electrons), like charges repel.
Example: A hydrogen atom consists of one proton and one electron.

Atomic Structure and Scale

Atoms are mostly empty space. If the nucleus were the size of a small ball, the electron would be about 5 football fields away. This explains why matter is not as dense as it could be, yet we cannot walk through walls due to the repulsion of electron clouds.

Scale: The nucleus is extremely small compared to the overall size of the atom.
Electron cloud: Electrons occupy a vast region around the nucleus, creating the atom's size.
Example: The mass of a hydrogen atom is approximately 1 atomic mass unit (amu).

Elements and the Periodic Table

Definition of Elements

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

Atomic number (Z): Number of protons in the nucleus.
Atomic mass: Sum of protons and neutrons.
Symbol: One- or two-letter abbreviation (e.g., H for hydrogen, O for oxygen).
Example: Helium (He) has 2 protons and typically 2 neutrons, so its atomic mass is 4.

Elements Essential for Life

Of the 92 naturally occurring elements, only a small subset is essential for life. The most abundant elements in living organisms are oxygen, carbon, hydrogen, and nitrogen.

Element	Symbol	Percentage of Body Mass (including water)
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	Various	<0.01%

Example: Iron (Fe) is a trace element essential for oxygen transport in blood.

Atomic Number, Mass Number, and Isotopes

Atomic Number and Mass Number

The atomic number is the number of protons in an atom, while the mass number is the sum of protons and neutrons.

Formula:
Example: Carbon-12 has 6 protons and 6 neutrons; mass number = 12.

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Some isotopes are stable, while others are radioactive and decay over time.

Stable isotopes: Do not change over time (e.g., Carbon-12, Carbon-13).
Radioactive isotopes: Unstable; decay spontaneously, emitting energy (e.g., Carbon-14).
Example: Carbon-12 (6 protons, 6 neutrons), Carbon-13 (6 protons, 7 neutrons), Carbon-14 (6 protons, 8 neutrons; radioactive).

Applications of Isotopes

Radioactive tracers: Used in medicine for diagnostic imaging by injecting a radioactive substance and detecting its movement in the body.
Radiometric dating: Scientists use the decay rate (half-life) of isotopes to determine the age of rocks and fossils.
Formula for half-life: where is the remaining quantity, is the initial quantity, is time, and is the half-life.

Origin of Elements

Formation of Elements

The elements found on Earth were formed in the Big Bang and in the interiors of stars. Heavier elements are produced by nuclear fusion in stars and distributed throughout the universe by supernova explosions.

Big Bang: Produced hydrogen, helium, and small amounts of lithium.
Stellar nucleosynthesis: Stars fuse lighter elements into heavier ones (e.g., carbon, oxygen).
Supernovae: Explosions of massive stars create and disperse elements heavier than iron.
"We are made of star stuff": The elements in our bodies originated in stars.

Electron Structure and Chemical Properties

Electron Shells and Orbitals

Electrons in an atom occupy specific energy levels called shells, which are further divided into orbitals. The arrangement of electrons determines an atom's chemical properties.

First shell: Holds up to 2 electrons (1s orbital).
Second shell: Holds up to 8 electrons (2s and three 2p orbitals).
Octet rule: Atoms are most stable when their outermost shell is full (usually 8 electrons).
Electron configuration: Electrons fill the lowest energy orbitals first.
Example: Carbon has 6 electrons: 2 in the first shell, 4 in the second shell.

Importance of Electron Arrangement

The chemical behavior of an atom is determined by the number and arrangement of its electrons, especially those in the outermost shell (valence electrons).

Valence electrons: Electrons in the outermost shell; involved in chemical bonding.
Atoms with incomplete outer shells tend to react to achieve stability.
Example: Sodium (Na) has one valence electron and readily loses it to achieve a full outer shell, forming Na+ ions.

Summary Table: Subatomic Particles

Particle	Charge	Location	Relative Mass
Proton	+1	Nucleus	1
Neutron	0	Nucleus	1
Electron	-1	Electron cloud	1/1836

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard introductory biology textbooks.