CH 2: Atoms, Ions, & Molecules

Matter, Atoms, Elements, and the Periodic Table

Introduction to Matter

Matter is a fundamental concept in anatomy and physiology, as all living and non-living things are composed of matter. Understanding the forms and properties of matter is essential for studying the chemical basis of life.

• Matter: Anything that has mass and occupies space.

• Forms of Matter:

  • Solid: Has a definite shape and volume (e.g., bone).

  • Liquid: Has a definite volume but takes the shape of its container (e.g., blood).

  • Gas: Has neither definite shape nor volume (e.g., oxygen).

Atoms and Elements

An atom is the smallest unit of an element that retains the chemical properties of that element. Elements are pure substances composed of only one type of atom and are organized in the periodic table.

• Atom: The smallest particle exhibiting chemical properties of an element.

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

• There are 92 naturally occurring elements, which make up all matter.

• Elements are organized in the periodic table of elements.

The Periodic Table of Elements

The periodic table arranges elements by increasing atomic number and groups them based on similar chemical properties. It is a key reference for understanding the elements found in the human body.

• Atomic number: Number of protons in an atom; unique to each element.

• Chemical symbol: Usually the first letter or first two letters of the element's name (e.g., C for carbon).

• Average atomic mass: The mass of both protons and neutrons in the atom.

Common Elements of the Human Body

The human body is primarily composed of a few major elements, with several minor elements present in smaller amounts. These elements are essential for physiological processes.

Major Elements	% Body Weight	Minor Elements	% Body Weight
Oxygen (O)	65.0	Sulfur (S)	0.25
Carbon (C)	18.5	Potassium (K)	0.20
Hydrogen (H)	9.5	Sodium (Na)	0.15
Nitrogen (N)	3.0	Chlorine (Cl)	0.15
Calcium (Ca)	1.5	Magnesium (Mg)	0.05
Phosphorus (P)	1.0	Iron (Fe)	0.006

Major elements collectively compose almost 99% of body weight, while minor elements collectively compose less than 1%.

Components of an Atom

Subatomic Particles

Atoms are composed of three types of subatomic particles, each with distinct properties and roles in atomic structure.

• Neutrons:

  • Mass of one atomic mass unit (amu).

  • No charge (neutral).

• Protons:

  • Mass of one amu.

  • Positive charge of one (+1).

• Electrons:

  • 1/1800th the mass of a proton or neutron.

  • Negative charge of one (−1).

  • Located at varying distances from the nucleus in regions called orbitals.

Determining the Number of Subatomic Particles

The number of protons, neutrons, and electrons in an atom can be determined using the atomic number and atomic mass.

• Proton number = atomic number

• Neutron number = atomic mass − atomic number

• Electron number = proton number (in a neutral atom)

• Example: Sodium (Na) has atomic number 11 and atomic mass 23. Neutron number = 23 − 11 = 12

Electron Shells

Electrons are arranged in shells around the nucleus, each with a specific energy level and capacity.

• First shell: Holds up to 2 electrons.

• Second shell: Holds up to 8 electrons.

• Shells closest to the nucleus must be filled first.

Orbitals are regions within shells where electrons are most likely to be found.

Summary Table: Subatomic Particles

Particle	Mass (amu)	Charge	Location
Proton	1	+1	Nucleus
Neutron	1	0	Nucleus
Electron	1/1800	−1	Orbitals (outside nucleus)

Example: The structure of the atom and the distribution of elements in the human body are foundational for understanding physiological processes such as cellular respiration, muscle contraction, and nerve impulse transmission.