Classification of Matter and Atomic Structure

1.2 Classifying Matter

Definition and Classification of Matter

Matter is defined as anything that has mass and occupies space. Understanding how matter is classified is fundamental in chemistry, as it helps distinguish between different types of substances and their properties.

• Elements: Pure substances consisting of only one type of atom. Examples include gold (Au) and oxygen (O2).

• Compounds: Pure substances composed of two or more different elements that are chemically bonded in a fixed ratio. For example, water ($H_2O$) is made of hydrogen and oxygen atoms bonded together.

• Mixtures: Physical combinations of two or more substances that are not chemically bonded. Mixtures can be further classified as:

  • Homogeneous Mixtures: Have a uniform composition throughout. An example is salt dissolved in water, forming a solution.

  • Heterogeneous Mixtures: Have a non-uniform composition; the different components are visible and can often be separated physically. An example is sand mixed with water.

States of Matter at the Particle Level

The physical state of matter is determined by the arrangement and movement of its particles:

• Solids: Particles are tightly packed in a fixed, orderly arrangement. They vibrate in place but do not move freely.

• Liquids: Particles are close together but not in a fixed position; they can move and slide past one another, allowing liquids to flow.

• Gases: Particles are far apart and move randomly and rapidly, filling the volume of their container.

Example: Ice (solid), liquid water, and water vapor (gas) all consist of $H_2O$ molecules, but their particle arrangements and movement differ.

1.7 – 1.8 Atomic Structure

The Nuclear Model of the Atom

Atoms are the basic units of matter, composed of three main subatomic particles: protons, neutrons, and electrons. The arrangement and properties of these particles determine the identity and characteristics of each element.

Subatomic Particle	Relative Charge	Relative Mass	Location
Proton	+1	~1 amu	Nucleus
Neutron	0	~1 amu	Nucleus
Electron	-1	~0 amu	Orbiting the nucleus

Atomic Number, Mass Number, and Atomic Mass

• Atomic Number (Z): The number of protons in the nucleus of an atom. It uniquely identifies an element. Example: Carbon has $Z = 6$.

• Mass Number (A): The total number of protons and neutrons in an atom's nucleus. $A = Z + N$ where $N$ is the number of neutrons.

• Atomic Mass: The weighted average mass of all naturally occurring isotopes of an element, usually expressed in atomic mass units (amu). $\text{Atomic Mass} = \sum (\text{fractional abundance} \times \text{isotope mass})$

Example: Chlorine has two main isotopes: $^{35}Cl$ and $^{37}Cl$. The atomic mass listed on the periodic table reflects the weighted average of these isotopes.