💏Intro to Chemistry Unit 2 Review

The idea that matter is made of tiny, indivisible particles goes back over 2,000 years. But it took centuries of experimental work before that idea became a testable scientific theory. John Dalton's atomic theory, developed in the early 1800s, was the first to connect the concept of atoms to actual chemical observations, and it laid the foundation for modern chemistry.

Ancient Origins

In the 5th century BCE, Greek philosophers Democritus and Leucippus proposed that all matter is made of tiny, indivisible units they called atomos (meaning "uncuttable"). This was a philosophical idea, not based on experiments, but it planted the seed for later scientific work.

Building Toward Dalton

Several key figures set the stage for Dalton's theory:

Robert Boyle (17th century) helped define what an element actually is and distinguished elements from compounds.
Antoine Lavoisier (18th century) established the law of conservation of mass through careful experiments, showing that mass is neither created nor destroyed in chemical reactions.
Joseph Proust (early 19th century) proposed the law of definite proportions based on experimental evidence, showing that compounds always contain the same elements in the same mass ratios.

Dalton's Atomic Theory

Dalton pulled these observations together into a coherent theory. His key postulates:

All matter is composed of tiny, indivisible particles called atoms.
Atoms of the same element are identical in mass and properties.
Atoms of different elements have different masses and properties.
Atoms cannot be created, divided, or destroyed in chemical reactions.
Compounds form when atoms of different elements combine in simple, whole-number ratios (for example, water is $H_2O$ , carbon dioxide is $CO_2$ ). These ratios can be represented using chemical symbols.

A quick note: we now know some of these postulates aren't perfectly accurate. Atoms can be split (nuclear reactions), and atoms of the same element can differ in mass (isotopes). But for understanding chemical reactions, Dalton's model was a huge step forward.

Laws Explained by Dalton's Theory

Law of Definite Proportions (Law of Constant Composition)

A pure compound always contains the same elements in the same proportions by mass, no matter where it comes from or how it was made. For example, water ( $H_2O$ ) always contains hydrogen and oxygen in a 1:8 mass ratio. Whether you collect water from a river or produce it in a lab, that ratio stays the same.

Law of Multiple Proportions

When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers. Here's how that works with carbon and oxygen:

In carbon monoxide ( $CO$ ): 12 g of carbon combines with 16 g of oxygen
In carbon dioxide ( $CO_2$ ): 12 g of carbon combines with 32 g of oxygen
The ratio of the oxygen masses (16 to 32) simplifies to 1:2, a small whole-number ratio

This pattern is exactly what Dalton's theory predicts: atoms combine in whole-number ratios, so the mass differences between compounds of the same elements follow whole-number patterns too.

Atomic Structure and Properties

Dalton's theory opened the door to later discoveries about what's inside atoms. A few terms you'll need going forward:

Atomic number: the number of protons in an atom's nucleus. This determines which element it is.
Atomic mass: the total mass of protons, neutrons, and electrons in an atom (though electrons contribute very little).
Valence: an atom's combining capacity, related to how its electrons are arranged.
Periodic table: a systematic arrangement of elements organized by atomic number and recurring chemical properties.

You'll explore subatomic particles and the periodic table in more detail later in this unit. For now, the key takeaway is that Dalton's theory gave chemistry its first real framework for understanding how and why elements combine the way they do.

💏Intro to Chemistry Unit 2 Review