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Atoms, Molecules and Ions Study Guide

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Updated on Sep 24, 2011

Introduction

John Dalton's atomic theory explains the solar system-type model of an atom with electrons orbiting around a compact nucleus with protons and neutrons. Atoms make up the elements, molecules, and compounds that ultimately create chemical processes. Each substance has its own unique name and molecular formula to describe its chemical properties.

Dalton's Atomic Theory

In 1808, John Dalton published A New System of Chemical Philosophy, which proposed his hypotheses about the nature of matter. Dalton's atomic theory explained that

  • all elements are made of tiny, indivisible particles called atoms (from the Greek atomos, meaning indivisible).
  • atoms of one element are identical in size, mass, and chemical properties.
  • atoms of different elements have different masses and chemical properties.
  • compounds are made up of atoms of different elements in a ratio that is an integer (i.e., whole number) or a simple fraction.
  • atoms cannot be created or destroyed. They can be combined or rearranged in a chemical reaction.

Subsequent experiments, notably those of J.J. Thomson (discoverer of the electron), E. Rutherford (who established that the atom was made of a dense, central core called a nucleus, positively charged by protons, and separated from moving electrons by empty space), and others such as A. Becquerel and Marie Curie (on the spontaneous disintegration of some nucleus with the emission of particles and radiation), were necessary, however, to complete the understanding of atoms.

The atomic weight (or mass) of an element is given by the weighted average of the isotopes' masses. Isotopes are atoms of an element that have different masses.

Dalton's atomic theory allowed scientists to understand and formulate three laws based on Dalton's hypothesis:

1. Law of conservation of mass: The law of conservation of mass is derived from Dalton's fifth hypothesis and states that mass cannot be created or destroyed. If the mass of the combined reactants is 20 grams, then the mass of the combined products must be 20 grams.
2. Law of definite proportions: The law of definite proportions is derived from Dalton's fourth hypothesis and states that different samples of the same compound always contain the same proportion by the mass of each element.Water (H2O) always has a ratio of 2 grams of hydrogen to 16 grams of oxygen regardless of the sample size.
3. Law of multiple proportions: The law of multiple proportions is also derived from Dalton's fourth hypothesis and states that if two elements combine to form multiple compounds, the ratio of the mass of one element combined with the 1 gram of the other element can always be reduced to a whole number. Hydrogen can combine with oxygen in two ways: water (H2O) and hydrogen peroxide (H2O2). The ratio of oxygen is 1:2.

Isotopes

Elements are defined by their atomic number. An element's atomic number is the number of protons in the atom and is sometimes written as a subscript of the elemental symbol (i.e., 11Na). Because the mass number defines the elemental symbol (sodium always has 11 protons and carbon always has 6 protons), the atomic number is frequently omitted.

Also important is the mass number of an element. The mass number is the sum of protons and neutrons (in the nucleus) of the atom and is written as a superscript of the element's symbol (23Na).

Isotopes are atoms of the same element with the same number of protons (same atomic number) but different number mass numbers (due to a different number of neutrons). Isotopes have identical chemical properties (the same reactivity) but different physical properties (i.e., some are radioactive, while others are stable). Consider the three isotopes of hydrogen in Table 2.1.

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