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# 2: Atoms and Elements

• 2.2: Early Ideas about the Building Blocks of Matter
The ancient Greeks proposed that matter consists of extremely small particles called atoms. Dalton postulated that each element has a characteristic type of atom that differs in properties from atoms of all other elements, and that atoms of different elements can combine in fixed, small, whole-number ratios to form compounds. Samples of a particular compound all have the same elemental proportions by mass.
• 2.3: Modern Atomic Theory and the Laws That Led to It
Dalton postulated that each element has a characteristic type of atom that differs in properties from atoms of all other elements, and that atoms of different elements can combine in fixed, small, whole-number ratios to form compounds. Samples of a particular compound all have the same elemental proportions by mass.
• 2.4: The Discovery of the Electron
Atoms, the smallest particles of an element that exhibit the properties of that element, consist of negatively charged electrons around a central nucleus composed of more massive positively charged protons and electrically neutral neutrons. Radioactivity is the emission of energetic particles and rays (radiation) by some substances. Three important kinds of radiation are α particles (helium nuclei), β particles (electrons traveling at high speed), and γ rays.
• 2.5: The Structure of The Atom
An atom consists of a small, positively charged nucleus surrounded by electrons. The nucleus contains protons and neutrons; its diameter is about 100,000 times smaller than that of the atom. The mass of one atom is usually expressed in atomic mass units (amu), which is referred to as the atomic mass. An amu is defined as exactly $$1/12$$ of the mass of a carbon-12 atom and is equal to 1.6605 $$\times$$ 10−24 g.
• 2.6: Protons, Neutrons, and Electrons in Atoms
• 2.7: Finding Patterns - The Periodic Law and the Periodic Table
The periodic table is used as a predictive tool that arranges of the elements in order of increasing atomic number. Elements that exhibit similar chemistry appear in vertical columns called groups (numbered 1–18 from left to right); the seven horizontal rows are called periods. The elements can be broadly divided into metals, nonmetals, and semimetals. Semimetals exhibit properties intermediate between those of metals and nonmetals.
• 2.8: The Average Mass of an Element’s Atoms
The mass of an atom is a weighted average that is largely determined by the number of its protons and neutrons, and the number of protons and electrons determines its charge. Each atom of an element contains the same number of protons, known as the atomic number (Z). Neutral atoms have the same number of electrons and protons. Atoms of an element that contain different numbers of neutrons are called isotopes. Each isotope of a given element has the same atomic number, but different mass number.
• 2.9: Molar Mass - Counting Atoms by Weighing Them
The chemical changes we observe always involve discrete numbers of atoms that rearrange themselves into new configurations. These numbers are far too large in magnitude for us to count , but they are still numbers, and we need to have a way to deal with them. We also need a bridge between these numbers, which we are unable to measure directly, and the weights of substances, which we do measure and observe. The mole concept provides this bridge, and is key to all of quantitative chemistry.
• 2.E: Atoms and Elements (Exercises)
These are homework exercises to accompany the Textmap created for Chemistry: A Molecular Approach by Nivaldo Tro.