Chapter 2

• Elements are composed of extremely small particles called atoms.
• All atoms of a given element are identical, having the same size, mass, and chemical properties. The atoms of one element are different from the atoms of all other elements.
• Compounds are composed of atoms of more than one element. For any given compound, the atoms present are always in the same ratio.
• A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not result in the creation or destruction of atoms.
• Proust’s law of definite proportions states that different samples of the same compound always contain its constituent elements in the same proportion by mass.
• law of multiple proportions. According to the law, if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers
• Proust was the first person to isolate sugar from grapes.
• law of conservation of mass, which is that matter can be neither created nor destroyed.
• atom as the basic unit of an element that can enter into chemical combination.
• atoms actually possess internal structure; that is, they are made up of even smaller particles, which are called subatomic particles.
• In the 1890s, many scientists became caught up in the study of radiation, the emission and transmission of energy through space in the form of waves.
• Chemical reactions usually involve a gain or loss of heat and other forms of energy.
• According to Albert Einstein, mass and energy are alternate aspects of a single entity called mass-energy. Chemical reactions usually involve a gain or loss of heat and other forms of energy. Thus, when energy is lost in a reaction, for example, mass is also lost. Except for nuclear reactions (see Chapter 19), however, changes of mass in chemical reactions are too small to detect. Therefore, for all practical purposes mass is conserved.
• A cathode ray tube with an electric field perpendicular to the direction of the cathode rays and an external magnetic field. The symbols N and S denote the north and south poles of the magnet. The cathode rays will strike the end of the tube at A in the presence of a magnetic field, at C in the presence of an electric field, and at B when there are no external fields present or when the effects of the electric field and magnetic field cancel each other.

  
• According to electromagnetic theory, a moving charged body behaves like a magnet and can interact with electric and magnetic fields through which it passes.
• J. J. Thomson, used a cathode ray tube and his knowledge of electromagnetic theory to determine the ratio of electric charge to the mass of an individual electron. The number he came up with was −1.76 × 10^8 C/g, where C stands for coulomb, which is the unit of electric charge
• Millikan found the charge of an electron to be −1.6022 × 10^−19 C.

  
• In 1895, German physicist Wilhelm Röntgen discovered that cathode rays caused glass and metals to emit a highly energetic form of radiation. These rays could penetrate matter, darken photographic plates, and cause substances to fluoresce. Since they were not deflected by magnets, unlike cathode rays, Röntgen concluded they did not contain charged particles. He named them "X rays" because their nature was unknown at the time.
• Marie Curie, suggested the name radioactivity to describe this spontaneous emission of particles and/or radiation.
• Radioactive decay produces three types of radiation: alpha (α) rays, beta (β) rays, and gamma (γ) rays. Alpha rays consist of positively charged particles (α particles) and are deflected by a positively charged plate. Beta rays are electrons (β particles) and are deflected by a negatively charged plate. Gamma rays (γ rays) are high-energy rays similar to X rays, with no charge, and are unaffected by external fields.

  
• Thomson proposed the "plum-pudding" model of the atom, where the atom is viewed as a uniform, positively charged sphere with electrons embedded throughout, like raisins in a cake.
• nucleus, which is a dense central core within the atom.
• The positively charged particles in the nucleus are called protons. In separate experiments, it was found that each proton carries the same quantity of charge as an electron and has a mass of 1.67262 × 10^−24 g—about 1840 times the mass of the oppositely charged electron.

  
• The mass of a nucleus constitutes most of the mass of the entire atom, but the nucleus occupies only about 1/10^13 of the volume of the atom. We express atomic (and molecular) dimensions in terms of the SI unit called the picometer (pm), where 1 pm = 1 × 10^−12 m
• A typical atomic radius is about 100 pm, whereas the radius of an atomic nucleus is only about 5 × 10^−3 pm.
• When Chadwick bombarded a thin sheet of beryllium with α particles, a very high-energy radiation similar to γ rays was emitted by the metal. Later experiments showed that the rays actually consisted of a third type of subatomic particles. Chadwick named these subatomic particles neutrons, because they proved to be electrically neutral particles with a mass slightly greater than that of protons
• In the helium nucleus there are two protons and two neutrons, but in the hydrogen nucleus there is only one proton and no neutrons; therefore, the ratio is 4:1.
• Familiarize

  
• The atomic number (Z) is the number of protons in the nucleus of each atom of an element.
• In a neutral atom the number of protons is equal to the number of electrons
• The mass number (A) is the total number of neutrons and protons present in the nucleus of an atom of an element.
• Most elements have two or more isotopes, atoms that have the same atomic number (Z) but different mass numbers (A).
• The chemical properties of an element are determined primarily by the protons and electrons in its atoms; neutrons do not take part in chemical changes under normal conditions.
• chemical properties change more markedly across a period due to the increasing nuclear charge and decreasing atomic radius, while chemical properties change less markedly down a group.
• molecule is an aggregate of at least two atoms in a definite arrangement held together by chemical forces (also called chemical bonds)
• ion is an atom or a group of atoms that has a net positive or negative charge.
• cation, an ion with a net positive charge
• anion is an ion whose net charge is negative due to an increase in the number of electrons
• Chemists use chemical formulas to express the composition of molecules and ionic compounds in terms of chemical symbols
• allotrope is one of two or more distinct forms of an element