Elements, Compounds, and Mixtures: An Atomic Overview
An element consists of only one kind of atom. An element is one kind of substance – the composition is fixed. Each element is unique because the properties of its atoms are unique. Molecules are structures containing two or more atoms chemically bound together. Diatomic molecules are two-atom molecules such as elemental oxygen. A compound is a type of matter in which two or more different elements are chemically bonded – the elements are present in fixed parts by mass (fixed mass ratios) – therefore it is also considered a substance because the composition is fixed. Properties of a compound are different than those of its component elements. A mixture is a group of two or more substances that are physically intermingled. They can very in their parts by mass, unlike compounds. They are not substances and retain the properties of their components. Can be separated by physical changes. FUP2.1: Diatomic molecules undergo a chemical change into compounds (two different elements). The Observations That Led to an Atomic View of Matter
Law of mass conservation states the total mass of substances do not change during a chemical reaction. The number of substances (and properties) may change, but the total amount of matter remains the same. Matter cannot be created nor destroyed. Definite Composition
Law of definite (or constant) composition states that a compound is composed of the same elements in the same parts (fractions) by mass or the part of the compounds mass which contributes to the compound’s total mass.
FUP2.2: 0.34t oxygen
Law of multiple proportions: if elements A and B react to form two compounds, the different masses f B that combine with a fixed mass of A can be express as a ratio of small whole numbers. Dalton’s Atomic Theory
Postulates of the Theory
1.All mater consists of atoms – indivisible particles which cannot be created nor destroyed. 2.Atoms of one element cannot be converted into other elements. 3.Atoms of an element are identical in mass and other properties. 4.Compounds result from the chemical combinations of a specific ratio of atoms of different elements. Explanation of Mass Laws
Mass conservation – atoms cannot be created nor destroyed (1) or converted to other elements (2). Each atom has a fixed mass, chemical reactions simply combine atoms into different substances (3). Definite composition – compounds are a combination of a specific ratio of different atoms (4) each which have a particular mass (3). Each element in a compound constitutes a fixed fraction of total mass. Multiple proportions – atoms of an element have the same mass (3) and are indivisible (1). Masses of element B that combine with a fixed mas of element A give a small, whole-number ratio since different numbers of B atoms combine with each A atom in different compounds. FUP2.3 (B) Two types of compounds exists with a definite ratio of elements A to elements B.
The Observations That Led to the Nuclear Atom Model
Dalton’s model explained the idea that masses of reacting elements could be explained in terms of atoms. Research in electricity led to the discovery of electrons – negatively charged particles. Discovery of the Electron
Cathode rays consist of negatively charged particles. Thompson discovered the existence of electrons by measuring the ratio of charged particles to hydrogen atoms. Millikian used an oil can to produce charged particles which were controlled by variably controlled charged plates. He concluded that the various charges of the droplets were a whole number multiple of a minimum charge. He also determined the charge of the electron and its mass. Discovery of the Nucleus
Rutherford concluded through his gold foil experiment that an atom is mostly space occupied by electrons, but in the center (nucleus) lies the mass of the atom. Chadwick discovered the neutron, an...