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Atoms, Elements, Compounds, and Mixtures Structure of the Atom Atoms are the smallest All atoms share the same particles into which matter can basic structure. Over the be divided and still maintain its years, scientists have characteristics. designed many different models for this structure. Atoms are the building blocks of the universe. There are 92 Each one was the best different kinds of atoms that model at the time, but as occur naturally, although more new evidence was are man-made. discovered, the models were revised to fit it. These building blocks can be joined together to create all of Scientists will probably revise the different kinds of matter we the current model as more can observe. evidence becomes available. What’s inside of an atom? ? Early Atomic Models The Greek scientist Democritus was the first person to say that all matter is made of atoms, but he also said that atoms are indivisible. All mater is made of atoms, and atoms of an element are alike. Different elements have different atoms. + = “Atoms are indivisible.” - Democritus In the early 1800’s, the English scientist John Dalton developed his Atomic Theory of Matter. It said four things: (1) All matter is made of atoms. (2) Atoms of an element are alike. (3) Atoms of different elements are different. (4) Atoms of different elements join together to make compounds. Atoms join to make compounds. - John Dalton In the late 1800’s, J. J. Thomson discovered that atoms were made of smaller particles. He discovered the electron and devised the “Plum Pudding” model of the atom. Positivelycharged matrix Negativelycharged electron - J. J. Thomson Recent Atomic Models In the early 1900’s, scientists discovered the positively-charged proton. In 1911, Rutherford discovered the nucleus of the atom, where protons are located. Atoms are mostly empty space. Electrons orbit the nucleus. - Niels Bohr Positively-charged protons are in the nucleus. - Rutherford Niels Bohr changed Rutherford’s model in 1913. Bohr said that electrons move around the nucleus in fixed energy levels. Electrons can move from one energy level to another. Bohr’s model has been called the “Solar System” model of the atom. Negative electron cloud The current model of atomic structure is called the “electron cloud” model. Electrons move within an energy level in an ever-changing path, not a fixed orbit. Neutrons bond with protons in the nucleus. Neutral neutrons are in nucleus with positive protons. - Electron Cloud Model Atomic Number and Mass Number All atoms of an element have the Atoms of the same element may same number of protons, and have different numbers of different elements have different neutrons. Atoms with extra or numbers of protons in their missing neutrons are called atoms. isotopes. The atomic number of an atom is An atom’s mass number is the the number of protons it has. If number of protons plus neutrons. the atom is neutral, the atomic number is also the number of Different isotopes of an element electrons. have different mass numbers. Atomic number of this neutral atom = 6. This means that it has 6 protons and 6 electrons. Mass number of this neutral atom is 14. This is the sum of the protons plus neutrons. Since there are 6 protons, there must be 14 - 6 = 8 neutrons. Elements An element is matter that is made of Chemical symbols are only one kind of atom. All of the abbreviations for the names of atoms of an element are alike. There elements. You can also use a are 92 naturally occurring elements. chemical symbol to represent one atom of an element. Elements can be identified by their properties. Some properties are color, Chemical symbols are the texture, density, malleability, ductility, same all over the world, no ability to dissolve in water, and ability matter what language is to conduct heat or electricity. spoken. The first letter is always capitalized, and if there At room temperature, most elements is a second letter it is never are solids, although a few are liquids capitalized. and some are gases. H Na Chemical symbol for the element Hydrogen, a flammable gas. Chemical symbol for the element Sodium, used in sodium vapor lamps. Chemical symbol for the element Chlorine, used in bleach and water treatment. Cl O Chemical symbol for the element Oxygen, needed for life. Compounds The atoms of most elements are very reactive. They chemically join together with atoms of other elements to form compounds. The properties of compounds are different than the properties of the elements that make the compound. When elements join to make compounds, they always join in the same proportion. H2O is water, not HO or H3O ! H is symbol for Hydrogen H2O Subscript 2 means that there are 2 Hydrogen atoms Water molecule O is symbol for Oxygen No subscript means one Oxygen atom Compounds are separated into two groups, depending on how their atoms are joined: (1) Molecular compounds are made of atoms that share electrons. (2) Ionic compounds are made of atoms that gain or lose electrons. Chemical formulas represent compounds. They show the kind and number or atoms in the compound. N is symbol for Nitrogen NH3 No subscript means one Nitrogen atom H is symbol for Hydrogen Subscript 3 means there are 3 Hydrogen atoms Ammonia Molecule Mixtures Mixtures are made when there are at least two different kinds of atoms that are in the same place but are not chemically combined. A mixture can be made with any proportion of ingredients, but a compound must be made in only one certain way. Components of a mixture keep their own properties, but those of a compound do not. Components of a mixture are not chemically combined, so they can be separated by physical means. This coffee is a mixture. This cake is a mixture. This basket of fruit is also a mixture. Types of Mixtures A homogeneous mixture is the same throughout. All parts of the mixture have the same proportion of ingredients. A heterogeneous mixture is different in different parts. Different regions of this kind of mixture have different proportions of the ingredients. An alloy is a homogeneous mixture of metals. All solutions are homogeneous mixtures. This cookie is heterogeneous. Soft drinks are homogeneous mixtures. The alloy these keys are made of is a homogeneous mixture. The crayons in the pail make a heterogeneous mixture. The milk is homogeneous. The gelatin mold is a heterogeneous mixture.