Atoms, Elements, Compounds, and Mixtures

Structure of the Atom  Atoms are the smallest particles into which matter can be divided and still maintain its characteristics.  Atoms are the building blocks of the universe. There are 92 different kinds of atoms that occur naturally, although more are man-made.  These building blocks can be joined together to create all of the different kinds of matter we can observe.  All atoms share the same basic structure. Over the years, scientists have designed many different models for this structure.  Each one was the best model at the time, but as new evidence was discovered, the models were revised to fit it.  Scientists will probably revise the current model as more 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.  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.  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. “Atoms are indivisible.” - Democritus Different elements have different atoms. += Atoms join to make compounds. - John Dalton Positively- charged matrix Negatively- charged electron - J. J. Thomson All mater is made of atoms, and atoms of an element are alike.

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.  Niels Bohr changed Rutherford’s model in 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.  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. Positively-charged protons are in the nucleus. - Rutherford Electrons orbit the nucleus. - Niels Bohr Negative electron cloud Neutral neutrons are in nucleus with positive protons. - Electron Cloud Model

Atomic Number and Mass Number  All atoms of an element have the same number of protons, and different elements have different numbers of protons in their atoms.  The atomic number of an atom is the number of protons it has. If the atom is neutral, the atomic number is also the number of electrons.  Atoms of the same element may have different numbers of neutrons. Atoms with extra or missing neutrons are called isotopes.  An atom’s mass number is the number of protons plus neutrons.  Different isotopes of an element 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 = 8 neutrons.

Elements  An element is matter that is made of only one kind of atom. All of the atoms of an element are alike. There are 92 naturally occurring elements.  Elements can be identified by their properties. Some properties are color, texture, density, malleability, ductility, ability to dissolve in water, and ability to conduct heat or electricity.  At room temperature, most elements are solids, although a few are liquids and some are gases.  Chemical symbols are abbreviations for the names of elements. You can also use a chemical symbol to represent one atom of an element.  Chemical symbols are the same all over the world, no matter what language is spoken. The first letter is always capitalized, and if there is a second letter it is never capitalized. Na Chemical symbol for the element Sodium, used in sodium vapor lamps. Chemical symbol for the element Chlorine, used in bleach and water treatment. Cl H Chemical symbol for the element Hydrogen, a flammable gas. 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. H 2 O is water, not HO or H 3 O !  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. H is symbol for Hydrogen Subscript 2 means that there are 2 Hydrogen atoms No subscript means one Oxygen atom N is symbol for Nitrogen No subscript means one Nitrogen atom H is symbol for Hydrogen Subscript 3 means there are 3 Hydrogen atoms NH3NH3 Ammonia Molecule H2OH2O Water molecule O is symbol for Oxygen

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. Soft drinks are homogeneous mixtures. This cookie is heterogeneous. The alloy these keys are made of is a homogeneous mixture. The milk is homogeneous. The crayons in the pail make a heterogeneous mixture. The gelatin mold is a heterogeneous mixture.