Atomic Structure Matter is made of tiny particles called atoms. There are three basic types of particles that make up an atom: Protons Neutrons Electrons The dense center of the atom is called the nucleus. The particles that are located in the nucleus are protons and neutrons. Protons are particles that have a positive charge. Neutrons have no charge and are therefore neutral. The electrical charge of the nucleus is positive. The nucleus contains most of the atom’s mass. The negatively charged particles of an atom that exist outside the nucleus are called electrons. They travel in an orbit usually close to the nucleus called an electron cloud. The structure of the different types of atoms and how they are joined together (how they are bonded to each other) determine what the substance is. Example: graphite, coal and diamonds are all made of atoms of carbon. An element is a substance that is made of only one type of atom and cannot be broken down into simpler substances by normal chemical or physical means. The number of protons in an atom will determine what kind of element it is. All atoms of the same element have the same number of protons. Hydrogen and Helium atoms Carbon atom Periodic Table of the Elements The periodic table is a listing of all the known elements that exist in the world. The number of protons in an atom is equal to the atomic number of the element. You will find it above the element’s symbol on the table. When you look at the table you will notice that the elements are listed in order of their atomic number from left to right. KEY Metals Semimetals Nonmetals A neutral atom is one that has an equal number of protons and electrons. The overall charge is therefore neutral. In a neutral atom: Atomic number = # of protons = # of electrons The atomic mass of an atom is equal to the number of protons plus the number of neutrons. You can determine the number of neutrons by subtracting the number of protons from the atomic mass. Atomic Structure of 14 Elements Element Symbol Atomic Mass Element Symbol Atomic Mass Number Number Number Number Hydrogen H 1 1 Calcium Ca 20 40 Helium He 2 4 Iron Fe 26 56 Oxygen O 8 16 Sulfur S 16 32 Carbon C 6 12 Sodium Na 11 23 Neon Ne 10 20 Chlorine Cl 17 35 Nitrogen N 7 14 Potassium K 19 39 Magnesium Mg 12 24 Argon Ar 18 40 Energy Levels An energy level represents the area in an atom where an electron is likely to be found. Each energy levels can hold only a limited number of electrons. The smallest, innermost energy level can hold only two electrons. The second energy level can hold up to eight electrons. The third energy level can hold up to 8 electrons. The 4th & 5th can hold up to 18 electrons. The 6th can hold up to 32 electrons. Electrons tend to occupy the lowest available energy level. The first energy level in an aluminum atom is filled by two electrons. The second energy level is also filled, by eight electrons. The third energy level has only three electrons, so it is not filled. The number of electrons in the outermost energy level determines the chemical behavior of the different elements. These outermost electrons are called valence electrons. Elements with the same number of valence electrons have similar chemical properties. For example, a sodium (Na) atom, with the atomic number 11 and a potassium (K) atom, with the atomic number 19, both have just one valence electron. Both sodium and potassium are highly reactive metals, which means that they combine easily with other elements. Elements such as helium (He), neon (Ne) and argon (Ar) are inert, which means that they do not easily combine with other elements. This is because they have full outermost energy levels. Classifying Elements Each element has a name and a symbol. The symbol is an abbreviated , or shortened version, of the element’s name. Aurum is the Latin name for gold. It is used to represent the element in chemical formulas and equations. The elements in the Periodic Table are arranged so that elements with similar properties are close together. Example: fluorine (F) and chlorine (Cl) are gases that react very easily with other elements, so they are close together in the table. The Periodic Table gets its name from the fact that the elements’ properties repeat themselves every few elements, or periodically. The vertical columns of elements are called groups. Some groups have special names: Group 1: The alkali metals Group 2: The alkaline earth metals Group 17: The halogens Group 18: The noble gases – unreactive, so they almost never react with other elements to make compounds. The Periodic Table Column = Group or Family 18 columns on the Periodic Table Row = Period 7 rows on the Periodic Table The horizontal rows of elements are called periods. The table also shows which elements are metals, non-metals, and metalloids. Metals All metals look shiny. Most metals are very hard (a few so soft you can cut them with a knife). Metals are also malleable, meaning that they can be bent, or beaten into different shapes without breaking. All metals except mercury are solids at room temperature because they generally have high melting and boiling points. All metals let heat and electricity pass through them easily, so they are good conductors of heat and electricity. Only a few metals are magnetic (ex. - iron). Nonmetals At room temperature most nonmetals are gases, some are solids and one (bromine) is a liquid. This is because nonmetals have a wide range of melting and boiling points. Nonmetals are not good conductors of electricity or heat (with the exception of carbon, which conducts electricity as well as a metal). All nonmetals are nonmagnetic. Metalloids A few elements, such as silicon, have some of the properties of metals and some of the properties of nonmetals. Since they are not clearly one or the other, they are called metalloids, or semimetals. They are used in making semiconductors – materials which can conduct some electricity better than an insulator can, but not as well as a metal. They are used in making electronic components and microchips. Compounds A compound is a substance that is made up of different elements. The atoms of the elements are joined together by chemical bonds. Bonds are chemical connections between atoms. Types of Bonds There are two basic types of bonds: Covalent Ionic The reason that bonds occur is that atoms “want” to have full outer electron shells. An atom gets a full outer shell by getting involved in one of the two types of bonds. Covalent Bonds Sharing one of more electrons is one type of bonding called a covalent bond. Example: water is formed when atoms of hydrogen share their electrons with an oxygen atom. A molecule is the smallest unit of a compound that has all the properties of the compound. Ionic Bonds The second type of bonding is when some atoms give up electrons, or ionic bonding. Example: salt is formed when the sodium gives up its electron and the chlorine takes it. An atom like this that has lost or gained an electron is called an ion. We write a + next to the element’s symbol if its atom has lost an electron and a – if it gains an electron. For example: Na becomes Na+ because it has more positive charge than negative and Cl becomes Cl- Opposites attract! The protons in the nucleus “miss” the “missing” electron and they stay close to it forming the bond. If two types of atoms bond together using either method, the result is a compound. In a chemical change, bonds are made or broken, which leads to a new substance, or substances being created. In a physical change, such as making a mixture or dissolving something, no bonds are affected. A mixture is a combination of Mixtures things where there are no chemical changes or bonds. There are two types of mixtures: Heterogeneous: Not mixed evenly Each component retains its own properties Examples: pizza, cereal and milk, rocks in the sand at the beach, banana splits Homogeneous: Mixed evenly throughout Properties are often different from their components Examples: milk, kool-aid, blood, lotion, window cleaner, glue A solution is an evenly mixed mixture (liquid), such as salt water. Mixtures can be separated by physical means. An isotope is an atom that has the same number of protons as another atom, but has a different number of neutrons. Atoms of an element can lose or gain electrons and still be the same element, but the atom is no longer neutral. If the number of electrons is less than the number of protons, the atom will have a positive charge. Atoms with more electrons have a negative charge. Atoms of the same element with a different number of electrons are called ions. gases!
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