Elements Element- A pure substance that cannot be broken down into any other substances by ordinary chemical or physical means Elements are pure substances, composed of only one type of atom Can be identified by specific chemical and physical properties 92 elements occur naturally, the rest have been synthetically created in labs There are currently 118 named elements Elements are often called the building blocks of matter because all matter is composed of one element or a combination of two or more elements

Atoms Atoms- The smallest part of an element that has all the properties of that element The basic particle from which all elements are made The building blocks of matter Made up of three main types of particles: Protons- Positively charged particles located in the nucleus Neutrons- Particles with no charge (neutral) found in the nucleus Electrons- Negatively charged particles that whirl around in a cloudlike region beyond the nucleus of the atom

Atomic Structure and Electrical Charge The number of protons (+ charges) in an uncombined atom is equal to the number of electrons (- charges) Uncombined atoms are those that have not been chemically combined with one or more other atoms This means that uncombined atoms are electrically neutral Isotopes- Atoms with the same number of protons but a different numbers of neutrons are called isotopes Isotopes are identified by their mass number (the number of protons and neutrons in the nucleus) Despite the different mass numbers, all isotopes of a particular element react the same way chemically

Molecules Atoms of elements can chemically combine to form larger particles called molecules Molecule- A combination of two or more atoms held together by chemical bonds Represented by chemical formulas (ex.- water H2O) Can be made of two or more of the same kind of atoms (ex.- atmospheric oxygen O2) Can be made of different types of atoms (ex.- water H2O)

Compounds Compound- substance made of atoms of different elements that are chemically combined in a set ratio (ex.- table salt NaCl) When elements are chemically combined, they form compounds having properties that are different from those of the uncombined elements A compound may be represented by a chemical formula, which shows the elements in the compound and the ratio of the atoms When atoms combine, only the electrons in the outermost energy shells combine This combining of elements to form new substances is called chemical bonding

Mixtures Mixture- Matter that is made of two or more substances- elements, compounds, or both- that are together in the same place but are not chemically combined Mixtures differ from compounds in two ways: Each substance in a mixture keeps its individual properties The parts of a mixture are not combined in a set ratio Components can be separated out by normal physical means

The Periodic Table Periodic Table- A table in which the elements are arranged in order of increasing atomic number The table has one square for each element (currently 118) Each square lists the element’s name, chemical symbol, atomic number, and average atomic mass Periodic Vocabulary: Chemical Symbol- A one or two letter representation of an element Atomic number- The number of protons in the nucleus of an atom Average Atomic mass- The average mass of an atom for a particular element, represents the number of protons and neutrons in the nucleus of an atom Usually not a whole number because it is based upon the combined percentages of all its isotopes (Atoms with the same number of protons but a different numbers of neutrons)

Organizing the Periodic Table Properties of an element can be predicted from its location in the periodic table Elements are placed in horizontal rows (called periods) based on atomic number Atomic number increases across a period Both the number of protons and the number of electrons increases by one as you move from one element to the next Elements are placed in vertical columns (called groups or families) based on having the same number of electrons in their outermost energy shell which gives them similar chemical properties

Atoms, Bonding, and the Periodic Table Electron Dot Diagrams The valence electrons of an atom are shown as dots around the symbol of the element. Here is a completed electron dot diagram for neon. Valence electrons- Electrons at higher energy levels that are involved in chemical bonding

Atoms, Bonding, and the Periodic Table 1 8 Periodic Table of the Elements The periodic table is arranged in order of increasing atomic number. The number of valence electrons also increases from left to right across a period. What is the number of valence electrons for each group? 4 2

Ionic Bonds another atom - Create ions (charged particles) Ionic bond- Chemical bond where one atom gives one of its electrons to another atom - Create ions (charged particles) How Ions Form An atom that loses one of its electrons becomes a positively charged ion. The atom that gains the electron becomes a negatively charged ion.

Ionic Bonds Formation of an Ionic Bond Follow the steps to see how an ionic bond forms between a sodium atom and a chlorine atom. Complete the electron dot diagrams.

Covalent Bonds Covalent Bonds- Chemical bonds that form when atoms share elwctrons - Atoms can form single, double, and triple covalent bonds by sharing one or more pairs of electrons.

Covalent Bonds Sharing Electrons By sharing 2 electrons in a covalent bond, each fluorine atom gains a stable set of 8 valence electrons. Which are the shared electrons that form a covalent bond between the 2 fluorine atoms?