Elements Elements are the primary types of matter Matter is anything that has mass and volume Elements can’t be broken down into more simple parts (that retain the properties of the element) An element is made up of only one type of atom Each element has unique physical properties (color, odor, density, melting point, etc.): no two are exactly alike Living things are made up mainly of CHON Each element is represented by a one or two letter symbol (Table 2.2): learn these

The Periodic Table Elements can be arranged into groups based on similar physical and chemical properties These groups are the columns of the periodic table There are 8 main groups, some of which have special names: - Group 1A = alkali metals - Group 2A = alkaline earth metals - Group 7A = halogens - Group 8A = noble gases

Some Group 1A Elements

Some Group 7A Elements

Metals, Nonmetals and Metalloids Elements are classified as metals, nonmetals or metalloids (as shown on periodic table) Each type has different physical properties: Metals: shiny, ductile/malleable, conduct heat and electricity, high m.p. and density Nonmetals: dull, brittle, poor conductors, low m.p. and density Metalloids: have some properties of each, used as semiconductors

The Atom Atoms are the building blocks of all matter An atom is the smallest particle that retains the properties of the element Atoms of different elements can combine to form compounds (discussed in Ch. 4) Chemical reactions involve changes in the atomic make-up of reacting substances (discussed in Ch. 6)

Structure of the Atom Atoms are mostly empty space They have 3 main sub-atomic particles: Protons: positively charged, in nucleus (mass = 1 amu) Neutrons: neutral, in nucleus (mass = 1 amu) Electrons: negatively charged, outside of nucleus (mass is very small) 1 amu is defined as 1/12 the mass of a carbon-12 atom

Atoms and Charge Atoms are always neutral - same number of protons as electrons Like charges repel and opposite charges attract Negative electrons are attracted to positive nucleus (keeps them around it) Positive protons in two nuclei repel each other, but negative electrons are attracted to positive nucleus of neighboring nucleus, so atoms in a substance are kept at an optimum distance from each other (bond length)

Atomic Number and Mass Number All atoms of an element have the same number of protons (and electrons) Atomic number (Z) = number of protons Periodic table is arranged in order of increasing Z Mass number (A) = number of protons plus number of neutrons

Isotopes and Atomic Mass Not all atoms of an element have the same mass number (why?) Because they can have a different number of neutrons Atoms of the same element with different mass numbers are called isotopes Each isotope has a specific isotopic mass (not the same as atomic mass or mass number) The atomic mass of an element, as reported on the periodic table, is a weighted average of all the isotopic atomic masses Atomic symbols are used to show an elements atomic number (subscript) and mass number (superscript)

Calculating the Atomic Mass of an Element

Example: If a sample of chlorine is found to contain 75.8% chlorine-35 and 24.2% chlorine-37, what is the approximate atomic mass of chlorine? % Isotope in Sample x Mass Number = Mass Contribution (75.8% = ) 75.8/100 x 35 =26.5 amu (24.2% = ) 24.2/100 x 37 =8.95 amu Atomic Mass of Chlorine =35.5 amu