Pure Substances • elements: (a) made of one kind of atoms, the simplest building blocks of matter (b) use the name or atomic symbol (Ex) hydrogen = H oxygen = O (2) compounds: (a) made of two or more different atoms chemically combined – molecules or formula units (b) use the name or chemical formula (Ex) water = dihydrogen monoxide = H2O potassium chloride = KCl
Atomic Symbols • Memorize the names and symbols of elements, Pg 74 • Unusual symbols • Cu = cuprum = copper • Au = aurum = gold • Ag = argentum = silver • Fe = ferrum = iron • Pb = plumbum = lead • Hg = hydrargyrum = mercury • K = kalium = potassium • Na = natrium = sodium • Sn = stannum = tin • W = wolfram = tungsten
Compounds to Elements Requires high energy such as electricity or UV light (Ex) H2O hydrogen gas and oxygen gas
Chemical Changes • Also called chemical reactions • Evidences for chemical changes • color change • heat released (exothermic) or absorbed (endothermic) = temperature change • precipitate form • gas formation • All chemical and physical changes must obey the law of conservation of mass
Postulates of Dalton’s Atomic Theory *postulate: 1) All elements are composed of indivisible particles called atoms. 2) Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 3) Atoms of different elements chemically combine in simple whole-number ratios to form compounds. (Ex) 2 hydrogen atoms combined with 1 oxygen atom = water (Ex) 2 hydrogen atoms combined with 2 oxygen atoms = hydrogen peroxide
4) Chemical reactions occur when atoms are separated, joined, or rearranged. (Ex) H2O H2 + O2 • Atoms of one element are never changed into atoms of another element as a result of a chemical reaction. (Ex) H2O H2 + O2 H and O elements in H and O elements in reactant in products
Atom • means “indivisible” in Greek *Greek philosopher Democritus (460 B.C.–370 B.C.) was among the first to suggest the concept of atom. • is the smallest particle of matter (1) There are about 120 different kinds of atoms (2) The collection of the same kind of atoms is called an element (Naturally, there are about 120 different elements) • All atoms have the same subatomic particles – neutrons, protons, and electrons *Subatomic means smaller than an atom (1) Protons and neutrons give the mass to an atom (2) Electrons give the volume to an atom
Only the number of protons (= atomic number) determines the element (Ex) atoms with 1 proton = hydrogen element atoms with 2 protons = helium element atoms with 3 protons = lithium element • The elements don’t change in chemical reactions *Means that the nucleus of an atom doesn’t change in chemical reactions • The radius of an atom is about 5 × 10−11 m (= 0.05 nm) to 2 × 10−10 m (0.2 nm) *Nanotechnology deals with particles composed of a few atoms * Atoms can be seen with a scanning tunneling microscope
Subatomic Particles *Why is a neutron heavier than a proton?
Cathode Ray Tube by J J Thomson Rutherford's Gold Foil
Describing an element 1) atomic symbol • one or two-letter abbreviation of the name (Ex) carbon = C; gold = Au 2) atomic number (Z) = the number of proton • Obtain from the periodic table – always the whole number • Written lower left of the atomic symbol (Ex) 6C (Ex) Find the atomic number of Cu, Ar, F, and K.
3) (atomic) mass number (A) = the number of protons + the number of neutrons • Describes what’s in the nucleus • Obtain from the periodic table by rounding the decimal number to the nearest whole number (Ex) With carbon, round 12.0107 to 12 • Written upper left of the atomic symbol (Ex) 4)Alternative way: name of element – atomic mass number (Ex) carbon-12 (Ex) What is the (atomic) mass number of Ag, Sr, Ni?
5) the number of neutrons = A ‒ Z (Ex) Find the number of neutrons in a phosphorus atom. (Sol) Z = 15; A= 31 the number of neutrons = 31 ‒ 15 = 16 *In an atom, # of protons = # of electrons (we say it is electrically neutral)
Practices • Find the number of protons, electron, and neutrons of the following atoms. • Ne • H • As • U • Cr • Rewrite the above symbols with the atomic number and the atomic mass number.
Atomic Models • Plum pudding model by J J Thomson Plum Pudding • Rutherford model (solar system model) • Modern model (electron orbital model)
Ion • the charged (+ or –) particle formed when an atom loses or gains one or more electrons • the total net charge (the sum of all positive and negative charges) is shown after the symbol upper right (Ex) Be2+ , F– • cation: positive ion formed when an atom loses electron (Ex) Na (11 p +11 e-) Na+ (11 p + 10 e-) (Ex) How many protons, neutrons, electrons are in a ion? • anion: negative ion formed when an atom gains electron (Ex) O (8 p + 8 e-) O2- (8 p + 10 e-) (Ex) How many protons, neutrons, electrons are in a ion?
Why are the mass numbers in the periodic table decimals? • average mass of isotopes • amu (atomic mass unit)
Isotope • atoms of the same element but with different number of neutrons • isotopes of hydrogen: hydrogen (1H), deuterium (2H), tritium (3H) • isotopes of oxygen are oxygen-16,oxygen-17, and oxygen-18. (Ex) Write the symbol for each, including the atomic number and mass number. • isotopes of chromium are chromium-50,chromium-52, and chromium-53. (Ex) How many neutrons are in each isotope, given that chromium has an atomic number of 24?
amu (atomic mass unit) • a mass unit for a very small mass • defined by assigning 12 amu to the mass of a carbon-12 atom • 1 amu = mass of 1/12 of a C-12 atom =1.66053886 × 10-27 kg (Ex) 1 proton = 1.007 amu ≈ 1 amu 1 neutron = 1.009 amu ≈ 1 amu 1 electron = 0.00054858 amu • The amu of other element is determined by comparing its weight to the weight of a carbon-12 atom. • The decimal number in the periodic table is the average amu of the isotopes of each element • The word, average, will be left out.
Example • A chromium atom has 24 protons, 24 electrons, and 24 neutrons. What is its amu? Use: C-12 = 12 amu • A boron atom has 5 protons, 5 electrons, and 5 neutrons. What is its amu?
Average Atomic Mass • natural abundance = the percentage of an isotope that exists in nature (Ex) 1H = 99.985 %; 2H = 0.015%; 3H = negligible (Ex) 35Cl = 75.77%; 37Cl = 24.23% (Ex) Find the average mass of hydrogen. (Ex) Find the average mass of chlorine.
Periodic Table shows • each element’s name and symbol • each element’s atomic number (= # of protons) • each element’s (average) amu • periods = rows • There are 7 periods ( period 1 to period 7) • groups or families = columns • There are 18 groups (group 1 – 18 or group 1A – 8A and group 1B – 8B)