Unit 2 The Atom (in general), Atomic Weights, The Mole, Stoichiometry

The Atom John Dalton ( ) Law of Conservation of Mass – a natural law stating that matter can neither be created nor destroyed; it can only be converted from one form to another; this also states that there is no detectable change in mass in an ordinary chemical reaction Law of Constant Composition (Definite Proportions) –the law that states that a compound always contains the same elements in the same proportions by mass Law of Multiple Proportions – a relation stating that when two elements A and B form two compounds, the relative amounts of B that combine with a fixed amount of A are in a ratio of small integers

The Atom John Dalton ( ) Develops postulates of the modern atomic theory. Atoms – the smallest particle of an element that can enter into a chemical reaction Elements – an element is composed of tiny particles called atoms Compounds – compounds are formed when atoms of two or more elements combine Reactions – in an ordinary chemical reaction, atoms move from one substance to another, but no atom of any element disappears or is changed into an atom of another element

The Atom Nuclear symbols A nuclear symbol should show the atomic number, mass number and charge of an atom around its symbol. K K X Symbol

The Atom The nuclear symbol for the atom here is: K K K Symbol

The Atom Why dont these two numbers match? K K K Symbol One is the mass number and the other is the atomic mass.

The Atom How are these nuclear symbols different? KK Potassium–40Potassium–39 The two potassiums have different mass numbers. This means there are 20 neutrons. This means there are 21 neutrons. ISOTOPES

The Atom How are these nuclear symbols different? KK +1 Potassium–39Potassium +1 ion – 39 The two potassiums have different charges. This charged ion has 18 electrons. This neutral atom has 19 electrons. IONS

Natural Abundance The amount of abundance of an elements various isotopes remains approximately the same. These are used to determine the mass number shown on the periodic table. Lithium Lithium-6 has an atomic abundance of 7.5% and an atomic mass of amu Lithium-7 has an atomic abundance of 92.5% and an atomic mass of amu What is lithiums atomic mass?

Natural Abundance The amount of abundance of an elements various isotopes remains approximately the same. The atomic mass of an element is determined by adding together the products of the amus of an isotope and its percent abundance This average atomic mass is the one that is represented as the mass of an element on the periodic table.

Atomic Weights amu – atomic mass unit the unit used to express atomic masses 1 / 12 of the mass of a carbon-12 atom 6.02x10 23 amu = 1 gram This means that any elements atom has mass in amus while that same elements mole has mass in grams… 1 atom of Cl = amu 1 mol of Cl = grams Whew!!! Thats important!

MOLES! 1 mole = Molar Mass grams 1 mole = 6.02x10 23 objects 1 mole = 22.4 Liters Molarity =. Moles Liters

Mole Practice 1. Calculate the formula mass of methylamine, CH 3 NH 2, to two decimal places. 1(C) 3(H) 1(N) + 2(H) 1(12.01) 3(1.01) 1(14.01) + 2(1.01) g

Mole Practice 2. What is the mass of the nitric acid molecule, HNO 3 ? 3. A sample of nitric acid contains mol HNO 3. How many grams is this? 1(H)+1(N)+3(O)=1(1.01)+1(14.01)+3(16.00)=63.02 amu

Mole Practice 4. The daily requirement of chromium in the human diet is 1.0x10 –6 g. How many moles of chromium does this represent? 5. The average daily requirement of the essential amino acid leucine, C 6 H 14 O 2 N, is 2.2 g for an adult. How many atoms of leucine are required daily?