Chapter 2 Atoms and Elements

Reacting Atoms when elements undergo chemical reactions, the reacting elements do not turn into other elements Dalton’s Atomic Theory 2H2(g) + O2(g)  2H2O(g) since the number of protons determines the kind of element, the number of protons in the atom does not change in a chemical reaction however, many reactions involve transferring electrons from one atom to another

Charged Atoms when atoms gain or lose electrons, they acquire a charge charged particles are called ions when atoms gain electrons, they become negatively charged ions, called anions (Cl-) when atoms lose electrons, they become positively charged ions, called cations (Na+) ions behave much differently than the neutral atom e.g., The metal sodium, made of neutral Na atoms, is highly reactive and quite unstable. However, the sodium cations, Na+, found in table salt are very nonreactive and stable

Ionic Charge from Group Numbers The charge of a positive ion is equal to its Group number. Group 1A(1) = 1+ Group 2A(2) = 2+ Group 3A(3) = 3+ The charge of a negative ion is obtained by subtracting 8 or 18 from its Group number. Group 6A(16) = 6 - 8 = 2- or 16 - 18 = 2-

Atomic Structures of Ions Nonmetals form anions For each negative charge, the ion has 1 more electron than the neutral atom F = 9 p+ and 9 e-, F ─ = 9 p+ and 10 e- Metals form cations For each positive charge, the ion has 1 less electron than the neutral atom Na atom = 11 p+ and 11 e-, Na+ ion = 11 p+ and 10 e-

Ions and the Periodic Table

Examples Predict the charge and identify the number of proton and electron from that ion Mg  Al  O  P 

Atomic Mass: The Average Mass of an Element’s Atoms Atomic mass is sometimes called atomic weight or standard atomic weight (amu) The atomic mass of each element is directly beneath the element’s symbol in the periodic table. It represents the average mass of the isotopes that compose that element, weighted according to the natural abundance of each isotope.

Collection Terms When we count large numbers of objects, we often use units such as 1 dozen objects = 12 objects. 1 gross objects = 144 objects. A collection term states a specific number of items. In chemistry, how do chemists know the number of atoms in an element or compound? As chemists, we often need to know the number of atoms in a sample of a given mass. Why? Because chemical processes happen between particles. Therefore, if we want to know the number of atoms in anything of ordinary size, we count them by weighing.

The Mole: A Chemist’s “Dozen” The chemist’s “dozen” is the mole (abbreviated mol). A mole is the measure of material containing 6.02214 × 1023 particles: 1 mole = 6.02214 × 1023 particles This number is Avogadro’s number. The second, and more fundamental, thing to understand about the mole is how it gets its specific value. The value of the mole is equal to the number of atoms in exactly 12 grams of pure C-12. 12 g C = 1 mol C atoms = 6.022 × 1023 C atoms

A Mole of Atoms A mole is a unit of measurement used in chemistry to express amounts of a chemical substance, the same number of particles as there are carbon atoms in 12.0 g of carbon. a collection term “dozen” 1 mole = NA = 6.022 x 1023 of anything Avogadro’s Number = 6.0221421 x 1023

Converting between Number of Moles and Number of Atoms Converting between number of moles and number of atoms is similar to converting between dozens of eggs and number of eggs. For atoms, you use the conversion factor 1 mol atoms = 6.022 × 1023 atoms. The conversion factors take the following forms:

Converting between Mass and Amount (Number of Moles) To count atoms by weighing them, we need one other conversion factor—the mass of 1 mol of atoms. The mass of 1 mol of atoms of an element is the molar mass. An element’s molar mass in grams per mole is numerically equal to the element’s atomic mass in atomic mass units (amu). The lighter the atom, the less mass in 1 mol of atoms.

Relationship Between Moles and Mass The mass of one mole of atoms is called the molar mass The molar mass of an element, in grams, is numerically equal to the element’s atomic mass, in amu

Examples Give the molar mass for each A. 1 mole of Li atoms = ________ g 1 mole of Co atoms = ________g 1 mole of S atoms = ________g

Converting between mass, moles and atoms g C mol C mol C g C g C mol C atoms

Examples Assuming all pennies are pure copper and each has a mass of 2.5 g Without doing calculation, determine the number atoms of copper present in 1 mole How many pennies does it take to make a mole?

Examples In a 3.0 moles of O2 molecules How many oxygen molecules are there ? How many oxygen atoms are there?

Examples Calculate the moles of carbon in 0.0265 g of pencil lead Calculate the mass (in grams) of 0.473 moles of titanium