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

The Atom

John Dalton (1766-1844) 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 (1766-1844) 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 symbolsA nuclear symbol should show the atomic number, mass number and charge of an atom around its symbol.K

19

39.0983

Mass Number A

Atomic Number Z XSymbol

The Atom

The nuclear symbol for the atom here is:

K19

39.0983K

Symbol

Mass Number 40

Atomic Number 19

The Atom

Why don’t these two numbers match?

K19

39.0983K

Symbol

Mass Number 40

Atomic Number 19

One is the mass number and the other is the atomic mass.

The Atom

How are these nuclear symbols different?

K40

19

39

19 KPotassium–40 Potassium–39

The two potassiums have different mass numbers.

This means

there are 20 neutrons.

This means

there are 21 neutrons.

ISOTOPE

S

The Atom

How are these nuclear symbols different?

K 39

19 K+1

Potassium–39 Potassium +1 ion – 39The two potassiums have different charges.

This charged

ion has 18 electrons.

This neutral

atom has 19 electrons.

IONS39

19

Natural Abundance

The amount of abundance of an element’s various isotopes remains approximately the same.

These are used to determine the mass number shown on the periodic table.

LithiumLithium-6 has an atomic abundance of 7.5% and an atomic mass of 6.015122 amu

Lithium-7 has an atomic abundance of 92.5% and an atomic mass of 7.016003 amu

What is lithium’s atomic mass?

Natural AbundanceThe amount of abundance of an element’s various isotopes remains approximately the same. The atomic mass of an element is determined by adding together the products of the amu’s of an isotope and its percent abundanceThis average atomic mass is the one that is represented as the mass of an element on the periodic table.

LithiumIn order to determine the atomic mass, change the percentages to decimals and multiply.

(0.075)(6.015122 amu)

(0.925)(7.016003 amu)

ADD

6.9409 amu

Atomic Weights

‘amu’ – atomic mass unit

the unit used to express atomic masses

1/12 of the mass of a carbon-12 atom

6.02x1023 amu = 1 gram

This means that any element’s atom has mass in amu’s while that same element’s mole has mass in grams…

1 atom of Cl = 35.45 amu

1 mol of Cl = 35.45 grams

Whew!!!

That’s

importa

nt!

MOLES!

1 mole = Molar Mass grams

1 mole = 6.02x1023 objects

1 mole = 22.4 Liters

Molarity = .MolesLiters

Mole Practice

1. Calculate the formula mass of methylamine, CH3NH2, to two decimal places. 1(C) 3(H) 1(N)+ 2(H)

1(12.01) 3(1.01) 1(14.01)+ 2(1.01) 31.07 g

Mole Practice

2. What is the mass of the nitric acid molecule, HNO3?

3. A sample of nitric acid contains 0.253 mol HNO3. 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, C6H14O2N, is 2.2 g for an adult. How many atoms of leucine are required daily?