Chemistry 100

Chapter 2

Atoms, Molecules and Ions

Law of Conservation of Matter

By 1800, chemists had noted that the mass of reactants equals the mass of products - provided you capture any escaping gas

Matter is not created or destroyed in a chemical reaction

Law of Constant Composition

Chemists (notably Proust) The relative amount (percentage) of each

element in a compound was the same no matter how the compound was made

These two laws lead Dalton to revive the Atomic Theory Matter is made up of small, indivisible

particles

Dalton’s Atomic Theory

An element is composed of atoms. All atoms of a given element are the

same. Atoms of different elements are different

and have different properties.

Dalton’s Theory-II

Atoms are not changed, created or destroyed in a chemical reaction.

Compounds are the combination of more than one element. A given compound has the same relative

number and kind of atoms.

Law of Multiple Proportions

Accorrding to Dalton’s Theory Two elements (A and B) form two distinct

compounds The amounts of B combining with a fixed amount

of A would be a small whole number ratio. Water: 1 g hydrogen + 8 g oxygen Hydrogen peroxide: 1 g hydrogen +16 g

oxygen

Atomic Structure

Roentgen discovered X-rays (1895) Becquerel discovered radioactivity

(1896) J. J. Thompson discovered the

electron (1897) Rutherford classified radioactivity

emissions: alpha (), beta () and gamma ()

Alpha, beta, gamma

Rutherford’s findings: Alpha are positive particles (+2) ;

heavier than electrons Beta are high speed electrons;

negatively charged particles (-1) Gamma are neutral rays Alpha particles are nucleus of He atom

Thompson’s Model

“Plum pudding” model

A cloud of positive charge holding the negatively charge electrons in place

Rutherford’s alpha experiment

Scattering of alpha particles by gold foil Most particles were undeflected Some were deflected by large angles

Rutherford’s Explanation

Rutherford’s Model

Centre - the nucleus - is small but positively charged

Most of the atom is empty spaces Electrons rotate about the nucleus -

like the solar system

Modern Model

Additional experiments showed Nucleus consists of protons (positive) and

neutrons (neutral) Electrons (negative) exist around the

nucleus Number of protons = number of electrons

Mass of Elementary Particles

Protons and neutrons have a mass of about 1 amu

Electrons have very small mass Most of the mass of an atoms comes from

nucleus (1 amu is 1.66054 10-24 grams)

Quantum Mechanical Model

Atomic Mass & Atomic Number

A

SyZ

A is Mass Number,protons + neutrons

Z is Atomic Number,number of protons

Isotopes

All atoms of a given element have the same number of protons. All Carbon atoms have 6 protons (and 6 electrons).

The number of protons is different for each element.

Atoms of a given element that differ in the number of neutrons are called isotopes.

Examples : carbon-12 and carbon-14C126 C146

Atomic Masses

By international agreement, the carbon-12 atom is defined as having a mass of exactly 12 atomic mass units (amu’s).

All atomic masses are referenced to this standard.

The Periodic Table

A typical entry in the periodic table

Ca20

40.078

Atomic mass

Atomic number

Periodic Table (II)

Elements in the periodic table are arranged in Groups or families – they have similar

chemical and physical properties Metals – towards the left Nonmetals – towards the right Metalloids – in the middle region

Atomic Masses in the Periodic Table

Question: why is the mass of C in the periodic table reported as 12.01 amu and not as 12.000 … amu, exactly?

Another example: the atomic mass of Cl is = 35.453 amu’s. We would expect Cl to be 35 amu?

Ionic Compounds

Tables of common ions in textbook (pages 62 and 64).

Ionic compounds Cation name followed by anion name, e.g.,

sodium bromide (NaBr) Multiple ion types

FeCl2 – iron (II) chloride

FeCl3 – iron (III) chloride

Binary Molecular Compounds

Binary compounds containing two nonmetals name of the first element in the formula

followed by the stem of the name of the second element with the suffix -ide.

The number of atoms of each element in the compound is indicated by a prefix.

mono di tri tetra

1 2 3 4

Some common names must be committed to memory.

Examples – nitrous oxide, N2O, and nitric oxide, NO.

Formulas and Names of Acids

An acid usually is a compound of hydrogen and a nonmetal or a polyatomic anion.

Treat the hydrogen atoms of the acid as H+ ions.

For acids containing monatomic anions, When these acids are found in water solution, add

the prefix hydro- and the suffix -ic to the stem of the name of the anion

Hydrofluoric acid (HF), hydrochloric acid (HCl)

Acids From Polyatomic Anions

If the anion name ends in ‘ate’, the ‘ate’ in the name of the anion is replaced by ‘ic acid ’ The acid of the sulfate ion is sulfuric acid (H2SO4)

The acid of the nitrate ion is nitric acid (HNO3)

If the anion names end in ‘ite’, we change the suffix to –ous and add the word acid. The acid of the nitrite is called nitrous acid, HNO2

The acid of the hypochlorite ion is called hypochlorous acid, HClO

Average Atomic Masses

Most elements in nature exist as mixtures of isotopes.

Atomic masses reported in the periodic table - weighted averages of the different isotopes.

The amount of each isotope in a sample of as an element may vary considerably with the source of the sample. This is the reason why some elements in the

periodic table have few significant figures for their mass.

Organic Compounds

Many organic compounds have complex three dimensional structures chains, and/or rings branches.

The highlighted groups are called functional groups. They are primarily responsible for the

chemical and some physical properties of the molecules.

Methane (CH4)

Ethane (C2H6)

Propane (C3H8)

Butane (C4H10)

Alkanes

C

H

HH

H

C

H

HH

C

H

HH

CH

HH

C

H

H

C

H

HH

CC

CH

C

H

HH

H

HH

H

HH

Methanol (CH3OH)

Ethanol (CH3CH2OH)

1-Propanol (CH3CH2CH2OH)2-propanol (CH3CH(OH)CH

3)

Alcohols

C

H

OHH

H

C

H

HH

C

H

HOH

CH

HH

C

H

H

C

H

OHH

CC

CH

H

HH

HOH

H

H

Methyl Amine (CH3NH2)

Dimethyl amine ((CH3)2NH)

Ethyl Amine (CH3CH2NH2)

Organic Amines

C

HNH2

HH

NH

CH3

CH3

C

H

HC

H

NH2H

H

Formic (Methanoic) acid (HCOOH)

Acetic (Ethanoic) Acid (H3COOH)

Propionic (Propanoic) Acid (H3CCH2COOH)

Organic Acids

C

O

OHH

C

H

HH

C

O

OH

CH

HH

C

H

H

C OH

O