Chemistry-Ch01 the Atom

8/9/2019 Chemistry-Ch01 the Atom

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11 The atomThe atom


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CONTENTS

1.1 Why study chemistry

1.2 Atoms, molecules, ions, elements and

compounds1.3 The atomic theory

1.4 The structure of the atom

1.5 The periodic table of the elements

1.6 Electrons in atoms


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Learning OutcomesLearning Outcomes

Able to differentiate between ions,

molecules, ionic and molecular

compound. Able to name compounds and write

their formulas


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1 The atom1 The atom

Chemistry is the science concerned

with the study of matter

Matter is anything that takes up spaceand has mass

Chemists are people trained in the

science of chemistry

Central to this study is the way matter isconstructed from the simplest building

block, the atom


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1.1 Why study chemistry?1.1 Why study chemistry?

Chemistry is a discipline that

encompasses many fields

Doctors, dentists, pharmacists,biochemists and geneticists all need a

knowledge of various chemical principles

The majority of sciences have some

chemistry component

Chemistry is also present in areas other

than science e.g. law


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1.1 Why study chemistry?1.1 Why study chemistry?

Why do people choose to be

chemists?

No simple answer

Probably true to say that most chemists

are fascinated by the nature of matter

and seek an understanding of it usingchemical principles


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1.2 Atoms, molecules, ions,1.2 Atoms, molecules, ions,

elements and compoundselements and compounds

Atoms

Discrete chemical species comprising a

central positively charged nucleus

surrounded by 1 or more negatively

charged electrons

Atoms are always electrically neutral


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Molecules

Collections of atoms with a definite

structure held together by covalent

bonds

Covalent bonds involve the sharing of

electrons between neighbouring atoms


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Ions

Chemical species that have either a

positive or negative electric charge Cations are ions with a positive charge

e.g. Na+

Anions are ions with a negative charge

e.g. Cl


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Elements

Collections of one type of atom only

There are 117 (at the moment)

The periodic table (next slide) lists all the

elements


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Compounds

Substances containing two or more elements in

definite and unchanging proportion

Compounds may be composed of molecules or

a covalently bonded network of atoms

Do not have individual molecules of an ionic

compound

Sodium chloride, NaCl, simply represents the smallest

repeating unit in an enormous 3D array of Na+ and Cl

ions


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1.3 The atomic theory1.3 The atomic theory

Law of conservation of mass

No detectable gain or loss of mass

occurs in chemical reactions. Mass is

conserved.

Law of definite proportions

In a given chemical compound, theelements are always combined in the

same proportions by mass.


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A sample of molybdenum disulfide contains 1.50 g

of Mo for each 1.00 g of S. If a different sample

contains 2.50 g of S, what mass of Mo does it

contain?

The law of definite proportions states that the

proportions of Mo and S by must be the same in

both samples.

x = 3.75 g Mo

Sg50.2

x

Sg00.1

Mog50.1!


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Daltons Atomic Theory

1. Matter consists of tiny particles.

2. Atoms are indestructible. In chemical

reactions, the atoms rearrange but they

do not themselves break apart.

3. In any sample of a pure element, all

atoms are identical in mass and other

properties.


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Daltons Atomic Theory

4. The atoms of different elements differ

in mass and other properties.

5. When atoms of different elements

combine to form a given compound,

the constituent atoms in the compoundare always present in the same fixed

ratio.


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Chemical equations describe

chemical reactions

Reactants go on the left hand side Products go on the right hand side

Reactants and products are separated

by a forward arrow

2H2g + O2g 2H2Ol

reactants products


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2H2g + O2g 2H2Ol The law of conservation of mass

requires the same number of each typeof atom on each side of the arrow

When this is the case the reaction is

described as balanced

Physical states are also specified inchemical equations

Gases, liquids and solids are abbreviated

g, l and s


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Law of multiple proportions

Whenever two elements form more than

one compound, the different masses of

one element that combine with the same

mass of the other element are in the ratio

of small whole numbers


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Each molecule has 1 sulfur

atom, so each has thesame mass of sulfur

Oxygen ratio is 3 to 2 both

by number of atoms and

mass


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Chemists no longer talk

of atomic theory, we

talk of atomic fact

Scanning tunnel

microscopy and atomic

force microscopy allow

us to view andmanipulate individual

atoms



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1.4 The structure of the atom1.4 The structure of the atom

Atoms are comprised of a nucleus and

surrounding electron(s)

The nucleus occupies less

than 0.1% of the

total atomic volume

The nucleus is

comprised ofprotons and

neutrons


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Atomic Structure

Comparison of the proton, neutron and

electron

Particle Charge Mass (amu)

Proton Positive (+1) 1.0073

Neutron Neutral 1.0086

Electron Negative (-1) 5.4858 v 10-4

1 amu = 1.66054 v 10-

1 g = 6.02214 v 102


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X is the chemical symbol for any

element

Z is the atomic number, this is the

number of protons in the nucleus A is the mass number, this is the

number of protons plus the number

or neutrons in the nucleus

XAZ


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In a neutral atom the atomic number is

also equal to the number of electrons

H hydrogen

1 proton (Z = 1)

1 electron0 neutrons (A = 1) H

1

1


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2

1

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1Example 1

Find the number of protons, electrons and

neutrons in the following isotope:

The number of proton is 20;

The number of electrons in a neutral atom is

20;The number of neutrons : 40 - 20 = 20.

Ca40

20


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1Example 2

The atomic number sulfur is 16. Write the

nuclear-isotope symbols for the four isotopes

of sulfur with 16, 17, 18 and 20 neutrons,

respectively.

All isotopes will have 16 protons. The mass

number of each isotope is the sum of its numberof neutrons plus its number of protons.

16 + 16 = 32; 16 + 17 = 33; 16 + 18 = 34; 16 + 20 = 36

SSSS36

16

34

16

33

16

32

16,,,


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Example 3

What is the symbol for an ion with 26 protons,

30 neutrons and 24 electrons?

Mass number of isotope = 26 + 30 = 56

Atomic number 26

Check with periodic table: p Fe

Ions has two more protons than electrons, it

has a net charges of 2+.

Complete symbol for this ion = Fe2+


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Isotopes

Atoms of an element with the same

number of protons but different numbers

of neutrons

Radioactive isotopes have unstable

nuclei which undergo spontaneousdecay to more stable nuclei

Nuclide

Any atomic nucleus

Radioactive nuclei are called radionuclides


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The chemical symbol is indicative of

the atomic number, so it is common to

see a shorthand version

is written as 1H

Atomic mass unit (u) The mass (1.666 054 1027 kg) equal to

the mass of one atom of12C The masses of all atoms are measured relative

to this

Average atomic masses account for isotopic

abundances

H11

12

1


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1.5 The periodic table of1.5 The periodic table of

elementselements

Elements may be ordered on the

basis of increasing atomic number (Z)

The periodic table of elementsillustrates this ordering

The periodic table is organised into:

Horizontal rows called periods

Vertical columns called groups


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1.5 The periodic table of1.5 The periodic table ofelementselements


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1.5 The periodic table of1.5 The periodic table of

elementselements

Metals

Generally good conductors of heat and

electricity, are malleable and ductile and

have a metallic lustre

Non-metals

Elements that do not have these

characteristics Metalloids

Lie somewhere between metals and

non-metals


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1.6 Electrons in atoms1.6 Electrons in atoms

Many of the chemical properties of an

atom and its chemical reactivity are

determined by the electrons

Electrons occupy regions of space called

orbitals

Each orbital has a characteristic electrondistribution and energy


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An electronic transition occurs when an

atom absorbs a specific amount of

energy and an electron is promoted to a

higher energy orbital to form an excitedstate.

Orbitals have definite energies. This is a

fundamental principle of quantum

mechanics call quantisation.


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Electrons have a single negative charge

Electrons have an intrinsic propertycalled spin

Spin can only have two values spin up

spin down

Each orbital within an atom can contain amaximum of 2 electrons, 1 spin up and1 spin down


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Electrons constitute the chemical bonds

that hold atoms together

Covalent chemical bonds usually consist

of 1, 2 or 3 pairs of electrons sharedbetween atoms

Chemical reactions often involve

reorganising these electrons in bond-

making and bond-breaking processes

Redox reactions involve transfer of one or

more electrons between chemical species


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Atoms are the fundamental building block

of all matter

The existence of atoms was proposed on

the basis of: The law of conservation of mass

The law of definite proportions

The law of multiple proportions

The atom is comprised of three subatomic

particles; the electron, proton and neutron

SummarySummary


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Elements comprise only a single type of

atom. The periodic table arranges all

known elements in order of increasing

atomic number

Electrons occupy regions of space called

orbitals

Energies of electrons in an atom are

determined by the energies of the

orbitals, so electrons in atoms have only

certain well-defined energies

SummarySummary


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Chemistry-Ch01 the Atom

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