Atoms, Elements,

Molecules & Compounds

.

All substances are made of atomsAll substances are made of very tiny particles called atoms.

hydrogen (H) and oxygen (O)

atoms

carbon (C) and hydrogen (H)

atoms

Carbon (C), nitrogen (N),

hydrogen (H), oxygen (O) and

sulphur (S) atoms

Iron (Fe), aluminium (Al),

silicon (Si), oxygen (O) and boron (B) atoms

Many substances are made up of different types of atoms.

An element is a substance made up of only one type of atom.

Atoms in elements

Copper (Cu) is an element made up of copper atoms only.

Carbon (C) is an element made up ofcarbon atoms only.

Helium (He) is an element made up ofhelium atoms only.

There are about one hundred Elements that form all substances.

How to write symbols for

elementsTwo important rules should be followed when writing the

symbols of elements so that there is no confusion.

1. The first letter of an element’s symbol is always

a capital letter.

2. If there are two letters in the element’s symbol,

the second letter is always a small letter.

e.g. N (not n) for nitrogen

e.g. Co for cobalt (not CO)

No, Watson! It was carbon monoxide

poisoning – not cobalt.

The symbol for many of the more common elements uses

just the first letter of the name.

Symbols of elementsA standard set of symbols is used to represent elements:

hydrogenH =carbonC =fluorineF =

oxygenO =nitrogenN =

Others elements have

the first two letters.

lithiumLi =aluminiumAl =heliumHe =

Some of the symbols are not

always as you might expect.

leadPb =goldAu =silverAg =

iodineI =

Molecules

A molecule is formed of a group of 2 atoms or more of the

same element that are joined together.

Eg. H2, O2, N2, Cl2, Br2

An element is a substance made up of only one type of atom.

Eg. H, O, N, Cl, Br

Oxygen is an

element made up of

oxygen atoms only.

Q. How many atoms

are there in an

oxygen molecule?

Compounds

Q. How many atoms

are there in a water

molecule?

A compound is formed of a group of different elements that

are joined together.

Eg. H2O, CO2, NH3

Q. How many elements

are there in a water

molecule?

Molecules versus Compounds

All compounds are molecules but not all

molecules are compounds.

Molecular hydrogen (H2), molecular oxygen (O2) and

molecular nitrogen (N2) are not compounds because each

is composed of a single element. Water (H2O), carbon

dioxide (CO2) and methane (CH4) are compounds because

each is made from more than one element.

Q. Which is a molecule and which is a compound?

NaCl, H2SO4, Cl2,

Atomic Number, Mass

Number & Isotopes

.

10

Subatomic Element

Particle Symbol Charge Relative

Mass

Electron e- 1- 0

Proton p+ + 1

Neutron n 0 1

11

Location of Subatomic Element

Electrons

nucleusProtons

Neutrons

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

XA

Z

Mass Number

Atomic Number

Element Symbol

2.3

Atomic number & Mass number

8 15 30

O P Zn16 31 65

8 p+ 15 p+ 30 p+

8 n 16 n 35 n

8 e- 15 e- 30 e-

Isotopes are atoms of the same element (X) with

different numbers of neutrons in the nucleus

2.3

Isotopes

Proton 1 1 1

Neutron 0 1 2

Electron 1 1 1

How many protons, neutrons, and electrons are in C146 ?

How many protons, neutrons, and electrons are in C116

?

Do You Understand Isotopes?

2.3

Period

Gro

up

Alk

ali M

eta

l

Noble

Gas

Halo

gen

Alk

ali E

arth

Me

tal

2.4

2.5

2.5

Electropositive elements

are the elements which have tendency to

lose electrons. e.g. Metals (Na, K, Al, Mg)

Electronegative elements are

those which have tendency to attract the

electrons of the bond. e.g. Non metals

(Cl, Br, I)

In the periodic table from left to right

Electropositivity decreases

Electronegativity increases upto group

7A

2.5

There are three major states of matter: solid,

liquid, and gas.

Solid: is something that has a definite shape and

volume.

Liquid: has a definite volume but takes the

shape of its container.

Gas: takes the shape of its container and it

expands to fill the entire container.

States of matter

A molecule is an aggregate of two or more atoms in a

definite arrangement held together by chemical bonds

H2 H2O NH3 CH4

A diatomic molecule contains only two atoms

H2, N2, O2, Br2, HCl, CO

A polyatomic molecule contains more than two atoms

O3, H2O, NH3, CH4

2.5

An ion is an atom, or group of atoms, that has a net

positive or negative charge.

cation : is an ion with a positive charge (+ve)

If a neutral atom loses one or more electrons

it becomes a cation.

Anion: is an ion with a negative charge (-ve)

If a neutral atom gains one or more electrons

it becomes an anion.

Na11 protons

11 electrons Na+ 11 protons

10 electrons

Cl17 protons

17 electrons Cl-17 protons

18 electrons

2.5

A monatomic ion contains only one atom

A polyatomic ion contains more than one atom

2.5

Na+, Cl-, Ca2+, O2-, Al3+, N3-

OH-, CN-, NH4+, NO3

-

13 protons, 10 (13 – 3) electrons

34 protons, 36 (34 + 2) electrons

Do You Understand Ions?

2.5

How many protons and electrons are in Al2713 ?3+

How many protons and electrons are in Se7834

2- ?

2.6

Molecular formula: it shows the exact number of

atoms of each element in the smallest unit of a

substance

Empirical formula: it shows the simplest

whole-number ratio of the atoms in a substance

H2OH2O

molecular empirical

C6H12O6 CH2O

O3 O

N2H4 NH2

2.6

Ionic compounds consist of a cation (+ve) and an

anion (-ve)

• the formula is always the same as the empirical formula

• the sum of the charges on the cations and anions in each

formula unit must be equal zero

The ionic compound NaCl

2.6

Some Polyatomic Ions (Table 2.3)

NH4+

ammonium SO42-

sulfate

CO32-

carbonate SO32-

sulfite

HCO3- bicarbonate NO3

- nitrate

ClO3- chlorate NO2

- nitrite

Cr2O72-

dichromate SCN-

thiocyanate

CrO42-

chromate OH- hydroxide

2.7

Chemical Nomenclature

• Ionic Compounds

– often a metal + nonmetal

– anion (nonmetal), add “ide” to element name

BaCl2 barium chloride

K2O potassium oxide

Mg(OH)2 magnesium hydroxide

KNO3 potassium nitrate

2.7

• Transition metal ionic compounds

– indicate charge on metal with Roman numerals

FeCl2 2 Cl- -2 so Fe is +2 iron(II) chloride

FeCl3 3 Cl- -3 so Fe is +3 iron(III) chloride

Cr2S3 3 S-2 -6 so Cr is +3 chromium(III) sulfide

2.7

Anion Cation

HI hydrogen iodide

NF3 nitrogen trifluoride

SO2 sulfur dioxide

N2Cl4 dinitrogen tetrachloride

NO2 nitrogen dioxide

N2O dinitrogen monoxide

Molecular Compounds

2.7

TOXIC!

Laughing gas

Acid can be defined as a substance that yields

hydrogen ions (H+) when dissolved in water.

e.g. HCl (hydrogen chloride orhydrochloric acid)

It dissolves in water to (H+ Cl-)

Oxoacid is an acid that contains hydrogen, oxygen,

and another element.

HNO3 nitric acid

H2CO3 carbonic acid

H2SO4 sulfuric acid

2.7

Base can be defined as a substance that yields

hydroxide ions (OH-) when dissolved in water.

NaOH Sodium hydroxide

KOH Potassium hydroxide

Ba(OH)2 Barium hydroxide

2.7

Na+ OH-

K+ OH-

Ba++ (OH)2-