Faculty of Allied Medical Sciences
General Chemistry
(MGGC-101)
Atoms, molecules and ions
Supervision:
Prof.Dr.Shehata El-Sewedy
Dr.Fatma Ahmed
Outcomes
By the end of this lecture, the
students will be able to
1-Recognize the significant figures in
measurements
2-To differentiate between International System of
Units
3-To know Dalton’s atomic theory
4-Understand Atomic number, Mass number and
Isotopes
5-Know the meaning of molecule and its types
6-learn to the meaning of ion and its types
3. Significant figures in measurements
In any measurement, there is always a degree on
uncertainty
Rules to determine the number of significant figures
►The numbers recorded in a measurement are called significant
digits or significant numbers.
1. Digits other than a zero are always significant.
854
► 3 sig figures and 3.6
► 2 sig figures
2- Zeros between non-zero digits  are Significant.
► 4 sig figures
and 30.76
► 4 sig figures
3- Zeros at the beginning of number  are not significant.
012
► 2 sig.fig.
and 0.012 ► 2 sig.fig
4- Zeros at the end of number and after the decimal point  are
Significant.
19.60
►
4 sig figures
5- Zeros at the end without decimal point  May or may not be
significant. (Use exponential notation).
Example: 500 could have 3 sig figures ► 5.00 X 102
or have 2 sig figures ► 5.0 X 102
or have 1sig figure ► 5 X 102
Example
Significant figures in calculations
1- In multiplication and division
The result must have the same number of S.F. as the number
having the least S. F. in the calculation.
e.g.
7.5023  2.00 = 15.0046
wrong
5 S.F. 3 S.F 
15.0 (only 3 S.F. Correct
34.103  0.0310 = 10.57193

10.6 (only 3 S.F.)
wrong
correct
2- In addition and subtraction
The result must have the same number of decimal
digits as the number with the least decimal digits in
calculation.
e.g. 25.23 + 21.3207 = 46.5507
2 d.d.
4 d.d

46.55 (only 2 decimals)
10.31 + 0.002438 = 10.307562

10.31 (only 2 decimals)
Rounding
Look at the leftmost digit to be dropped, then
1-If this digit is greater than 5, add 1 to the last
digit to be retained. Thus, rounding 1.2161 to 3
significant figures gives 1.22.
2-If digit is less than five, drop it, Thus, rounding
1.2141 to 3 significant figures gives 1.21.
3-If this digit is 5 :
15.55 = 15.6
odd
15.45 = 15.4
even
Example:
Perform the following calculations and round the
answers to the correct number of significant figures:
a.
b.
c.
d. 37.76 + 3.907 + 226.4
Solution:
The factor 5.8 has the fewest significant figures;
therefore the answer should be reported to two
significant figures. Round the answer to 3.6
The number with the least number of decimal
places is 5.41. Therefore, round the answer to two
decimal places, to 5.01
The answer is 0.37. Note how you have lost one
significant figure in the subtraction.
The answer is 268.1
Units of measurement
Measurement always consists of two parts, a number
and a unit.
•1-Metric system
2- International System of Units (SI system)
This system is commonly used in all countries.
Fundamental SI units
Some unit conversations:
1 kilogram = 1  103 gram
1 megagram = 1  106 gram
60 second = 60  1015 femto second
Femto second = 1  10–15 second
Pico meter = 1  10–12 meter
1 meter = 1  1012 pico meter
Derived SI units
Units which are not fundamental SI
units called derived.

Area = (Length)2

Volume = (Length)3

Liter = 103 cm3 = 103 mL

Velocity = m/s (length/time)

Density = g/cm3 (mass/volume)
Atoms, molecules and ions
DALTON’S ATOMIC THEORY (1808)
1-Elements are composed of extremely small particles
called atoms.
2-All atoms of a given element are identical, having the
same size, mass and chemical properties. The atoms of one
element are different from the atoms of all other elements.
3- Compounds are composed of atoms of more than one
element. In any compound, the ratio of the numbers
of atoms of any two of the elements present is either
an integer or a simple fraction.
4- A chemical reaction involves only the separation,
combination, or rearrangement of atoms
16 X
+
8Y
8 X2Y
SUBATOMIC PARTICLES
Mass
Particle
Symbol
Charge
Electron
e-
-
9.11X10-28 g
Proton
p+
+
1.67X10-24 g
Neutron
n
0
1.67X10-24 g
Atomic number and Mass number
An atom is neutral
The net charge is zero
Number of protons = Number of electrons
Atomic number = Number of electrons
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Mass Number
Atomic Number
A
Z
X
Element Symbol
SUBATOMIC PARTICLES IN SOME ATOMS
65
30
Zn
30 p+
35 n
30 e-
31
15
P
15 p+
16 n
15 e-
16
8
O
8 p+
8n
8 e-
Isotopes:
The isotopes of an element have the same atomic no.
(same chemical properties) but differ in their mass
no. (different physical properties) i.e they have same
no. of protons but different no. of neutrons.
Example: the isotopes of hydrogen
12C,
13C
Eg.
The 2 atoms have the same at.no. (equal no. of
protons)
But they differ in their mass no. (different no. of
neutrons)
12C: at.no.= 6,mass no.=12, no. of
neutrons=6
13C: at. no. =6, mass no.=13, no. of
neutrons=7
16O,
17O,
18O
Eg. O atom
8p,
8p,
8p
8n,
9n,
10n
8e,
8e,
8e
Molecules and Ions
A molecule is an aggregate of two or more atoms held
together by chemical forces
H2
H2O
NH3
CH4
The forces that hold atoms together in molecules
are called chemical bonds.
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
Molecules can be represented in several different
ways:
Chemical formula
Represent the type and actual number of atoms in
a molecule,
for example, carbon dioxide CO2 (it has 1 carbon
atom and 2 oxygen atoms).
Structral formula
Formula by which the individual bonds are shown
(may or may not show the actual shape of
molecule).
Example
Ions
An ion is an atom, or group of atoms, that has a net
positive or negative charge. An ion may be a cation or
an anion.
Cation: ion with a positive charge. If a neutral atom
loses one or more electrons it becomes a cation.
Anion: ion with a negative charge. If a neutral atom gains one or
more electrons it becomes an anion.
Quiz time
What is the number of protons and
electrons in S2 –
What is the number of protons and
electrons in aluminum ion, Al3+
What is the number of protons, neutron and
electrons in
16O,
17O,
18O
How many Significant figures and decimal points in
Student Question
What the SI units of
1-time
3-temperature
4-amount of substance
2-length
5-mass
Answer the following operation
18.5+31.3=
3.2+5.09+3.007=
36.36÷4.04=
4.053÷4.00=
27.00x2.00=
0.0040x3001.37=
How many Significant figures in
1)32.01
2)5.010
3)32.00
4)200
5)8.1
Calculate the no. of proton, neutron and electrons for
Define and give example:
Cation
Anion
Isotope
Structural formula
Assignments
Group A and Group
B
Types of Ions
Aya Salah- Omnia Gamal- Hanan
Farg-Sarah Fasal Hashem
Uses of cations in industry
Saad Mohamad El-sawi
Mohamad Ahmad Fouad
Mohamad Moawad
El hussein Mohsen -Adel Ahmed
Mahmoud Abd El- Mageed
Applications of Isotopes
Mana Allah Gamal- Malak AtefYasmine Abd ElHameed
Applications of anions
Ali Hasan – Mahmoud MohamadAmir Mohamad- Mervat Abd El-HalimMervat Essam – Yasmine Rabee
RECOMMENDED TEXTBOOKS:
1-Raymond Chang. Chemistry. 10th ed.
2009
2-Zumdehl. International edition. 2009