- Atoms consist of a nucleus containing protons and neutrons surrounded by electrons in orbitals. - The atomic number is the number of protons, which identifies the element. The mass number is the total number of protons and neutrons. - Isotopes are atoms of the same element with different numbers of neutrons. The relative atomic mass takes into account the natural abundance of isotopes.

1. Structure of
the Atom
Introductory Chemistry
Atomic Structure
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2. Atoms are tiny particles!!
NUCLEUS ELECTRONS
PROTONS NEUTRONS
POSITIVE
CHARGE
ATOM
POSITIVE
CHARGE
PROTONS
NEUTRAL
CHARGE
NEUTRONS
NUCLEUS
NEGATIVE CHARGE
ELECTRONS
ATOM
Mostly empty space!!

3. An atom consists of
• a nucleus (of protons and neutrons)
• electrons in space about the nucleus.
The Anatomy of an Atom
Nucleus
Electron cloud

4. Subatomic
components
Relative
Mass
Charge
nucleus
Proton 1 +1
Neutron 1 0
Orbital
cloud Electron 0.005 -1
Features of the Atom

5. A-Z notation
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12
6 C
mass number A
atomic number Z
element symbol
The atomic number
equals the number of
protons. Each element
has a unique atomic
number.

6. Mass Number
• mass number A = protons + neutrons
• always a whole number
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• NOT the value given on
the Periodic Table!

7. Atomic Number
• atomic number z = protons
• always a whole number
© Addison-Wesley Publishing Company, Inc.
• identifies the element

8. Practice: Lithium has the atomic number 3
and mass number 7
– # of protons:
– # of neutrons:
– # of electrons:
3
4
3

9. Practice: Lithium has the atomic number 3
and mass number 7
– electron arrangement (configuration) 2, 1
Lithium is in Group 1
on the Periodic Table

10. Practice: electron arrangements of the first
20 elements (2,8,8,2)
• Chlorine-37
– atomic #:
– mass #:
– # of protons:
– # of electrons:
– # of neutrons:
17
37
17
17
20
Cl37
17

11. Ions
• ions are electrically charged atoms
Neutral atom
negative ionpositive ion
lose electrons gain electrons
p+ > e-
p+ < e-
cation anion

12. Practice: determine the required values for
the positive calcium ion 41 Ca +2
41 Ca+2
– atomic #:
– mass #:
– # of protons:
– # of electrons:
– # of neutrons:
20
41
20
18
21

13. © Addison-Wesley Publishing Company, Inc.
Isotopes: Atoms of the same element with different
mass numbers.
carbon-12 and
carbon-14 are
isotopes
similar chemical properties
stable
radioactive

14. Average Relative Atomic Mass AR
• a weighted average of
all isotopes of an
element
100
(%)(mass(mass)(%) )

Avg.
Atomic
Mass
• this value is found on the Periodic Table
• based on the %
abundance data from
mass spectrometer

15. Average Relative Atomic Mass AR or RAM
What is the relative atomic mass of chlorine?
Chlorine has two naturally occurring isotopes,
Cl-35 and Cl-37. 75% of chlorine atoms are 35Cl.
What % of chlorine atoms is 37Cl?

16. Average Relative Atomic Mass AR or RAM
What is the relative atomic mass of chlorine?
Chlorine has two naturally occurring isotopes,
Cl-35 and Cl-37. 75% of chlorine atoms are 35Cl.
What % of chlorine atoms is 37Cl?
Avg.
Atomic =
Mass

17. Another way of looking at it
Mass Number of Isotope
Out of 100 atoms, how
many have this mass?
What would be the total
mass of these atoms?
35Cl
37Cl

18. Another way of looking at it
Mass Number of Isotope
Out of 100 atoms, how
many have this mass?
What would be the total
mass of these atoms?
35Cl 75
37Cl 25

19. Another way of looking at it
Mass Number of Isotope
Out of 100 atoms, how
many have this mass?
What would be the total
mass of these atoms?
35Cl 75 35 × 75 = 2625
37Cl 25 37 × 25 = 925

20. Another way of looking at it
Mass Number of Isotope
Out of 100 atoms, how
many have this mass?
What would be the total
mass of these atoms?
35Cl 75 35 × 75 = 2625
37Cl 25 37 × 25 = 925
Total mass of 100 atoms 3550

21. Another way of looking at it
Mass Number of Isotope
Out of 100 atoms, how
many have this mass?
What would be the total
mass of these atoms?
35Cl 75 35 × 75 = 2625
37Cl 25 37 × 25 = 925
Total mass of 100 atoms 3550
Average mass of 1 atom 35.5