1. 4.3 How Atoms Differ

2. 4.3 Atomic Number – number of protons in an atom
On periodic table -top left number (whole #).
Identifies element.
Ex. He = 2
Has 2 protons
Elements are arranged by increasing atomic number.

3. Since atoms are neutral: # protons (+) = # electrons (-)
What is the atomic number for Li?
How many protons?
Electrons?
Ans. 3 protons, 3 electrons
Use the periodic table to find the atomic number for oxygen.
Ans. 8 protons, 8 electrons

4. Isotopes – atoms of the same element having different number of neutrons
Have same # protons & electrons
(same element)
Have different masses
(since different # neutrons)
What element is shown in the diagram?
How many protons and
electrons in each isotope?
How many neutrons?
What is the mass?

6. Mass # = # protons + # neutrons Ex. B-11 (boron with mass# of 11)
Mass of electrons is not counted (negligible).
# neutrons = mass # - atomic #
# neutrons = = 6
Mass on periodic table
average mass of all the
isotopes of a given element.
Boron has 2 isotopes,
B-10 and B-11
Which is more common?

7. Given the Isotope C-14: (red = protons, black = neutrons)
1) Find the atomic #.
2) Find the # protons.
3) Find the # electrons.
4) Find the neutrons.
5) What is the mass #?
6) What is carbon’s
atomic mass?
APE: atomic# = #protons = #electrons (neutral)
1) 6 2) 6 3) 6 4) 14-6=8 5) 14 6)

8. C-12 Isotope Find the atomic #, protons, & electrons.
What is its mass #?
How many neutrons?
12-6 = 6
What is the atomic mass?
Compare to the C-14
isotope.

9. Compare the 3 isotopes of hydrogen: H-1 H-2 H-3
How many p+ for each?
What is the atomic #?
How many e-?
What is the mass#
How many no?
What is the atomic mass for H?
amu

10. Isotopic Notation 14 C or 14 C 6 Used to show mass # (top)
and atomic # (bottom)
1) Find atomic #, number protons, number of electrons.
2) How many neutrons?
Ans. 1) 6,6,6 2)14-6=8

11. Quiz
1) What is lithium’s atomic number?
2) How many protons & electrons does Li have?
3) What does N-13 mean?
Find the # protons, at.#, neutrons, electrons.
4) Write the name of the isotope having 8 protons and 9 neutrons.
5) Write the isotopic notation for this isotope.
17O 8

12. Fill in the Chart: Symbol Isotope Name Protons Neutrons Electrons 31P15 8 7
Al-27

13. IONS – Charged Particles (Atoms with a charge)
Negative ions are formed when electrons are gained.
Ex. F – At. # = 9
F-1 has 10 electrons
Positive ions are formed when electrons are lost.
Ex. Mg – At. # = 12
Mg2+ has 10 electrons

14. * Find p+, e-, n0 for isotopes of ions:Ca
20 protons
20 electrons if neutral
18 electrons has a +2 charge (lost 2)
21 neutrons P
15 p+, 15 e-, (if neutral)
+ 3 = 18 electrons

15. This is nearly equal to mass of a proton or neutron:
Mass of Individual Atoms – since atomic masses are so tiny, scientists assign atomic mass units (amu).
1 amu = 1/12 the mass of 1 atom of C –12.
This is nearly equal to mass of a proton or neutron:
1 p+ = amu
1 n0 = amu
1 e- = amu

16. Atomic Mass is a weighted average. To find a weighted average:
(% as decimal)(Mass 1)+ (% as dec.)(Mass 2)
Ex. Find the average mass of football team if:
92.0% weigh 200. lbs % weigh 180. lbs.
Ave. mass = (0.920)(200) + (0.0800)(180) =
= 198 lbs.

17. Atomic mass of element – Weighted average mass of the isotopes of that element.
Isotopes % Abundance Atomic Mass
Cl % amu
Cl % amu
At.Mass= (%Cl-35 as dec.)(Mass) +(%Cl-37)(Mass)
= ( )(34.969) + ( )(36.966)
= amu amu
= amu
Check answer on Periodic Table √
Ans. Closest to Cl-35 (76% abund.)

18. Quiz Review 1) Look over all vocabulary.
2) Look over scientists & contributions: Democritus, Dalton, Rutherford
3) Know parts of the atom. (Atom chart)
4) Know how to read isotopic symbols.
5) Be able to use symbol & isotope name to find protons, neutrons & electrons.
6) Find atomic mass & element, given isotopes & % abundance.

19. The End