Chemistry 4.3 Notes

Distinguishing Among Atoms

What we know…

Pg. 110 – Atoms are composed of electrons, protons, and neutrons. Protons and neutrons make up the small, dense nucleus. Electrons surround the nucleus and occupy most of the volume of the atom.

**How do atoms differ from each other then?

I. Atomic Number

A. Def – the # of protons found in an atom.1. Found in the nucleus2. Above symbol on P.T.3. Atoms differ from each other due to the number of protons.4. Ex: Hydrogen 1 proton5. Ex: Oxygen 8 protons

6. He Be

Ne Mo

Pb Sn Ba

Atomic Number

2

4

10

42

82

50

56

II. Mass NumberA. Def – the total # of neutrons and protons

in the nucleus of an atom.1. Found above chemical symbol on P.T.2. Formula: Mass # = protons + neutrons3. Ex: Na 234. Ex: Al 27

5. B Li Rn Cr Ti Zn

Mass Number

11

7

222

52

48

65

III. What about Electrons?

A. **Assume electrons = the # of protons in an atom**

1. All atoms have a charge of zero

2. Complete the following table in your notes.

Name SymbolAtomic Number

Protons NeutronsMass

NumberElectrons

Scandium

Copper

Krypton

Neon

Phosphorus

Gallium

Manganese

Name SymbolAtomic Number

Protons NeutronsMass

NumberElectrons

Scandium Sc 21 21 24 45 21

Copper Cu 29 29 35 64 29

Krypton Kr 36 36 48 84 36

Neon Ne 10 10 10 20 10

Phosphorus P 15 15 16 31 15

Gallium Ga 31 31 39 70 31

Manganese Mn 25 25 30 55 25

Symbol Protons Electrons Neutrons Atomic # Mass #

W

76 128

45

41

57

150

III. Isotopes

A. Def – same atom but different # of neutrons.

1. This alters the mass #

2. Chemically the same

3. Ex: Hydrogen

3 isotopes = H-1, H-2, H-3

4. Drawing an isotope

1st put in # of protons

2nd use mass # to determine the # of neutrons

5. Draw the 3 isotopes of hydrogen

6. Draw the 3 isotopes of NeonNe-20

Ne-21

Ne-22

7. Draw the 2 isotopes of Chlorine (35 and 37)

IV. Atomic MassA. Def – a weighted average of all the isotopes in a sample

of a given element.1. reflects mass2. reflects relative abundance

B. Atomic Mass Unit1. Def – 1/12th of a carbon-12 atom2. Prevents using scientific notation

ex: 3.155 x 10-22 g = fluorine3. Why is the atomic mass not a whole number?

isotopes4. There are isotopes for almost every element

5. Ex: Chlorine 35.453

2 isotopes (Cl-35 and Cl-37)

What would the atomic mass be?

36.000 right? Wrong

we need the relative abundance

6. Cl-35 has an abundance of 75.77%. Cl-37 has an abundance of 24.23%.

35 x .7577 = 26.52

37 x .2423 = 8.965

35.485

7. Hydrogen has 3 isotopes, H-1, H-2, H-3. H-1 occurs 99.99% of the time, H-2 occurs 0.015% of the time, and H-3 is negligible (can only be made in the lab). What is the atomic mass?

1.030