Chapter 1: Where are the Electrons?

Atoms

1.  How many electrons are there in each of the following : N^- , O^2-, F^- and C⁺ ?

N has 7 electrons so N^- has 7+1 = 8, 6 valence and 2 core
O has 8 electrons so O^2- has 8+2 = 10, 8 valence and 2 core
F has 9 electrons so F^- has 9+1 = 10, 8 valence and 2 core
C has 6 electrons so C⁺ has 6-1 = 5, 3 valence and 2 core

2. What is the atomic mass unit for an atom of bromine?

79.904 amu, which is the weighted average of the isotopic masses for ⁷⁹Br (50.7%) and ⁸¹Br (49.3%).

3.  How many protons and neutrons are there is each of the following : ¹H, ²H, ¹²C, ¹⁶O, ³⁵Cl and ³⁷Cl ?

The atomic number, Z, which defines the number of protons is defined by the elemental symbol. The mass numbers are given as superscripts before the atomic symbol. Note that because atoms are neutral, the number of positive protons also defines the number of negative electrons surrounding the nucleus.

Orbitals

1.   Which quantum number defines an orbitals energy level for (a) the hydrogen atom and (b) other atoms ?

(a) For the hydrogen atom, the principle quantum number, n, defines the energy level.
(b) For atoms other than hydrogen atom, the principle quantum number, n, and the angular momentum quantum number, l, define the orbital energy levels.

           First determine what quantum numbers are indicated by the name 3p.

Any of the following 'sets' of quantum numbers will be valid:

 

n	l	ml
3	1	1
3	1	0
3	1	-1

First consider their electron configurations (core, valence)

Both have a total of four, occupied, valence orbitals, one s orbital and three p orbitals.

First determine what quantum numbers are indicated by the name 3s.

Then note that you want the quantum numbers for an electron so m_s must also be considered!
Any of the following 'sets' of quantum numbers are valid.

 

n	l	ml	ms
3	0	0	-1/2
3	0	0	+1/2

Use the information given in the box below when answering the next three questions:
 

1.  Which of the electron configurations above best describes the ground state electron configuration of Cl^-? (a)
2.  Which of the electron configurations above is possible for the ground state of P?  (e)
3.  Which of the electron configurations above describes an excited state of Mg⁺? (c)

4.  Is the following electron configuration consistent with Hund's rule of maximum multiplicity?

Yes, all electrons unpaired before pairing

No, two electrons in a p orbital have the same four quantum numbers

Did you draw the core or did you draw the valence electron configuration by mistake ?

Did you draw the valence or did you draw the core electron configuration by mistake ?