Lesson 3: Electron Configuration and Periodic Properties

Valence electrons

electron in an atom’s outermost energy level; determines the chemical properties of an atom 

The periodic table is divided into four blocks:

s, p, d, and f.  

The blocks correspond to which

energy sublevel in the element is filled or partially filled with valence electrons. 

s-block elements consist of groups

1 and 2, plus the element helium. 

The p-block consists of groups

13–18

groups 13-18 fill the

p orbitals. 

Elements in blocks s and p (groups 1, 2, and 13–18) are called the

 representative elements or the main group elements.  

In group 18, both the s and p orbitals of the period’s principal energy level are

 

 completely filled. 

Group 1 elements have . 

one valence electron

group 2 elements have

two valence electrons

The energy level of an element’s valence electrons indicates the

 period on the periodic table in which it is found. 

Valence electrons determines: 

The group an element belongs to  

Why elements in the same group behave similarly  

Elements in the same group have the same number of valence electrons, so they react in similar ways

 

Chemical reactivity 

atoms gain, lose, or share valence electrons to reach a stable electron configuration 

The number of valence electrons in an atom is related to

its position on the periodic table 

Properties repeat with increasing atomic number because 

electron configurations repeat 

Elements in the same group 

lose, gain, or share electrons in similar ways 

because Elements in the same group lose, gain, or share electrons in similar ways 

 they have similar chemical properties.

Alkali metals are all the

elements in group 1, except hydrogen.  

They have 1 valence electron 

partially filled s orbital with one electron

They lost this electron easily to form

positively charged, X⁺ ion 

Group 1 Metals are soft metals 

(we can cut/slice these metals with a knife)  

 most reactive metals 

alkali metals

alkali metals lose

only 1 valence electron 

alkali metals are stored in

kerosene

Alkali Metals are stored in kerosene to

prevent their reaction with moisture and oxygen from air 

Reactivity of group 1 metals increases

down the group 

The outer electron is  further from the positively charged nucleus (less strongly attracted); hence, it is

easier to lose the outer electron 

Alkaline earth metals are the

elements in Group 2 

Alkaline Earth Metals have 

2 valence electrons (filled s orbital with two electrons) 

Alkaline Earth Metals are

are harder, denser, and stronger  

Alkaline Earth Metals are reactive metals, but 

less reactive than Group 1 

Alkaline Earth Metals form

X⁺² ions by losing two valence electrons 

Losing two electrons requires

 

 more energy 

The transition elements are divided into 

Transition metals (d block) and inner transition metals (f block) 

Elements in groups 3–12 are known as the 

transition metals. 

There are exceptions, but d-block elements usually have

filled outermost s orbitals and filled or partially filled d orbitals. 

The five d orbitals can hold 10 electrons, so the d-block spans

ten groups on the periodic table. 

The f-block contains the

inner transition metals. 

f-block elements have

filled or partially filled outermost s orbitals and filled or partially filled 4f and 5f orbitals. 

The seven f orbitals hold

14 electrons, and the inner transition metals span 14 groups. 

 

Inner Transition metals (f block): 

The two sets of inner transition metals, known as:  

-the lanthanide series and  

-the actinide series 

are located along the bottom of the periodic table 

The lanthanides are the 14 elements with atomic numbers from

58 (cerium, Ce) to 71 (lutetium, Lu)

 

The actinides are the 14 elements with atomic numbers from 

90 (thorium, Th) to 103 (lawrencium, Lr) 

Halogens  are the elements in

group 17 

halo

salt

gen

make

Halogens have _ valence electrons

7 valence electrons

Halogens gain

1 electron easily to form negatively charged, X^- ion 

Halogens are nonmetals or metal

They are nonmetals 

Most reactive non-metals are

halogens

halogens react with metals to form

salt (ionic compounds) 

In halogens Reactivity decreases

down the group (Fluorine being  

the most reactive) 

Halogens gain only 1 electron to complete the 

outermost s and p sublevels (8 valence electrons; octet rule) 

Group 18 are known as

Noble gases

Nobel gases valence electron amount

Have 8 valence electrons (complete s and p outermost orbitals,

Exception for Nobel Gases - valence electron

helium which has only 2  ve

Nobel Gases - reactivity

Are extremely unreactive due to their complete outer energy level   

Nobel Gases are used in

various applications (e.g., helium in balloons, argon in filament light bulbs, etc…) 

Group 18 valence electron configuration

is ns²np⁶ e

Group 18 electron configuration exception

helium 1s² as its electrons only occupy the first energy level