What do you notice about the number of energy levels or shells as you move from left to right?

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Trends in the Number of Valence Electrons

The table below for the main group elements is set out just like the Periodic Table of the elements.

The electronic configuration in shell notation is given for an atom of each of the elements.

The electrons in the valence shell (highest energy level) are given in red.

Can you see a pattern, or trend, in the electronic configuration of the atoms in each group and in each period?

Group 1
(IA) Group 2
(IIA) Group 13
(IIIA) Group 14
(IVA) Group 15
(VA) Group 16
(VIA) Group 17
(VIIA) Group 18
(VIIIA or 0) 1 valence electron 2 valence electrons 3 valence electrons 4 valence electrons 5 valence electrons 6 valence electrons 7 valence electrons 8 valence electrons
(EXCEPTION He has 2 valence electrons)

	symbol electron configuration in shells (valence shell electrons, the number of electrons in the highest energy level, shown in red)
Li 2,1	Be 2,2		B 2,3	C 2,4	N 2,5	O 2,6	F 2,7	Ne 2,8
Na 2,8,1	Mg 2,8,2		Al 2,8,3	Si 2,8,4	P 2,8,5	S 2,8,6	Cl 2,8,7	Ar 2,8,8
K 2,8,8,1	Ca 2,8,8,2		Ga 2,8,18,3	Ge 2,8,18,4	As 2,8,18,5	Se 2,8,18,6	Br 2,8,18,7	Kr 2,8,18,8
Rb 2,8,18,8,1	Sr 2,8,18,8,2		In 2,8,18,18,3	Sn 2,8,18,18,4	Sb 2,8,18,18,5	Te 2,8,18,18,6	I 2,8,18,18,7	Xe 2,8,18,18,8
Cs 2,8,18,18,8,1	Ba 2,8,18,18,8,2		Tl 2,8,18,32,18,3	Pb 2,8,18,32,18,4	Bi 2,8,18,32,18,5	Po 2,8,18,32,18,6	At 2,8,18,32,18,7	Rn 2,8,18,32,18,8
Fr 2,8,18,32,18,8,1	Ra 2,8,18,32,18,8,2

You should see two patterns, or trends:

• Elements in the same Group have the same number of valence shell electrons.
  GroupNumber of
  valence electrons

  1 (IA)	2 (IIA)	13 (IIIA)	14 (IVA)	15 (VA)	16 (VIA)	17 (VIIA)	18 (VIIIA or 0)
  1	2	3	4	5	6	7	8 2

• Across a period of the Periodic Table from left to right the number of valence electrons increases.2
  GroupNumber of
  valence electrons Trend:

  1 (IA)	2 (IIA)	13 (IIIA)	14 (IVA)	15 (VA)	16 (VIA)	17 (VIIA)	18 (VIIIA or 0)
  1	2	3	4	5	6	7	8 2
  lowest number of valence electrons	→	→	→	→	→	→	highest number of valence electrons

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In the table below, the electronic configuration of only the valence electrons is given using subshell notation.

Can you find a pattern, or trend, in the energy levels occupied by the valence electrons?

Period Group 1
(IA) Group 2
IIA Group 13
(IIIA) Group 14
(IVA) Group 15
(VA) Group 16
(VIA) Group 17
(VIIA) Group 18
(VIIIA or 0) Energy Level of Valence Electrons Trend 1 1
(K shell) Lowest Energy
↓ 2 2
(L shell) ↓ 3 3
(M shell) ↓ 4 4
(N shell) ↓ 5 5
(O shell) ↓ 6 6
(P shell) ↓ 7 7
(Q shell) ↓
Highest Energy

		symbol position of valence shell electrons (energy level of valence electrons shown in red)
		He 1s2
Li 2s1	Be 2s2		B 2s22p1	C 2s22p2	N 2s22p3	O 2s22p4	F 2s22p5	Ne 2s22p6
Na 3s1	Mg 3s2		Al 3s23p1	Si 3s23p2	P 3s23p3	S 3s23p4	Cl 3s23p5	Ar 3s23p6
K 4s1	Ca 4s2		Ga 4s24p1	Ge 4s24p2	As 4s24p3	Se 4s24p4	Br 4s24p5	Kr 4s24p6
Rb 5s1	Sr 5s2		In 5s25p1	Sn 5s25p2	Sb 5s25p3	Te 5s25p4	I 5s25p5	Xe 5s25p6
Cs 6s1	Ba 6s2		Tl 6s26p1	Pb 6s26p2	Bi 6s26p3	Po 6s26p4	At 6s26p5	Rn 6s26p6
Fr 7s1	Ra 7s2

You should see two patterns, or trends, in the energy levels of the valence electrons:

• The valence shell electrons of elements in the same period of the Periodic Table occupy the same energy level:
  PeriodEnergy Level of
  Valence ElectronsValence Shell

  1	1	K
  2	2	L
  3	3	M
  4	4	N
  5	5	O
  6	6	P
  7	7	Q

• Down a Group of the Periodic Table from top to bottom, the energy of the valence electrons increases:
  Energy Level of Valence Electrons

  in a Group

  Trend

  2	lowest energy
  3	↓
  4	↓
  5	↓
  6	↓
  7	highest energy

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The periodic table is organized like a big grid. Each element is placed in a specific location because of its atomic structure. As with any grid, the periodic table has rows (left to right) and columns (up and down). Each row and column has specific characteristics. For example, magnesium (Mg) and calcium (Mg) are found in column two and share certain similarities while potassium (K) and calcium (Ca) from row four share different characteristics. Magnesium and sodium (Na) also share qualities because the are in the same period (similar electron configurations).

You've got Your Periods...

Even though they skip some squares in between, all of the rows read left to right. When you look at the periodic table, each row is called a period (Get it? Like PERIODic table.). All of the elements in a period have the same number of atomic orbitals. For example, every element in the top row (the first period) has one orbital for its electrons. All of the elements in the second row (the second period) have two orbitals for their electrons. As you move down the table, every row adds an orbital. At this time, there is a maximum of seven electron orbitals.

...and Your Groups

Now you know about periods going left to right. The periodic table also has a special name for its vertical columns. Each column is called a group. The elements in each group have the same number of electrons in the outer orbital. Those outer electrons are also called valence electrons. They are the electrons involved in chemical bonds with other elements.

Every element in the first column (group one) has one electron in its outer shell. Every element in the second column (group two) has two electrons in the outer shell. As you keep counting the columns, you'll know how many electrons are in the outer shell. There are exceptions to the order when you look at the transition elements, but you get the general idea. Transition elements add electrons to the second-to-last orbital.

For example, nitrogen (N) has the atomic number seven. The atomic number tells you there are seven electrons in a neutral atom of nitrogen. How many electrons are in its outer orbital? Nitrogen is in the fifteenth column, labelled 'Group VA'. The 'V' is the Roman numeral for five and represents the number of electrons in the outer orbital. All of that information tells you there are two electrons in the first orbital and five in the second (2-5).

Phosphorus (P) is also in Group VA which means it also has five electrons in its outer orbital. However, because the atomic number for phosphorus is fifteen, the electron configuration is 2-8-5.

Two at the Top

Hydrogen (H) and helium (He) are special elements. Hydrogen, in its neutral form, does not have a neutron. There is only one electron and one proton. You probably won't find atomic hydrogens floating around by themselves. Atomic hydrogen wants to combine with other elements to fill its outer shell. Your chemistry work will most likley use molecular hydrogen (H2) or hydrogen ions (H+, protons).

Helium (He) is different from all of the other elements. It is very stable with only two electrons in its outer orbital (valence shell). Even though it only has two electrons, it is still grouped with the noble gases that have eight electrons in their outermost orbitals. The noble gases and helium are all "happy," because their valence shell is full.