Electronic Arrangements

Electronic arrangements

Electronic arrangement refers to the manner in which electrons are arranged in an atom. An atom contains a central nucleus containing protons and neutrons, and a cluster of electrons revolving in orbits around the nucleus. These electrons are grouped in shells.

Bohr's Theory on the arrangement of electrons in atoms can be summarized as follows

1. Electrons are in orbit around the nucleus of the atom.
2. The electron orbits are grouped together in shells; a shell is a group of orbits occupied by electrons with approximately equal energy.
3. The electrons in shells distant from the nucleus have higher energy than those in shells close to the nucleus.
4. Electrons fill the shells starting with the first shell, which is closest to the nucleus. Shells are numbered 1, 2, 3, 4, etc. outwards from the nucleus. The letters K, L, M and N respectively starting from the nucleus may represent the shells.
5. The maximum possible number of electrons in a shell numbered n is 2n².
6. The first shell can only contain up to 2 electrons. The second shell can contain a maximum of 8 electrons. The third shell can contain up to 18 electrons.
7. In the outermost shell of any atom, the maximum number of electrons possible is 8.
8. The outer electrons of some atoms can be removed easily to form ions.
9. Chemical bonding between atoms to form molecules involves the electrons in the outer shell only.

Electronic arrangement of some atoms

Diagrams

Element	Symbol	Number of Electrons	1st Shell	2nd Shell	3rd Shell	4th Shell	Electron configuration
Hydrogen	H	1	1				1
Helium	He	2	2				2
Lithium	Li	3	2	1			3:1
Beryllium	Be	4	2	2			2:2
Boron	B	5	2	3			2:3
Carbon	C	6	2	4			2:4
Nitrogen	N	7	2	5			2:5
Oxygen	O	8	2	6			2:6
Fluorine	F	9	2	7			2:7
Neon	Ne	10	2	8			2:8
Sodium	Na	11	2	8	1		2:8:1
Magnesium	Mg	12	2	8	2		2:8:2
Aluminium	Al	13	2	8	3		2:8:3
Silicon	Si	14	2	8	4		2:8:4
Phosphorus	P	15	2	8	5		2:8:5
Sulfur	S	16	2	8	6		2:8:6
Chlorine	Cl	17	2	8	7		2:8:7
Argon	Ar	18	2	8	8		2:8:8
Potassium	K	19	2	8	8	1	2:8:8:1
Calcium	Ca	20	2	8	8	2	2:8:8:2