Chemical Properties of Atoms

Chemical Properties of Atoms

Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. The chemical properties of the atom are determined by the number of protons, in fact, by number and arrangement of electrons. The configuration of these electrons follows from the principles of quantum mechanics. The number of electrons in each element’s electron shells, particularly the outermost valence shell, is the primary factor in determining its chemical bonding behavior. In the periodic table, the elements are listed in order of increasing atomic number Z.

The total number of protons in the nucleus of an atom is called the atomic number (or the proton number) of the atom and is given the symbol Z. The number of electrons in an electrically-neutral atom is the same as the number of protons in the nucleus. The total electrical charge of the nucleus is therefore +Ze, where e (elementary charge) equals to 1,602 x 10-19coulombs. Each electron is influenced by the electric fields produced by the positive nuclear charge and the other (Z – 1) negative electrons in the atom.

It is the Pauli exclusion principle that requires the electrons in an atom to occupy different energy levels instead of them all condensing in the ground state. The ordering of the electrons in the ground state of multielectron atoms, starts with the lowest energy state (ground state) and moves progressively from there up the energy scale until each of the atom’s electrons has been assigned a unique set of quantum numbers. This fact has key implications for the building up of the periodic table of elements.

Electron Affinity

In chemistry and atomic physics, the electron affinity of an atom or molecule is defined as:

the change in energy (in kJ/mole) of a neutral atom or molecule (in the gaseous phase) when an electron is added to the atom to form a negative ion.

X + e → X + energy        Affinity = – ∆H

In other words, it can be expressed as the neutral atom’s likelihood of gaining an electron. Note that, ionization energies measure the tendency of a neutral atom to resist the loss of electrons. Electron affinities are more difficult to measure than ionization energies.

A fluorine atom in the gas phase, for example, gives off energy when it gains an electron to form a fluoride ion.

F + e → F        – ∆H = Affinity = 328 kJ/mol

To use electron affinities properly, it is essential to keep track of sign. When an electron is added to a neutral atom, energy is released. This affinity is known as the first electron affinity and these energies are negative. By convention, the negative sign shows a release of energy. However, more energy is required to add an electron to a negative ion which overwhelms any the release of energy from the electron attachment process. This affinity is known as the second electron affinity and these energies are positive.

Affinities of Non metals vs. Affinities of Metals

  • Metals: Metals like to lose valence electrons to form cations to have a fully stable shell. The electron affinity of metals is lower than that of nonmetals. Mercury most weakly attracts an extra electron.
  • Nonmetals: Generally, nonmetals have more positive electron affinity than metals. Nonmetals like to gain electrons to form anions to have a fully stable electron shell. Chlorine most strongly attracts extra electrons. The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative values.

Electronegativity

Electronegativity, symbol χ, is a chemical property that describes the tendency of an atom to attract electrons towards this atom. For this purposes, a dimensionless quantity the Pauling scale, symbol χ, is the most commonly used.

The electronegativity of fluorine is:

χ = 4.0

In general, an atom’s electronegativity is affected by both its atomic number and the distance at which its valence electrons reside from the charged nucleus. The higher the associated electronegativity number, the more an element or compound attracts electrons towards it.

The most electronegative atom, fluorine, is assigned a value of 4.0, and values range down to cesium and francium which are the least electronegative at 0.7.

electron affinity and electronegativity

Ionization Energy

Ionization energy, also called ionization potential, is the energy necessary to remove an electron from the neutral atom.

X + energy → X+ + e

where X is any atom or molecule capable of being ionized, X+ is that atom or molecule with an electron removed (positive ion), and e is the removed electron.

A nitrogen atom, for example, requires the following ionization energy to remove the outermost electron.

N + IE → N+ + e        IE = 14.5 eV

The ionization energy associated with removal of the first electron is most commonly used. The nth ionization energy refers to the amount of energy required to remove an electron from the species with a charge of (n-1).

1st ionization energy

X → X+ + e

2nd ionization energy

X+ → X2+ + e

3rd ionization energy

X2+ → X3+ + e

Ionization Energy for different Elements

There is an ionization energy for each successive electron removed. The electrons that circle the nucleus move in fairly well-defined orbits. Some of these electrons are more tightly bound in the atom than others. For example, only 7.38 eV is required to remove the outermost electron from a lead atom, while 88,000 eV is required to remove the innermost electron. Helps to understand reactivity of elements (especially metals, which lose electrons).

In general, the ionization energy increases moving up a group and moving left to right across a period. Moreover:

  • Ionization energy is lowest for the alkali metals which have a single electron outside a closed shell.
  • Ionization energy increases across a row on the periodic maximum for the noble gases which have closed shells

For example, sodium requires only 496 kJ/mol or 5.14 eV/atom to ionize it. On the other hand neon, the noble gas, immediately preceding it in the periodic table, requires 2081 kJ/mol or 21.56 eV/atom.

Ionization energy
Source: wikipedia.org License: CC BY-SA 3.0
1
H

Hydrogen

Nonmetals

2
He

Helium

Noble gas

3
Li

Lithium

Alkali metal

4
Be

Beryllium

Alkaline earth metal

5
B

Boron

Metalloids

6
C

Carbon

Nonmetals

7
N

Nitrogen

Nonmetals

8
O

Oxygen

Nonmetals

9
F

Fluorine

Nonmetals

10
Ne

Neon

Noble gas

11
Na

Sodium

Alkali metal

12
Mg

Magnesium

Alkaline earth metal

13
Al

Aluminium

Post-transition metals

14
Si

Silicon

Metalloids

15
P

Phosphorus

Nonmetal

16
S

Sulfur

Nonmetal

17
Cl

Chlorine

Nonmetal

18
Ar

Argon

Noble gas

19
K

Potassium

Alkali metal

20
Ca

Calcium

Alkaline earth metal

21
Sc

Scandium

Transition metals

22
Ti

Titanium

Transition metals

23
V

Vanadium

Transition metals

24
Cr

Chromium

Transition metals

25
Mn

Manganese

Transition metals

26
Fe

Iron

Transition metals

27
Co

Cobalt

Transition metals

28
Ni

Nickel

Transition metals

29
Cu

Copper

Transition metals

30
Zn

Zinc

Transition metals

31
Ga

Gallium

Post-transition metals

32
Ge

Germanium

Metalloids

33
As

Arsenic

Metalloids

34
Se

Selenium

Nonmetal

35
Br

Bromine

Nonmetal

36
Kr

Krypton

Noble gas

37
Rb

Rubidium

Alkali metals

38
Sr

Strontium

Alkaline earth metals

39
Y

Yttrium

Transition metals

40
Zr

Zirconium

Transition metals

41
Nb

Niobium

Transition metals

42
Mo

Molybdenum

Transition metals

43
Tc

Technetium

Transition metals

44
Ru

Ruthenium

Transition metals

45
Rh

Rhodium

Transition metals

46
Pd

Palladium

Transition metals

47
Ag

Silver

Transition metals

48
Cd

Cadmium

Transition metals

49
In

Indium

Post-transition metals

50
Sn

Tin

Post-transition metals

51
Sb

Antimony

Metalloids

52
Te

Tellurium

Metalloids

53
I

Iodine

Nonmetal

54
Xe

Xenon

Noble gas

55
Cs

Caesium

Alkali metals

56
Ba

Barium

Alkaline earth metals

57-71

 

Lanthanoids

 

72
Hf

Hafnium

Transition metals

73
Ta

Tantalum

Transition metals

74
W

Tungsten

Transition metals

75
Re

Rhenium

Transition metals

76
Os

Osmium

Transition metals

77
Ir

Iridium

Transition metals

78
Pt

Platinum

Transition metals

79
Au

Gold

Transition metals

80
Hg

Mercury

Transition metals

81
Tl

Thallium

Post-transition metals

82
Pb

Lead

Post-transition metals

83
Bi

Bismuth

Post-transition metals

84
Po

Polonium

Post-transition metals

85
At

Astatine

Metalloids

86
Rn

Radon

Noble gas

87
Fr

Francium

Alkali metal

88
Ra

Radium

Alkaline earth metal

89-103

 

Actinoids

 

104
Rf

Rutherfordium

Transition metal

105
Db

Dubnium

Transition metal

106
Sg

Seaborgium

Transition metal

107
Bh

Bohrium

Transition metal

108
Hs

Hassium

Transition metal

109
Mt

Meitnerium

 

110
Ds

Darmstadtium

 

111
Rg

Roentgenium

 

112
Cn

Copernicium

 

113
Nh

Nihonium

 

114
Fl

Flerovium

 

115
Mc

Moscovium

 

116
Lv

Livermorium

 

117
Ts

Tennessine

 

118
Og

Oganesson

 

57
La

Lanthanum

Lanthanoids

58
Ce

Cerium

Lanthanoids

59
Pr

Praseodymium

Lanthanoids

60
Nd

Neodymium

Lanthanoids

61
Pm

Promethium

Lanthanoids

62
Sm

Samarium

Lanthanoids

63
Eu

Europium

Lanthanoids

64
Gd

Gadolinium

Lanthanoids

65
Tb

Terbium

Lanthanoids

66
Dy

Dysprosium

Lanthanoids

67
Ho

Holmium

Lanthanoids

68
Er

Erbium

Lanthanoids

69
Th

Thulium

Lanthanoids

70
Yb

Ytterbium

Lanthanoids

71
Lu

Lutetium

Lanthanoids

89
Ac

Actinium

Actinoids

90
Th

Thorium

Actinoids

91
Pa

Protactinium

Actinoids

92
U

Uranium

Actinoids

93
Np

Neptunium

Actinoids

94
Pu

Plutonium

Actinoids

95
Am

Americium

Actinoids

96
Cm

Curium

Actinoids

97
Bk

Berkelium

Actinoids

98
Cf

Californium

Actinoids

99
Es

Einsteinium

Actinoids

100
Fm

Fermium

Actinoids

101
Md

Mendelevium

Actinoids

102
No

Nobelium

Actinoids

103
Lr

Lawrencium

Actinoids