# Why does electronegativity increase across a period?

What explains why electronegativity increases as you move across a period? Does it have something to do with the shielding effect of added electrons?

• Good question. In reality electrons will be revolving around the nucleus in high speed. It is difficult to imagine, how electron will be added to electronegative element. I don't know whether some piling up of electrons would be there or not, when electron is added under such high speed. It is also to be noted that electron in high speed shows wave nature and not particle nature. As I am yet college student I haven't got convinced with those wave and particle nature. – Immortal Player Nov 25 '13 at 14:25
• I had a simple doubt that, even though atom will neutral, how could there be electric field, which is the reason for attraction of electrons of other elements by an electronegative atom. When I learnt coulomb's law, I got the answer. When the distance between two dipole charges is zero, there will be no electric field and no attraction or repulsion. As in atom we have protons and electrons separated by a certain distance, electric field will not be zero, so there will be attraction or repulsion. Though this was not the answer for your question, I thought it would be helpful for you. – Immortal Player Nov 25 '13 at 14:37

The electronegativity is the tendency of an atom or a functional group to attract electrons (or electron density) towards itself.

This is because the attraction of bonding electrons by an atom increases with nuclear charge (Atomic Number) and decrease of atomic radius. Both these factors operate as we move to the right in period.

Does it have something to do with the shielding effect of added electrons?

The electron cloud in the inner orbits act as a shield and reduces the nuclear attraction to the outer orbits. Because of the shielding effect, the tendency of nuclear attraction reduces and thus electronegativity reduces.

ashu's answer is good, but it does not get to the real reason that electronegativity increases across periods. Electronegativity is dependent on other properties that are more intuitively dependent on position within a period.

The absolute (Mulliken) electronegativity $\chi$ (which correlates with the more familiar, but more abstract, Pauling electronegativity) of an atom is the arithmetic mean of its first ionization energy and its first electron affinity

$$\chi_A = (E_{i,A}+E_{ea,A})/2$$

These values behave in a predictable manner related to the effective nuclear charge idea ashu mentions. Consider the data from the second period (all values in kJ/mol):

    Ei      Ea     X
Li  520     60     290
Be  900     0      450
B   800     27     414
C   1087    122    605
N   1402    ~0     701
O   1314    141    728
F   1681    328    1005
Ne  2081    ~0     1041


Notice that $E_i$ increases and $E_{ea}$ also increases (though not as regularly) from lithium to neon. The electronegativities also increase.

Across the period, electronegativity increases. This is mainly due to the increase in effective nuclear charge. As the number of core electrons remain constant, the number of protons increases. The protons actually draw those electrons closer to the nucleus.

• That was already said in earlier answers. – Mithoron Feb 9 '16 at 23:00

Along a period the effective nuclear charge increases thus decreasing the atomic radius. This favours the increase in electronegativity and therefore it increases along a period from left to right.

Electronegativity means tendency to attract electron,along the period atomic size decreases,atomic size decreases means there is more attraction of nucleus towards electron,more attraction of nucleus means there is more positive charge,and more positive charge means there is more tendency to attract electron.thus electronegativity increases.

Well the electronegativity increases across the period because the electrons are being added onto the same energy level,this increases the number of electrons of an atom,the increase of electrons of an atom also leads to increase in clear charge,in fact the nuclear charge increases more,this leads to attraction of electrons by the nuclear charge,hence electrons get attracted towards the nuclear charge hence electrons are not easily lost!