Video: Applying Knowledge of Electronegativity and Atomic Radius and Their Trends on the Periodic Table

For statements I and II, state for each if they are true or false. I) Alkali metals are the least electronegative elements in the periodic table. II) electronegativity generally decreases as the atomic radius increases across a period of the periodic table. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements one and two, state for each if they are true or false. 1) Alkali metals are the least electronegative elements in the periodic table. 2) electronegativity generally decreases as the atomic radius increases across a period of the periodic table. If both are true, state if two is a correct explanation for one.

The alkali metals are the metals to be found in group one of the periodic table. The element hydrogen, sometimes placed at the top of group one, is not a metal and it’s not an alkali metal. So, the alkali metals are lithium, sodium, potassium, rubidium, cesium, and francium although there are no stable isotopes of francium, so it’s usually removed for practical reasons. Meanwhile, the electronegativity of an element indicates the strength of attraction between atoms of that element and shared electrons in bonds.

If we have a more electronegative atom bonded to a less electronegative atom, the more electronegative atom will have a stronger force of attraction on the electrons in the bond, meaning that the electrons will lie closer to the more electronegative atom than the less electronegative atom. So, the more electronegative atom will be slightly negative, and the less electronegative atom will be slightly positive.

There’s a trick we can use on the periodic table. As we go left to right or bottom to top, elements become progressively more electronegative although the pattern isn’t perfect and we have to make exceptions for the noble gases. Elements like helium, neon, and argon are so unreactive that they don’t have defined electronegativity values. On the other hand, alkali metals do. And being most to the left of the periodic table, as a group, they represent the least electronegative elements in the periodic table. So, statement one is true. Statement two may help reveal exactly why.

Statement two says that electronegativity generally decreases as the atomic radius increases across a period of the periodic table. Each row of the periodic table is a period, so let’s have a look at what happens to the atomic radius moving across a period. Here, I’ve chosen period two, but any period will do. As we go left to right across a period, the atomic number increases. The atomic number of an element is simply the number of protons in the nucleus of an atom or ion of that element. This means that as we move left to right across a period, the charge of the nucleus of an atom of each element increases. As we move across the period, there are more and more electrons in the atoms. But the electrons are being added to the same shell.

So, as the nuclear charge increases, those electron shells are being compacted. So, a fluorine atom is much smaller than a lithium atom. So, this means the atomic radius actually increases right to left. Increasing the charge in the nucleus and making the atom smaller make it easier for that atom to attract electrons in bonds. While having a larger atom with a lower-charged nucleus will diminish the electronegativity. At the far left, alkali metals have the worst combination of these factors. And it is generally true that electronegativity decreases as the atomic radius increases across a period of the periodic table. The key thing to identify here was that atomic radius increases right to left, not left to right.

Now, we can move on to the last part of the question. While atomic radius isn’t a complete picture of electronegativity, the fact that it increases left to right across a period gives us a good idea of why electronegativity decreases. So, it is true that two is a correct explanation for one.

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