Electronegativity
Some elements share electrons willingly. Others refuse to share at all. And then there is fluorine, which grabs electrons like a toddler who will not let go of a toy. Electronegativity measures exactly this tendency - how strongly an atom pulls on shared electrons when it is bonded to another atom.
This distinction matters: electron affinity describes an isolated atom gaining an electron, while electronegativity describes a bonded atom competing for electrons already being shared. Both reflect an atom’s appetite for electrons, but electronegativity is the property that governs real chemical bonds.
The Pauling Scale
Linus Pauling developed the most commonly used electronegativity scale. Values range from 0.7 (cesium) to 4.0 (fluorine). These values are dimensionless - they are relative numbers, not measured in any particular unit.
Noble gases are typically not assigned electronegativity values. Since they rarely form bonds, there is no tug-of-war to measure. You will never be asked for the electronegativity of neon or argon.
Electronegativity Values for Common MCAT Elements
| Element | Symbol | EN Value |
|---------|--------|----------|
| Fluorine | F | 4.0 |
| Oxygen | O | 3.5 |
| Nitrogen | N | 3.0 |
| Chlorine | Cl | 3.0 |
| Carbon | C | 2.5 |
| Sulfur | S | 2.5 |
| Hydrogen | H | 2.1 |
| Phosphorus | P | 2.1 |
| Sodium | Na | 0.9 |
| Cesium | Cs | 0.7 |
Periodic trends, and the single cause behind them
Scroll sideways to see the whole map.
The one cause underneathAcross a period, protons are added but the new electrons go into the same shell, so they shield each other poorly and effective nuclear charge climbs. Down a group, a whole new shell is added and the outer electrons are both further out and screened by everything beneath. Every trend on this page is one of those two sentences.
The two ionisation-energy dipsBe to B falls because boron's outermost electron is in a higher-energy 2p rather than 2s. N to O falls because oxygen is the first period-2 element forced to put two electrons in one 2p orbital, and their mutual repulsion makes one easier to remove. Both dips are on the plot, not smoothed away.
Ionic radiusCations are always smaller than their parent atom, often dramatically so when a whole shell is lost: Na is 186 pm, Na⁺ is 102 pm. Anions are always larger, because added electrons increase repulsion without adding any protons.
You do not need to memorize every value. Know that F > O > N = Cl > C = S > H, and you will handle most MCAT questions. If the question demands a precise value, the passage will provide it.
The Periodic Trend
Electronegativity follows the same directional trend as ionization energy and electron affinity:
- Across a period (left to right): electronegativity increases. Higher effective nuclear charge means the nucleus pulls more strongly on shared electrons.
- Down a group (top to bottom): electronegativity decreases. The valence electrons are farther from the nucleus and shielded by more inner electron shells, so the atom’s pull weakens.
Fluorine sits in the upper right corner of the periodic table (excluding noble gases), which is exactly where both trends converge to produce the maximum value. Cesium sits in the lower left corner, where both trends converge to produce the minimum.
Electronegativity Difference and Bond Type
Here is where electronegativity becomes truly powerful on the MCAT. The difference in electronegativity between two bonded atoms tells you what type of bond they form.
| EN Difference | Bond Type | Electron Sharing |
|---------------|-----------|-----------------|
| Less than 0.5 | Nonpolar covalent | Electrons shared equally (or nearly so) |
| 0.5 to 1.7 | Polar covalent | Electrons shared unequally |
| Greater than 1.7 | Ionic | Electrons effectively transferred |
These cutoffs are approximate, not absolute. Some sources use slightly different boundaries. But for the MCAT, this three-tier classification is what you need.
Worked Examples
Example 1: What type of bond forms between H (EN = 2.1) and Cl (EN = 3.0)?
EN difference = 3.0 - 2.1 = 0.9. This falls in the 0.5 to 1.7 range, so HCl has a polar covalent bond. The shared electrons spend more time near chlorine, giving Cl a partial negative charge and H a partial positive charge.
Example 2: What type of bond forms between Na (EN = 0.9) and Cl (EN = 3.0)?
EN difference = 3.0 - 0.9 = 2.1. This exceeds 1.7, so NaCl has an ionic bond. The electrons are not shared at all - they are transferred from sodium to chlorine.
Example 3: What type of bond forms between C (EN = 2.5) and H (EN = 2.1)?
EN difference = 2.5 - 2.1 = 0.4. This falls below 0.5, so C-H bonds are considered nonpolar covalent. This is why hydrocarbons are nonpolar molecules - every C-H bond is essentially nonpolar.
Connecting Electronegativity to Molecular Polarity
Bond polarity and molecular polarity are not the same thing. A molecule can have polar bonds but still be nonpolar overall if the bond dipoles cancel due to symmetry. CO₂ has two polar C=O bonds, but because the molecule is linear and symmetric, the dipoles point in opposite directions and cancel. Water also has two polar O-H bonds, but the bent geometry means the dipoles do not cancel, making water a polar molecule.
The MCAT tests this distinction frequently. Always consider both the EN difference (bond polarity) and the molecular geometry (whether dipoles cancel) when determining if a molecule is polar.
Polar covalent bond. An EN difference of 1.2 falls in the 0.5 to 1.7 range. The electrons are shared but unequally - they spend more time near the more electronegative atom, creating partial charges (delta+ and delta-) on the bonded atoms.
Noble gases rarely form chemical bonds. Electronegativity measures how strongly an atom attracts shared electrons in a bond. Since noble gases have full valence shells and generally do not participate in bonding, there is no tug-of-war to measure. No bond means no electronegativity.
Fluorine, with an electronegativity of 4.0. Fluorine sits in the upper right corner of the periodic table (excluding noble gases), where both the across-a-period trend and the down-a-group trend converge to maximize electronegativity.