B. Matter, Chemical Trends, and Chemical Bonding · Matter, Chemical Trends, and Chemical BondingElectronegativity and Analysing Trends Across a Period
B2.1 B2.2 B3.3 A1.6 A1.8 A1.11About 70 minutes
Learning goals- I can define electronegativity and describe its trends.
- I can summarize all the periodic trends and explain them with effective nuclear charge and distance.
- I can analyse data for the elements in a period to identify general trends and exceptions.
- I can communicate data analysis with graphs and clear statements.
Words to know
electronegativity (EN)
A unitless measure of the ability of an atom in a bond to attract shared electrons.
Pauling scale
The most common electronegativity scale, from about 0.7 to 4.0 (fluorine).
metallic character
How readily an element loses electrons and shows metallic properties.
periodic trend
A regular change in a property across a period or down a group.
exception (anomaly)
A data value that does not follow the general trend.
A tug-of-war over electrons
When two atoms share a pair of electrons in a bond, both nuclei pull on that pair. If one atom pulls harder, the electrons spend more of their time near it. Chemists needed a number to describe how hard each atom pulls, and the American chemist Linus Pauling gave them one in 1932: electronegativity.
Electronegativity is the last of the periodic properties in this unit. It will be the key tool in the bonding lessons that follow. In this lesson you will also do what expectation B2.2 asks: analyse real data on the properties of elements across a period and find the trends yourself.
Electronegativity
Electronegativity (EN) is a measure of the ability of an atom in a chemical bond to attract the shared electrons towards itself. It has no units. On the Pauling scale:
- Fluorine has the highest value, 4.0.
- Oxygen is 3.4, chlorine 3.2, nitrogen 3.0.
- Metals have low values: sodium 0.9, potassium 0.8, caesium 0.8.
- Helium, neon and argon are usually given no value, because they do not normally form bonds.
Trends: electronegativity increases across a period (Zeff increases and atoms get smaller, so the nucleus pulls bonding electrons harder) and decreases down a group (bonding electrons are farther from the nucleus and more shielded).
Electronegativity is different from electron affinity. Electron affinity is a measured energy for a single isolated atom gaining an electron. Electronegativity describes an atom's pull on electrons it shares in a bond. The two follow similar trends.
All the trends in one picture
Every property in this unit so far follows from the same two ideas: effective nuclear charge (increases across a period) and distance (increases down a group).
- Atomic radius: decreases across, increases down.
- Ionization energy: increases across, decreases down.
- Electron affinity (energy released): generally increases across, decreases down.
- Electronegativity: increases across, decreases down.
- Metallic character (how easily an element loses electrons and behaves like a metal): decreases across, increases down.
A useful summary: elements in the top right (excluding noble gases) hold electrons most strongly; elements in the bottom left hold them least strongly.
Analysing data across a period
Scientists find trends by organizing data in tables and graphs, then asking: what is the general pattern, and are there exceptions? Here is real data for period 3.
| Property | Na | Mg | Al | Si | P | S | Cl | Ar |
|---|
| Electron arrangement | 2,8,1 | 2,8,2 | 2,8,3 | 2,8,4 | 2,8,5 | 2,8,6 | 2,8,7 | 2,8,8 |
| Covalent radius (pm) | 166 | 141 | 121 | 111 | 107 | 105 | 102 | 106 |
| First IE (kJ/mol) | 496 | 738 | 578 | 787 | 1012 | 1000 | 1251 | 1521 |
| Electronegativity | 0.9 | 1.3 | 1.6 | 1.9 | 2.2 | 2.6 | 3.2 | — |
| Melting point (°C) | 98 | 650 | 660 | 1414 | 44 (white P) | 115 | −102 | −189 |
| Type | metal | metal | metal | metalloid | non-metal | non-metal | non-metal | noble gas |
Worked exampleExample 1: Finding trends and exceptions in data
Use the period 3 table to describe the trend in first ionization energy, and identify any exceptions.
- Read the row from left to right: 496, 738, 578, 787, 1012, 1000, 1251, 1521.
- Overall, the values rise from 496 to 1521 kJ/mol: the general trend is an increase.
- Look for values that go the wrong way: Al (578) is lower than Mg (738), and S (1000) is lower than P (1012).
- Decide whether these are mistakes: the same dips appear in period 2 (B, O), so they are a real pattern, explained by sub-shells in SCH4U.
IE₁ generally increases across period 3, with two small exceptions (Al and S).
Worked exampleExample 2: Not every property follows a simple trend
Describe the pattern in melting point across period 3. Does it follow the same kind of trend as electronegativity?
- Melting points: 98, 650, 660, 1414, 44, 115, −102, −189 °C.
- They rise from Na to a peak at Si (1414 °C), then drop sharply for P, S, Cl and Ar.
- This is not a steady increase or decrease. Melting point depends on the structure of the solid: metals and silicon have atoms bonded in a continuous network, while P₄, S₈, Cl₂ and Ar are separate small molecules or atoms held by weak forces.
- Electronegativity, by contrast, increases steadily.
Melting point rises to a maximum at silicon, then falls. Unlike electronegativity, it depends on how the atoms are bonded together, not just on single-atom properties.
Worked exampleExample 3: Ranking by electronegativity
Without a table, rank O, S, Na and Cl from lowest to highest electronegativity. Then check with values.
- Na is a group 1 metal (bottom left of these): lowest.
- S and Cl are in period 3; Cl is farther right, so Cl > S.
- O is above S in group 16, so O > S. O is also nearer the top right than Cl.
- Values: Na 0.9, S 2.6, Cl 3.2, O 3.4.
Data analysis skills (A1.6, A1.8)
When you analyse data: (1) organize it in a table or graph; (2) state the general trend in words; (3) identify exceptions and check whether they are real (repeated in other periods, larger than measurement error) or likely errors; (4) explain the trend with a model (here, Zeff and distance). Never ignore an exception just because it is inconvenient.
Just for funWhy does fluorine always win at tug-of-war? It has the strongest grip on the table: 4.0 on the Pauling scale. Francium and caesium, down in the bottom left corner, barely hold the rope.
Hands-on activityGraph and analyse period 3
You need: graph paper or a spreadsheet program, a ruler and a pencil.
- Using the period 3 table, make three graphs with the elements (Na to Cl or Ar) across the bottom: (a) covalent radius, (b) electronegativity, (c) melting point.
- Under each graph, write one sentence giving the general trend and one sentence naming any exceptions.
- Compare graph (a) with graph (b). As radius goes down, what happens to electronegativity? Explain in one sentence using effective nuclear charge.
- Graph (c) looks very different. Write a hypothesis to explain why (hint: think about what is holding the particles together in each solid).
- Optional: look up the same properties for period 2 (Li to F) in a reliable source, such as a textbook or a chemistry data booklet, and see whether the patterns repeat. Record your source.
What to look for: radius falls while electronegativity rises (larger Zeff pulls electrons closer and harder); melting point peaks at Si. A good hypothesis mentions that Si atoms form a network of strong bonds, while Cl₂ and Ar are small separate particles.
Your turn
1Define electronegativity.
The ability of an atom in a chemical bond to attract shared electrons towards itself.
2Which element has the highest electronegativity? What is its value on the Pauling scale?
3Rank in order of increasing electronegativity: N, Li, C, F.
Li (1.0) < C (2.6) < N (3.0) < F (4.0).
4Which is more electronegative, Cl or Br? Explain.
Cl (3.2 vs 3.0): its bonding electrons are in a shell closer to the nucleus.
5Why are helium, neon and argon usually not given electronegativity values?
They do not normally form bonds, and electronegativity describes an atom in a bond.
6State the trend in metallic character across a period and down a group.
Metallic character decreases across a period and increases down a group.
7From the period 3 table, which element has the highest melting point, and what kind of element is it?
Silicon (1414 °C), a metalloid.
More practice: warm-up
1Who created the electronegativity scale used in this lesson?
2Does electronegativity increase or decrease down a group?
3Which is more electronegative, O or S?
4Which corner of the periodic table has the most metallic elements?
5What is the electronegativity of sodium?
More practice: on your own
1Explain the difference between electron affinity and electronegativity.
Electron affinity is the energy change when an isolated gaseous atom gains an electron. Electronegativity is a unitless measure of how strongly an atom in a bond pulls on shared electrons.
2From the period 3 data, does radius have an exception? (Look at Ar.) Suggest why this value may not be comparable.
Ar (106 pm) is larger than Cl (102 pm). Argon does not form covalent bonds normally, so its radius is measured or estimated differently; values from different methods are not directly comparable.
3Use Zeff and distance to explain why the electronegativity of Cs (0.8) is so much lower than that of F (4.0).
Cs: Zeff ≈ +1 and its valence shell is shell 6, very far from the nucleus. F: Zeff ≈ +7 in shell 2. F's nucleus pulls bonding electrons with a much larger net charge from much closer.
4Predict whether the melting point of potassium is higher or lower than sodium's (98 °C). Then check: potassium melts at 63 °C. Was your prediction supported?
Lower is a reasonable prediction: larger atoms with a weaker hold on their valence electron form weaker metallic bonding. The data (63 °C < 98 °C) support it.
5A student reports electronegativities for period 2 as Li 1.0, Be 1.6, B 2.0, C 2.6, N 3.0, O 3.4, F 4.0. Describe the trend and state whether there are exceptions.
Electronegativity increases steadily from Li to F with no exceptions, which matches the expected increase in Zeff.
Challenge
1Explain why ionization energy shows small exceptions across a period but electronegativity does not.
Ionization energy is measured for one electron in an isolated atom, so it is sensitive to which sub-shell that particular electron is in. Electronegativity is a derived average measure of attraction for bonding electrons and changes smoothly with Zeff and size.
2Germanium (Ge) is below silicon. Predict (with reasons) whether Ge has a larger or smaller radius, higher or lower IE₁, and higher or lower EN than Si. Check: Ge radius 120 pm, IE₁ 762 kJ/mol, EN 2.0. Which prediction did the data not support?
Expected: larger radius (new shell), lower IE (farther valence electrons), lower EN. Data: radius 120 > 111 ✔; IE 762 < 787 ✔; EN 2.0 > 1.9 ✘. The EN prediction is not supported: the filled transition-metal shell in period 4 shields poorly, so Ge's Zeff is higher than the simple model predicts. Real data sometimes reveal effects a simple model leaves out.
3Design a graph that best shows that ionization energy and atomic radius are related. What would you put on each axis, and what shape do you expect?
Put atomic radius on the x-axis and first IE on the y-axis, one point per element (for example, the first 20 elements, excluding noble gases whose radii are measured differently). Expect a downward trend: larger atoms have lower ionization energies.
Quiz
1. Electronegativity is best described as
2. Which element has the lowest electronegativity?
3. Which list is in order of increasing electronegativity?
5. In the period 3 data, aluminium's first ionization energy (578 kJ/mol) is lower than magnesium's (738 kJ/mol). The best interpretation is that
6. Across period 3, the melting point
Big idea: This lesson completes the periodic-trends vocabulary of expectation B2.1 (electronegativity) and practises B2.2: analysing real data across a period to identify trends. It also practises the scientific-investigation skills of organizing data and judging exceptions (A1.6, A1.8).
How to help: Work through the period 3 table together. Ask, "What happens to each row as you go right? Are there any surprises?" Let your teen decide whether a surprise is a real exception or a likely error.
Common difficulties:
- Confusing electronegativity with electron affinity (one is about bonds, one is about isolated atoms).
- Expecting every property to follow a smooth trend. Melting point depends on structure, which changes across the period.
- Memorizing arrows without a reason. Push for the explanation: "Zeff goes up, so..." or "a new shell, so...".