For a neutral sodium atom with Z = 11, which statement is correct?
Z gives protons; in a neutral atom it also gives electrons. Sodium therefore has 11 of each.
Chapter 1 · The Periodic Table
Atomic number & mass number
Two small numbers tell a complete nuclear story: an element’s identity, its electrons, and the particles packed in its nucleus.
Z = protons = electrons
A = protons + neutrons
A = protons + neutrons
A minus Z gives N.
Read the larger number above; the identity number below.
01Define atomic and mass number.
02Calculate protons, electrons and neutrons.
03Read AZX notation.
04Use Z for configuration, group and period.
01
The atom’s three particlesStart at the centre
Inside the nucleus
Protons (p) have positive charge. Neutrons (n) have no charge. Both live in the nucleus.
Outside the nucleus
Electrons (e) have negative charge and occupy shells around the nucleus.
Memory image
Think of the nucleus as a house. Its occupants are protons and neutrons; electrons are outside in shells.
Think of the nucleus as a house. Its occupants are protons and neutrons; electrons are outside in shells.
02
Atomic number · ZThe element’s identity number
The textbook definition is exact: “The atomic number of an element is equal to the number of protons present in the nucleus of an atom of that element.”
Atomic number (Z) = number of protons (p)
For a neutral atom: Z = p = e
Why it matters
No two different elements have the same atomic number. Change the number of protons and you change the element itself.
C: Z = 6 · N: Z = 7 · O: Z = 8
What Z unlocks
It gives the number of electrons in a neutral atom. Arrange them in shells, then use occupied shells for period and valence electrons for group.
Atomic number
→Number of electrons
→Electronic configuration
→Shells + valence e⁻
→Period + group
03
Mass number · ACount the heavy nucleus particles
The textbook definition: “The mass number of an element is the sum of the number of protons and neutrons present in the nucleus of its atom.”
Mass number = protons + neutrons
A = p + n
Neutrons = A − Z
Electrons are not included: the mass number counts the particles in the nucleus. Atomic number counts only protons; mass number counts protons and neutrons together.
Phosphorus has A = 31 and Z = 15. How many neutrons does it have?
n = A − Z = 31 − 15 = 16 neutrons.
Which particles are included in mass number?
A = p + n. Electrons are not included.
04
Read the notationAbove is A; below is Z
2311Na
23 is the mass number, A.
11 is the atomic number, Z.
Na is the element symbol.
General form: AZX
A is Above.
Z is below.
For 2311Na: protons = 11, electrons = 11 and neutrons = 23 − 11 = 12. For carbon 126C, all three values are 6, 6 and 6. For aluminium 2713Al, they are 13, 13 and 14.
Interactive · atom calculator
Give it A and Z
Use the notation to calculate the particles in a neutral atom.
Protons—
Electrons—
Neutrons—
Enter whole numbers where A is at least Z.
In 2311Na, write which number is A and which is Z.
A = 23 (mass number) and Z = 11 (atomic number).
05
Table 1.4The first twenty elements
Use the formula n = A − Z. The source table prints a dash for hydrogen’s neutrons; the calculation is 1 − 1 = 0.
| Element | Symbol | Z | A | Neutrons |
|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 0 |
| Helium | He | 2 | 4 | 2 |
| Lithium | Li | 3 | 7 | 4 |
| Beryllium | Be | 4 | 9 | 5 |
| Boron | B | 5 | 11 | 6 |
| Carbon | C | 6 | 12 | 6 |
| Nitrogen | N | 7 | 14 | 7 |
| Oxygen | O | 8 | 16 | 8 |
| Fluorine | F | 9 | 19 | 10 |
| Neon | Ne | 10 | 20 | 10 |
| Sodium | Na | 11 | 23 | 12 |
| Magnesium | Mg | 12 | 24 | 12 |
| Aluminium | Al | 13 | 27 | 14 |
| Silicon | Si | 14 | 28 | 14 |
| Phosphorus | P | 15 | 31 | 16 |
| Sulphur | S | 16 | 32 | 16 |
| Chlorine | Cl | 17 | 35 or 37 | 18 or 20 |
| Argon | Ar | 18 | 40 | 22 |
| Potassium | K | 19 | 39 | 20 |
| Calcium | Ca | 20 | 40 | 20 |
Tap-sorter · choose the correct number
Oxygen-16: neutrons
Potassium-39: neutrons
Chlorine-37: neutrons
Calcium-40: protons
06
Patterns for lighter elementsUseful observations, not magic rules
Even atomic number
A is often 2Z
Examples: He 4, C 12, O 16, Ca 40
Textbook exceptions: Beryllium (2Z = 8, listed A = 9) and Argon (2Z = 36, listed A = 40).
Odd atomic number
A is often 2Z + 1
Examples: Li 7, B 11, F 19, Na 23
Textbook exceptions: Nitrogen (2Z + 1 = 15, listed A = 14) and Hydrogen (2Z + 1 = 3, listed A = 1).
Important
These are observed patterns stated for lighter elements. Do not apply them blindly to every element.
These are observed patterns stated for lighter elements. Do not apply them blindly to every element.
Which pair is listed as exceptions to the even-Z pattern?
Beryllium and argon are the stated exceptions to the even atomic-number pattern.
07
Method and worked examplesFour dependable steps
1. Z gives protons.
2. Z gives electrons in a neutral atom.
3. n = A − Z.
4. Use the electrons for electronic configuration.
Worked example · Z = 15, A = 31
Find protons, electrons, neutrons and configuration. This element is phosphorus.
Step 1 · p = Z = 15.
Step 2 · In a neutral atom, e = 15.
Step 3 · n = A − Z = 31 − 15 = 16.
Step 4 · 15 electrons arrange as 2, 8, 5.
Final answer: p 15, e 15, n 16, configuration 2, 8, 5.
Final answer: p 15, e 15, n 16, configuration 2, 8, 5.
20 protons and 20 neutrons
Z = number of protons = 20. A = p + n = 20 + 20 = 40. Atomic number 20 is calcium.
3517Cl
p = 17, e = 17, n = 35 − 17 = 18; configuration = 2, 8, 7; three shells give Period 3 and seven valence electrons give Group 17.
An element has Z = 12. What is its electronic configuration?
Z = 12 means 12 electrons: 2, 8, 2. It is in Period 3, Group 2.
An atom has Z = 17 and configuration 2, 8, 7. Where is it?
Three occupied shells mean Period 3; seven valence electrons mean Group 17.
08
n/p ratio and common mistakesKeep the labels straight
The neutron-to-proton ratio is written n/p. The chapter states that an n/p ratio around 1 is stable (examples: sodium, potassium and calcium), while 1.5 and above is radioactive and unstable (example: uranium). For calcium, n/p = 20/20 = 1.
Tap a correction
“Neutrons = A + Z.”
Correct: n = A − Z.
“Calcium has mass number 40, so it has 40 neutrons.”
Correct: n = 40 − 20 = 20.
“Mass number includes electrons.”
Correct: mass number includes protons + neutrons, not electrons.
“A = 2Z always.”
Correct: it is only a stated lighter-element pattern and has exceptions.
“23 is Z in sodium notation.”
Correct: in 2311Na, 23 is A, and 11 is Z.
Why does a change in the number of protons change the element?
No two different elements have the same atomic number, so changing the proton count changes identity.
09
Concept check and practiceSay it, then write it
Define atomic number.
Atomic number is the number of protons present in the nucleus of an atom.
Define mass number.
Mass number is the sum of protons and neutrons present in the nucleus.
If Z = 16 and A = 32, how many protons, electrons and neutrons are present?
p = Z = 16; e = 16 in a neutral atom; n = 32 − 16 = 16.
Which formula finds the number of neutrons?
A minus Z gives N.
Concept check — all 12 answers
- Atomic number: number of protons in the nucleus.
- Mass number: sum of protons and neutrons in the nucleus.
- Z.
- A.
- 16 protons.
- 16 electrons.
- 32 − 16 = 16 neutrons.
- No two different elements have the same atomic number.
- 2, 8, 3.
- Group 17, Period 3.
- n = A − Z.
- Beryllium and argon.
Level 1 — Easy practice and answers
- Symbol for atomic number: Z.
- Symbol for mass number: A.
- Z = p = e for a neutral atom.
- A = p + n.
- n = A − Z.
- Carbon has 6 protons.
- Oxygen-16 has 8 neutrons.
- Sodium has mass number 23.
Level 2 — Application practice and answers
| Atom | Protons | Electrons | Neutrons |
|---|---|---|---|
| 73Li | 3 | 3 | 4 |
| 126C | 6 | 6 | 6 |
| 199F | 9 | 9 | 10 |
| 2311Na | 11 | 11 | 12 |
| 2713Al | 13 | 13 | 14 |
| 3115P | 15 | 15 | 16 |
| 3517Cl | 17 | 17 | 18 |
| 4020Ca | 20 | 20 | 20 |
Level 3 — ICSE-style questions and model answers
1. Z = 14, A = 28: p = 14, e = 14, n = 14, configuration = 2,8,4, Period 3, Group 14.
2. 19 protons and 20 neutrons: Z = 19, A = 39, e = 19, configuration = 2,8,8,1; element potassium.
3. Significance of atomic number: It gives protons and, in a neutral atom, electrons; distinguishes elements; gives electronic configuration and hence group and period.
4. Complete the table: Mg: 12,12,12; Cl: 17,17,18; Ar: 18,18,22; K: 19,19,20 (protons, electrons, neutrons).
10
One-page revisionKeep these four lines ready
ATOMIC NUMBER — Z
protons; also electrons in a neutral atom; identity; configuration; group and period
MASS NUMBER — A
protons + neutrons
CALCULATIONS
protons = Z · electrons = Z · neutrons = A − Z
LIGHT-ELEMENT PATTERN
even Z: A often 2Z (exceptions Be, Ar)
odd Z: A often 2Z + 1 (exceptions N, H)
Give reason: Atomic number is unique because no two different elements have the same number of protons. It gives electronic configuration because a neutral atom has as many electrons as protons. Occupied shells determine period; valence electrons help determine group.