Every element has a number
On the periodic table, every element has a number: hydrogen is 1, carbon is 6, oxygen is 8, and sodium is 11. These numbers aren’t arbitrary. Each one is the number of protons in the nucleus.
You haven’t studied the periodic table yet, so just look things up: The periodic table is taught in lesson 1.4.2. For now, all you need to know is that it lists every element in order of its number, and each box shows the element’s number, symbol, and name. Every element you’ll need in this lesson is listed below. Whenever the lesson says “look it up on the periodic table,” use this table:
| No. | Symbol | Name |
|---|---|---|
| 1 | H | hydrogen |
| 6 | C | carbon |
| 7 | N | nitrogen |
| 8 | O | oxygen |
| 9 | F | fluorine |
| 10 | Ne | neon |
| 11 | Na | sodium |
| 13 | Al | aluminum |
| 15 | P | phosphorus |
| 16 | S | sulfur |
| 17 | Cl | chlorine |
| 20 | Ca | calcium |
| 26 | Fe | iron |
Atomic number
The atomic number原子序数, written Z, is the number of protons in the nucleus. In a neutral atom:
The number of protons decides which element an atom belongs to: every atom with 6 protons is carbon, and every atom with 8 protons is oxygen. So the definition of an element from lesson 1.2.6 can now be made precise:
An element is a type of atom with a particular number of protons (a particular nuclear charge).
Mass number
Protons and neutrons each have a relative mass of about 1, and electrons are light enough to ignore. So adding up the protons and neutrons tells you roughly how heavy an atom is. That total is the mass number质量数, written A. The number of neutrons is written N:
Isotope notation
Writing the mass number at the upper left of the symbol and the atomic number at the lower left pins down exactly which atom you mean:
means 11 protons, mass number 23, 23 − 11 = 12 neutrons, and 11 electrons. You can also write it as sodium-23 (or Na-23). This notation is standard in high school chemistry and AP Chemistry, so it pays to get comfortable with it now.
Work each one out yourself, then click Check:
| Particle | Protons | Neutrons | Electrons | Mass number |
|---|---|---|---|---|
| phosphorus atom | 15 | 31 | ||
| sulfur atom | 16 | 32 |
Now use the atom builder to build each atom from its notation:
Not to scale: a real nucleus is only about 1/100,000 the diameter of its atom.
Example 1: Reading isotope notation
How many protons, neutrons, and electrons are in an atom of ?
- Read the bottom number. The atomic number Z = 17, so there are 17 protons.
- Read the top number. The mass number A = 35, so N = A − Z = 35 − 17 = 18 neutrons.
- Electrons. The atom is neutral, so electrons = protons = 17.
Example 2: Writing isotope notation
An atom has 26 protons and 30 neutrons in its nucleus. Which element is it? Write its isotope notation.
- Identify the element. Protons = atomic number = 26. On the periodic table (use the table at the start of this lesson), element 26 is iron (Fe).
- Find the mass number. A = Z + N = 26 + 30 = 56.
- Write the notation. Mass number at the upper left, atomic number at the lower left: , also written iron-56.
Common mistakes
Mistake 1: “Mass number = protons + electrons.” The mass number counts the protons and neutrons in the nucleus. Electrons are far too light to count.
Mistake 2: Swapping the two numbers. The mass number A (the bigger one) goes at the top left, and the atomic number Z (the smaller one) at the bottom left. The mass number is never smaller than the atomic number.
Mistake 3: “The number of neutrons decides the element.” The number of protons does. Atoms of the same element with different numbers of neutrons are still the same element (isotopes, next lesson).
Quick check
How many neutrons are in an atom of ?
N = A − Z = 27 − 13 = 14.
Which number decides what element an atom is?
An element is defined by its number of protons (its nuclear charge).
Summary
- Atomic number Z = nuclear charge = protons = electrons in a neutral atom. The number of protons decides the element.
- Mass number A = protons Z + neutrons N. Electrons don’t count.
- In isotope notation , the mass number goes at the upper left and the atomic number at the lower left.
- An element is a type of atom with a particular number of protons.
On to AP (for later)
AP topics 1.2 and 1.5 assume you’re fluent in isotope notation: seeing , you should immediately know it has 17 protons and 20 neutrons. AP questions also connect the atomic number to an element’s position on the periodic table to predict its properties.