Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.
What to Focus On
Core idea:Atomic Number starts by naming the element, charge, and relevant protons, neutrons, or electrons.
Common stuck point:Students often know a formula related to atomic number but skip the recognition step: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion? That leads to a correct-looking substitution attached to the wrong chemical model.
Sense of Study hint:Ask: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?
Worked Examples
Example 1
easy
Element X has 15 protons. Identify the element and determine the number of electrons in a neutral atom.
Answer
Phosphorus (P), 15 electrons
First step
1
Look up atomic number Z=15 on the periodic table: this corresponds to phosphorus (P).
Full solution
2
The number of protons equals the atomic number, so phosphorus has 15 protons in its nucleus.
3
A neutral atom carries no net charge, meaning the number of electrons equals the number of protons: 15 electrons.
The atomic number uniquely defines each element and equals the number of protons. In a neutral atom, the electron count matches the proton count.
Example 2
medium
Two atoms both have a mass number of 40. Atom A has 20 protons and Atom B has 19 protons. Are they the same element? How many neutrons does each have?
Example 3
medium
An ion has Z=20 and 18 electrons. State its charge and identify the species.
Example 4
hard
An atom is described as 2656Fe3+. State protons, neutrons, electrons, and atomic mass number.
Example 5
challenge
A neutral atom has its electrons arranged as 2,8,8,2. Find its atomic number and identify the element.
Practice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
easy
What is the atomic number of an element with 29 protons? Name the element.
Example 2
easy
An atom has atomic number 15. How many protons does it have? If the atom is neutral, how many electrons does it have?
Example 3
easy
What does the atomic number (Z) of an element count?
Example 4
easy
Carbon has atomic number 6. How many protons does a carbon atom have?
Example 5
easy
An atom has atomic number 1. What element is it?
Example 6
easy
Is the atomic number always a whole number?
Example 7
easy
Can the atomic number of an atom change during a chemical reaction?
Example 8
easy
What does atomic number tell you about an element's place in the periodic table?
Example 9
easy
Two atoms have atomic numbers 7 and 7. Are they the same element?
Example 10
easy
How many protons does a neutral atom with Z=11 and 12 neutrons have?
Example 11
medium
An atom has 17 protons and 20 neutrons. What is its atomic number and which element is it?
Example 12
medium
The atomic mass of chlorine is listed as 35.5. Why is the atomic number 17, a whole number, instead?
Example 13
medium
If a nucleus gains a proton through a nuclear process, how does its atomic number change and what happens to its identity?
Example 14
medium
Element A has Z=19 and element B has Z=20. Which is potassium and which is calcium?
Example 15
medium
A neutral atom has 16 electrons. What is its atomic number, and name the element?
Example 16
medium
Two atoms have Z=26 but mass numbers 54 and 56. Are they the same element, and what is their relationship?
Example 17
medium
An ion is S2− with 18 electrons. What is sulfur's atomic number?
Example 18
medium
A neutral atom has 13 protons. What is its atomic number, and name the element?
Example 19
medium
Elements are placed in order of increasing atomic number. Which comes first in the table, Z=7 or Z=8?
Example 20
challenge
An atom is described as having Z=12, mass number 24, and a 2+ charge. Give its protons, neutrons, and electrons.
Example 21
challenge
Mendeleev first ordered elements by atomic mass, but a few pairs were out of order. Why is ordering by atomic number better?
Example 22
challenge
An unknown neutral atom has 2 electrons in its first shell and 6 in its second. Find its atomic number and element.
Example 23
easy
Helium has atomic number 2. How many protons does a helium atom have?
Example 24
easy
What element has atomic number 26?
Example 25
easy
What element has atomic number 47?
Example 26
easy
True or false: a neutral atom has the same number of electrons as its atomic number.
Example 27
easy
Can two different elements share the same atomic number?
Example 28
easy
Beryllium has atomic number 4. Give the number of protons and the neutral electron count.
Example 29
medium
A neutral atom has 33 electrons. State its atomic number and identify the element.
Example 30
medium
An atom has Z=18 and 20 neutrons. Find the mass number.
Example 31
medium
Element X has Z=9. What ion does it most commonly form?
Example 32
medium
Element A is directly to the right of element B on the periodic table. How do their atomic numbers compare?
Example 33
medium
An atom has 47 protons, 60 neutrons, and 46 electrons. Find Z and the ion charge.
Example 34
medium
A neutral atom of element X has 18 electrons. Its atomic mass is 39.95amu. Which element is X?
Example 35
medium
If a nucleus emits an alpha particle (2 protons, 2 neutrons), how does Z change?
Example 36
medium
Tellurium has atomic mass 127.6amu and iodine has 126.9amu. Why does iodine come after tellurium on the periodic table?
Example 37
hard
An ion with charge −2 has 18 electrons. Find Z and identify the element.
Example 38
hard
Three species each have 10 electrons: F−, Ne, Na+. Order them by atomic number.
Example 39
hard
Why can two isotopes of an element have the same atomic number but different atomic masses?
Example 40
hard
Moseley's X-ray experiments showed that one nuclear quantity correlates with X-ray frequency. Which one?
Example 41
hard
An atom in group 17, period 3 of the periodic table. Find Z and name the element.
Example 42
challenge
A nuclear reaction emits a beta particle (an electron from the nucleus, as a neutron becomes a proton). How do Z and A change?