Atomic Number & Mass – Lecture Review

18 question-and-answer flashcards covering Moseley’s experiment, definitions of atomic number and mass number, particle relationships, element notation, and worked examples for carbon, potassium, and aluminum.

Who experimentally discovered the relationship between X-ray frequency and atomic number?

English physicist Henry Gwyn Jeffreys Moseley.

What did Moseley observe when different metals were bombarded with electrons?

Each metal emitted X-rays whose frequencies varied systematically with its atomic number.

The atomic number (Z) of an element represents the number of what sub-atomic particle?

Protons in the nucleus.

Why is the atomic number considered the "identity" of an atom?

Because no two different elements have the same number of protons.

In a neutral atom, how is the number of electrons related to the number of protons?

They are equal (P = E).

Define atomic mass (mass number, A).

The total number of protons plus neutrons in the nucleus.

Write the equation that relates neutrons (N), protons (P), and mass number (A).

N + P = A.

How do you calculate the number of neutrons in an atom?

Subtract the atomic number from the mass number: N = A − Z.

State the two equalities that hold for a neutral atom involving P, Z, and E.

P = Z and P = E.

For carbon-12, give Z, A, and the numbers of protons, neutrons, and electrons.

Z = 6, A = 12; 6 protons, 6 neutrons, 6 electrons.

Potassium-39 has an atomic number of 19. How many protons, neutrons, and electrons does it possess?

19 protons, 20 neutrons (39 − 19), and 19 electrons.

Aluminum-27 has Z = 13. How many neutrons does it contain?

14 neutrons (27 − 13).

What variable is plotted on the y-axis when illustrating Moseley’s law?

The square root of the X-ray frequency.

What key conclusion did Moseley’s work provide for arranging the periodic table?

Elements should be ordered by atomic number rather than atomic mass.

Write the standard nuclear symbol notation for an element showing mass number and atomic number.

${}^A_Z\text{Symbol}$ (A positioned as a superscript, Z as a subscript before the element symbol).

Which two sub-atomic particle counts are always added to obtain an atom’s mass number?

Protons and neutrons.

Complete the relationship: P = __ = E

Z (the atomic number).

What physical quantity did Moseley find to be directly proportional to the atomic number?

The square root of an element’s characteristic X-ray frequency.