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Atomic Structure - Atomic Number and Mass Number

Grade 9IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Atomic Number (ZZ) is the number of protons in the nucleus of an atom. It is unique to each element and determines its position on the Periodic Table.

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The Mass Number (AA) is the total number of protons and neutrons in the nucleus of an atom. These particles are collectively known as nucleons.

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Standard nuclide notation is written as ZAX^{A}_{Z}X, where XX represents the chemical symbol of the element.

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In a neutral atom, the number of protons is exactly equal to the number of electrons (p+=e−p^+ = e^-).

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Neutrons do not carry a charge but contribute to the mass and stability of the nucleus.

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Isotopes are atoms of the same element (same ZZ) that have different numbers of neutrons, resulting in different mass numbers (AA). For example, 612C^{12}_{6}C and 614C^{14}_{6}C are isotopes of carbon.

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Ions are formed when an atom gains or loses electrons. A cation (positive ion) has fewer electrons than protons, while an anion (negative ion) has more electrons than protons.

📐Formulae

A=Z+NA = Z + N

N=A−ZN = A - Z

Net Charge=(number of p+)−(number of e−)\text{Net Charge} = (\text{number of } p^+) - (\text{number of } e^-)

💡Examples

Problem 1:

Calculate the number of protons, neutrons, and electrons in a neutral atom of Sodium represented as 1123Na^{23}_{11}Na.

Solution:

Protons = 1111, Neutrons = 1212, Electrons = 1111.

Explanation:

The atomic number (ZZ) is 1111, so there are 1111 protons. Since the atom is neutral, there are also 1111 electrons. The number of neutrons is found by subtracting ZZ from the mass number (AA): 23−11=1223 - 11 = 12.

Problem 2:

An ion of Magnesium has a 2+2+ charge and a mass number of 2424. Given that the atomic number of Magnesium is 1212, determine the number of subatomic particles present.

Solution:

Protons = 1212, Neutrons = 1212, Electrons = 1010.

Explanation:

Protons are equal to Z=12Z = 12. Neutrons are A−Z=24−12=12A - Z = 24 - 12 = 12. Because the ion has a +2+2 charge, it has lost 22 electrons from its neutral state (12−2=1012 - 2 = 10).

Problem 3:

Identify the element and write the nuclide notation for an atom containing 2020 protons and 2222 neutrons.

Solution:

2042Ca^{42}_{20}Ca

Explanation:

An atomic number (ZZ) of 2020 corresponds to Calcium (CaCa). The mass number (AA) is the sum of protons and neutrons: 20+22=4220 + 22 = 42. Thus, the notation is 2042Ca^{42}_{20}Ca.

Atomic Number and Mass Number - Revision Notes & Key Formulas | IB Grade 9 Science