Periodic Table Mastery

Key ConceptsThis chapter covers how atomic number, mass number, relative atomic mass, and isotopes are used to read periodic table data. You need to distinguish these values accurately in exams.

Atomic number = number of protons in an atom. It identifies the element.

In a neutral atom, number of electrons = number of protons.

Mass number = protons + neutrons in one atom.

Neutrons = mass number - atomic number.

Relative atomic mass (Ar) is the weighted average mass of an element’s isotopes.

Isotopes have the same number of protons but different numbers of neutrons.

Isotopes have similar chemical properties because they have the same electron arrangement.

On most periodic tables, the larger decimal value is relative atomic mass, not mass number.

Before you continue: How can you calculate the number of neutrons when atomic number and mass number are given?

Key TermsAtomic number - The number of protons in an atom’s nucleus.

Mass number - The total number of protons and neutrons in one atom.

Relative atomic mass (Ar) - The weighted mean mass of an element’s isotopes.

Isotope - An atom of the same element with a different number of neutrons.

Proton - A positively charged particle in the nucleus.

Neutron - An uncharged particle in the nucleus.

Electron - A negatively charged particle outside the nucleus.

Active RecallAtomic number__________

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Mass number__________

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Relative atomic mass (Ar)__________

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Isotope__________

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Proton__________

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Neutron__________

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Electron__________

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Worked Examples1. Finding neutronsAn atom has atomic number 11 and mass number 23.

Protons = atomic number = 11.

Neutrons = mass number - atomic number.

Neutrons = 23 - 11 = 12.

Now you try: An atom has atomic number 17 and mass number 35. Find its neutrons.

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2. Interpreting isotopesCarbon-12 and carbon-14 are isotopes.

Carbon’s atomic number is 6, so both atoms have 6 protons.

Carbon-12 has 12 - 6 = 6 neutrons.

Carbon-14 has 14 - 6 = 8 neutrons.

They have the same chemical properties but different masses.

Now you try: Compare chlorine-35 and chlorine-37. State their proton and neutron numbers.

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3. Calculating relative atomic massAn element has two isotopes: 75% has mass 35, and 25% has mass 37.

Multiply each isotope mass by its abundance: (35 × 75) + (37 × 25).

Divide by 100: 3,550 ÷ 100.

Relative atomic mass = 35.5.

Now you try: An element is 80% isotope 10 and 20% isotope 11. Calculate Ar.

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Practice Questions(Easy) Define atomic number.

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(Easy) An atom has 19 protons and mass number 39. How many neutrons does it have?

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(Medium) Explain why isotopes of an element have the same chemical properties.

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(Medium) Chlorine-35 and chlorine-37 are isotopes. State one similarity and one difference.

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(Hard) Calculate Ar for an element containing 60% isotope 24 and 40% isotope 26.

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(Hard) An ion has 12 protons and 10 electrons. Identify its charge and explain why.

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