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Atoms, Elements and Compounds - Atomic structure and the Periodic Table

Grade 12IGCSEChemistry

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

🔑Concepts

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Atoms consist of a central nucleus containing protons (p+p^+) and neutrons (n0n^0), surrounded by electrons (e−e^-) orbiting in shells or energy levels.

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The Atomic Number (ZZ) is the number of protons in the nucleus. The Nucleon Number (AA), or mass number, is the total number of protons and neutrons.

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Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (e.g., 1735Cl^{35}_{17}Cl and 1737Cl^{37}_{17}Cl). They have identical chemical properties but different physical properties.

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Electronic configuration for the first 20 elements follows the pattern 2,8,8,22, 8, 8, 2. The number of outer-shell electrons determines the chemical properties and the Group number in the Periodic Table.

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The Periodic Table is arranged by increasing atomic number (ZZ). Periods (rows) indicate the number of shells, and Groups (columns) indicate the number of valence electrons.

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Group I (Alkali Metals) become more reactive down the group as the outer electron is further from the nucleus and more easily lost (MightarrowM++e−M ightarrow M^+ + e^-).

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Group VII (Halogens) become less reactive down the group as the attraction for an incoming electron decreases (X+e−ightarrowX−X + e^- ightarrow X^-).

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Group VIII (Noble Gases) have a full outer shell, making them chemically inert (unreactive) and monatomic.

📐Formulae

A=Z+N (where A=nucleon number, Z=protons, N=neutrons)A = Z + N \text{ (where } A = \text{nucleon number, } Z = \text{protons, } N = \text{neutrons)}

Ar=∑(isotope mass×percentage abundance)100A_r = \frac{\sum (\text{isotope mass} \times \text{percentage abundance})}{100}

Charge of Ion=Number of Protons−Number of Electrons\text{Charge of Ion} = \text{Number of Protons} - \text{Number of Electrons}

💡Examples

Problem 1:

Determine the subatomic particle count for an oxide ion, 816O2−^{16}_{8}O^{2-}.

Solution:

Protons = 88, Neutrons = 16−8=816 - 8 = 8, Electrons = 8+2=108 + 2 = 10.

Explanation:

The atomic number 88 indicates 88 protons. The mass number 1616 minus the atomic number gives 88 neutrons. The 2−2- charge means the atom has gained 22 electrons to achieve a stable electronic configuration of 2,82, 8.

Problem 2:

Calculate the relative atomic mass (ArA_r) of Copper, given that it consists of 69%69\% of 63Cu^{63}Cu and 31%31\% of 65Cu^{65}Cu.

Solution:

Ar=(63×69)+(65×31)100=4347+2015100=63.62A_r = \frac{(63 \times 69) + (65 \times 31)}{100} = \frac{4347 + 2015}{100} = 63.62

Explanation:

Relative atomic mass is the weighted average of the masses of the isotopes based on their natural abundance.

Problem 3:

Identify the element with electronic configuration 2,8,52, 8, 5 and its position in the Periodic Table.

Solution:

Element: Phosphorus (PP). Position: Group V, Period 3.

Explanation:

The total number of electrons is 1515 (2+8+52+8+5), which matches the atomic number of Phosphorus. There are 33 shells occupied (Period 3) and 55 electrons in the valence shell (Group V).