Atomic Structure & The Periodic Table

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Notes

Elements, Compounds & Mixtures

**Element**: substance made of atoms with the same number of protons; cannot be split chemically.
**Compound**: pure substance of two or more elements chemically combined; cannot be separated by physical means.
**Mixture**: combination of substances not chemically combined; separable by physical methods (e.g., filtration).
Examples: element = copper; compound = CO₂; mixture = sand and water.

Atomic Structure

Atoms consist of **protons**, **neutrons** (in nucleus) and **electrons** (in shells).
**Proton**: relative mass 1, charge +1; **Neutron**: relative mass 1, charge 0; **Electron**: relative mass 1/1840, charge -1.
**Atomic number (Z)** = number of protons; determines element and equals electrons in neutral atom.
**Mass number (A)** = total protons + neutrons; also called nucleon number.
Number of neutrons = mass number – atomic number.

Electronic Configuration

Electrons occupy **shells** (energy levels); first shell holds 2, second 8, third 8 (simplified).
**Electronic configuration** lists electrons per shell, e.g., carbon = 2,4.
**Outermost shell** (valence shell) determines chemical properties; full outer shell = stable (noble gas).
Number of occupied shells = **Period** number; number of outer electrons = **Group** number (for Groups I–VII).
Ions: e.g., Na⁺ has configuration 2,8 (lost one electron).

Isotopes

**Isotopes**: same element, same number of protons, different number of neutrons.
Isotopes have identical **chemical properties** (same outer electrons) but different **physical properties** (mass).
Notation: e.g., carbon-14 or ¹⁴C.

Relative Atomic Mass (Extended)

**Relative atomic mass (Aᵣ)** = weighted average mass of all isotopes compared to 1/12 mass of carbon-12.
Formula: Aᵣ = (Σ(% abundance × mass)) / 100.
Example: rubidium: (72×85 + 28×87)/100 = 85.6.

Periodic Table & Groups

Elements in same **Group** have same number of outer electrons and similar chemical properties.
**Noble gases** (Group 0/VIII) have full outer shells and are unreactive.
**Group number** for Groups I–VII equals number of outer electrons.
**Period number** equals number of occupied electron shells.

Bohr model of a sodium atom showing nucleus with 11 protons and 12 neutrons, and electron shells 2,8,1.

Practice questions

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1.Which option correctly describes the relative charges and masses of the subatomic particles?
Easy
- AProton +1, neutron 0, electron -1; relative mass of proton 1, neutron 1, electron 0.00054
- BProton +1, neutron 0, electron -1; relative mass of proton 0.00054, neutron 1, electron 0
- CProton 0, neutron +1, electron -1; relative mass of proton 1, neutron 1, electron 0.00054
- DProton +1, neutron 0, electron 0; relative mass of proton 1, neutron 1, electron 0.00054
2.The atomic number of element Q is 9 and its mass number is 19. In which group of the Periodic Table should element Q be placed?
Easy
- AGroup 0
- BGroup I
- CGroup VII
- DGroup II
3.The diagram shows the atomic structure of an element. Which element is it?
Easy
- ACarbon
- BBeryllium
- COxygen
- DBoron
4.Which two atoms are isotopes?
Easy
- AQ and S
- BQ and T
- CR and T
- DR and S
5.Which of the following statements is correct?
Easy
- AElements contain different atoms
- BCompounds consist of elements that are not chemically combined
- CMixtures consist of elements that are chemically combined
- DA mixture can contain elements and compounds
6.Which statement about carbon is correct?
Medium
- AIt has 4 electron shells.
- BIt has 2 outer shell electrons.
- CIt has 12 nucleons.
- DIt has an electron configuration of 2,8,2.
7.Two atoms, X and Y can be represented as shown: ³₇X and ³₈Y. Which row is correct for both atoms?
Medium
- Aelectron configuration of X: 2,1; electron configuration of Y: 2,1; Is Y an isotope of X? Yes
- Belectron configuration of X: 2,1; electron configuration of Y: 2,6; Is Y an isotope of X? Yes
- Celectron configuration of X: 2,5; electron configuration of Y: 2,1; Is Y an isotope of X? No
- Delectron configuration of X: 2,5; electron configuration of Y: 2,6; Is Y an isotope of X? No
8.Element X contains 12 of each sub-atomic particle. What is the correct Group number for element X?
Medium
- AGroup 0
- BGroup I
- CGroup VII
- DGroup II

Slides

Elements, Compounds & Mixtures

An element is a substance made of atoms with the same number of protons, and cannot be split into anything simpler by chemical means.
A compound is a pure substance made of two or more elements chemically combined, e.g. CO₂.
A mixture is a combination of substances not chemically combined, separable by physical methods.

Which of the following is a mixture?

Copper
Carbon dioxide
Brass
Water

Atomic Structure

Atoms contain protons, neutrons, and electrons.
Protons and neutrons are in the nucleus; electrons orbit in shells.
Proton: relative mass 1, charge +1. Neutron: relative mass 1, charge 0. Electron: relative mass 1/1840, charge -1.

The atomic number (Z) is the number of ____ in the nucleus. The mass number (A) is the total number of ____ and ____.

Calculating Subatomic Particles

In a neutral atom, number of electrons = number of protons = atomic number.
Number of neutrons = mass number – atomic number.
Example: Carbon-12 has 6 protons, 6 electrons, and 6 neutrons.

An atom has atomic number 11 and mass number 23. How many neutrons does it have?

Electronic Configuration

Electrons fill shells in order: first shell holds 2, second holds 8, third holds 8.
The electronic configuration is written as numbers separated by commas, e.g. carbon is 2,4.
The number of occupied shells equals the period number; the number of outer electrons equals the group number (for Groups I–VII).

Which of these are true?

The first shell can hold up to 2 electrons.
The third shell can hold up to 18 electrons at IGCSE level.
Elements in the same group have the same number of outer electrons.

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
They have the same chemical properties because they have the same number of outer electrons.
They have different physical properties (e.g. density, melting point) due to different masses.

Two atoms have 6 protons and 6 electrons, but one has 6 neutrons and the other has 8 neutrons. They are:

Different elements
Isotopes
Ions
Compounds

Relative Atomic Mass (Extended)

Relative atomic mass (Aᵣ) is the weighted average mass of all isotopes of an element compared to 1/12 of carbon-12.
Formula: Aᵣ = ( (% abundance × mass) for each isotope ) / 100.
Example: Rubidium-85 (72%) and Rb-87 (28%) gives Aᵣ = (72×85 + 28×87)/100 = 85.6.

The relative atomic mass of chlorine is 35.5. This is because chlorine has two isotopes: Cl-35 and Cl-____. The abundance of Cl-35 is about 75% and Cl-37 is about ____%.

Noble Gases

Noble gases (Group VIII/0) have full outer shells of electrons.
They are very unreactive (inert) because they are stable.
Examples: helium (2), neon (2,8), argon (2,8,8).

Which element has the electronic configuration 2,8,8?

Neon
Argon
Chlorine
Potassium

Relationship to Periodic Table

The number of occupied shells = period number.
The number of outer electrons = group number (for Groups I–VII).
Example: Chlorine (2,8,7) is in Period 3, Group VII.

Which statements are correct?

An element with configuration 2,8,2 is in Group II.
An element with configuration 2,8 is in Period 2.
The group number is always the number of outer electrons.

Ions

Atoms can lose or gain electrons to form ions with a full outer shell (like a noble gas).
Positive ions (cations) form when electrons are lost; negative ions (anions) form when electrons are gained.
Example: Na (2,8,1) loses 1 electron to become Na⁺ (2,8).

A sodium ion (Na⁺) has ____ electrons. Its electronic configuration is ____.

Summary

Atoms consist of protons, neutrons, and electrons.
Atomic number = protons; mass number = protons + neutrons.
Electronic configuration determines group and period.
Isotopes have same protons, different neutrons; same chemical properties.

Which of the following is true about isotopes?

They have different numbers of protons.
They have the same chemical properties.
They have the same physical properties.
They are different elements.

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