Atomic Structure And The Periodic Table

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Notes

Elements, Compounds & Mixtures

An **element** is a substance made of atoms with the same number of protons; it cannot be split chemically.
A **compound** is a pure substance of two or more elements chemically combined; it cannot be separated by physical means.
A **mixture** is a combination of substances not chemically combined; it can be separated by physical methods like filtration or evaporation.
Examples: element – oxygen; compound – carbon dioxide (CO₂); mixture – sand and water.

Atomic Structure

Atoms consist of a **nucleus** $(protons + neutrons)$ surrounded by **electrons** in shells.
**Protons** have relative mass 1 and charge +1; **neutrons** have mass 1 and charge 0; **electrons** have negligible mass and charge –1.
The **atomic number (Z)** is the number of protons; it defines the element.
The **mass number (A)** is the total number of protons + neutrons.
Number of neutrons = mass number – atomic number.
In a neutral atom, number of electrons = number of protons.

Electronic Configuration

Electrons fill shells in order: first shell holds 2, second holds 8, third holds 8 (simplified model).
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).
Elements in the same group have the same number of outer-shell electrons.
**Noble gases** have full outer shells and are unreactive.

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 electron arrangement.
Physical properties (e.g., density, melting point) may differ due to different masses.
Example: carbon-12 (⁶ protons, ⁶ neutrons) and carbon-14 (⁶ protons, ⁸ neutrons).

Atomic structure of carbon-12: 6 protons, 6 neutrons, 2 electrons in first shell, 4 in second shell.

Practice questions

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1.Which of the following is a pure substance made up of two or more elements chemically combined?
Easy
- AElement
- BCompound
- CMixture
- DSolution
2.What is the relative mass of a proton?
Easy
- A0
- B1
- C $\frac{1}{1840}$
- D2
3.The atomic number is the number of protons in the nucleus of an atom.
Easy
True or false?
4.What is the electronic configuration of a neutral magnesium atom?
Medium
5.An atom of element X has atomic number 29 and mass number 63. How many neutrons does it have?
Medium
- A31
- B37
- C40
- D34
6.Complete the sentence:
Medium
Isotopes are atoms of the same element that have the same number of protons but a different number $of ____$ .
7.Match each subatomic particle to its charge.
Hard
- Proton
- Neutron
- Electron
- 1+
- 0
- 1-
8.Arrange the following elements in order of increasing atomic number: chlorine, sodium, carbon, calcium.
Hard
- Carbon
- Sodium
- Chlorine
- Calcium

Slides

Elements, Compounds & Mixtures

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

Which of the following is a mixture?

Copper(II) sulfate
Carbon dioxide
Sand and water
Calcium carbonate

Atomic Structure

Atoms consist of protons, neutrons, and electrons.
Protons and neutrons are in the nucleus; electrons orbit in shells.
Protons have relative mass 1 and charge +1; neutrons have mass 1 and charge 0; electrons have negligible mass and charge -1.

The number of protons in an atom is called $the ____$ number, and the total number of protons and neutrons is $the ____$ number.

Calculating Subatomic Particles

Number of protons = atomic number.
Number of electrons = atomic number (in a neutral atom).
Number of neutrons = mass number - atomic number.

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 (simplified).
The electronic configuration lists the number of electrons in each shell, e.g., carbon is 2,4.
The outermost shell is the valence shell; atoms are stable when it is full.

The electronic configuration of oxygen (atomic number 8) $is ____$ .

Electronic Configuration & the Periodic Table

The number of occupied shells equals the period number.
The number of outer electrons equals the group number (for Groups I-VII).
Elements in the same group have the same number of outer electrons.

An element has electronic configuration 2,8,2. Which group is it in?

Group 2
Group 8
Group 12
Group 2,8,2

Noble Gases

Noble gases $(Group \frac{8}{0})$ have full outer shells and are unreactive.
Atoms react to achieve a full outer shell, often attaining a noble gas configuration.

Which of these are true about noble gases?

They have full outer electron shells.
They are highly reactive.
They are in Group 8 of the Periodic Table.

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 electron configuration.
Physical properties (e.g., density) may differ due to mass differences.

Carbon-12 and carbon-14 are isotopes. What is the same about them?

Number of neutrons
Number of protons
Mass number
Physical properties

Summary: Atomic Notation

Atomic notation shows mass number (A) top left and atomic number (Z) bottom left.
Example: ¹²C means carbon with mass 12 and atomic 6.
Isotopes are written as element name or symbol followed by mass number, e.g., carbon-14.

An isotope of hydrogen has 1 proton and 2 neutrons. Its mass number $is ____$ .

Flashcards

What is an element?

A substance made of atoms that all contain the same number of protons and cannot be split into anything simpler by chemical means.

What is a compound?

A pure substance made up of two or more elements chemically combined. Compounds cannot be separated into their elements by physical means.

What is a mixture?

A combination of two or more substances (elements and/or compounds) that are not chemically combined. Mixtures can be separated by physical methods such as filtration or evaporation.

What are the three subatomic particles and their relative masses?

Proton (relative mass 1), neutron (relative mass 1), electron (relative mass $\frac{1}{1840})$ .

What are the charges of proton, neutron, and electron?

Proton: 1+, neutron: 0 (neutral), electron: 1-.

Where are protons and neutrons located in an atom?

They are located at the centre of the atom, which is called the nucleus.

What is the atomic number (proton number)?

The number of protons in the nucleus of an atom. It determines the element and its position on the Periodic Table.

What is the mass number (nucleon number)?

The total number of protons and neutrons in the nucleus of an atom.

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

Number of neutrons = mass number – atomic number (proton number).

In a neutral atom, how do the numbers of protons and electrons compare?

In a neutral atom, the number of protons equals the number of electrons.

What is the maximum number of electrons in the first, second, and third electron shells?

First shell: 2 electrons, second shell: 8 electrons, third shell: 8 electrons (simplified model).

What is the electronic configuration of carbon (atomic number 6)?

2,4 (2 electrons in the first shell, 4 in the second shell).

What is the electronic configuration of sodium (atomic number 11)?

2,8,1 (2 in first shell, 8 in second, 1 in third).

How does the electronic configuration relate to the period of an element?

The number of occupied shells (number of notations) equals the period number.

How does the electronic configuration relate to the group of an element (Groups I-VII)?

The number of electrons in the outer shell (last notation) equals the group number.

Why are noble gases unreactive?

They have full outer shells of electrons, making them very stable.

What are isotopes?

Atoms of the same element with the same number of protons but different numbers of neutrons.

Why do isotopes of the same element have the same chemical properties?

They have the same number of electrons in their outer shells, which determines chemical behaviour.

How do isotopes differ in physical properties?

Isotopes have different masses due to different numbers of neutrons, affecting physical properties like density, boiling point, and melting point.

What is the relative atomic mass unit based on?

One relative atomic mass unit is equal to $\frac{1}{12}$ the mass of a carbon-12 atom.

What is the valence shell?

The outermost shell of an atom, which contains the valence electrons.

What is the electronic configuration of a sodium ion (Na+)?

2,8 (sodium atom loses one electron, leaving 10 electrons).

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