Chapter 4. The Periodic Table: Arrangement of Electrons

Element

A substance that cannot be split into simpler substances by chemical means

Robert Boyle, what did he do?

First chemist to properly define the meaning of the term element

Compound

Elements combined together in chemical reactions

What did Humphry Davy do?

Discovered elements by passing electricity through compounds containing those elements

Mendeleev’s Periodic Law

When elements are arranged in order of increasing relative atomic mass, the properties of the elements are repeated at regular intervals in other elements.

Dimitri Mendeleev, what did he do?

Arranged elements in order of increasing relative atomic mass, placed elements with similar properties under each other in groups resulting in the reversal of the order of some elements and gaps. He was also able to predict the properties of undiscovered elements since he knew the properties of the other elements in the particular group.

Henry Moseley, what did he do?

Developed a method of determining the atomic number of an element using X-rays, this led to the creation of the modern periodic table.

Atomic Number

The number of protons in the nucleus of that atom

Modern Periodic Table

Arrangement of elements in order of increasing atomic number

Modern Periodic Law

When elements are arranged in order of increasing atomic number, properties of elements are repeated at regular intervals in other elements

Differences between Mendeleev’s Periodic Table and the Modern Periodic Table

• There are no gaps in the modern periodic table

• In Mendeleev’s Periodic Table elements are arranged in order of increasing relative atomic mass

• Mendeleev included the transition elements with the other elements instead of giving them a separate block

Advantages of the modern periodic table/arranging elements in order of atomic number

• No need to reverse the order of some elements to force them into the correct groups

• All elements are placed in groups with similar properties

• To date, there are no gaps in the modern periodic table

Mass number of an element

Sum of the number of protons and neutrons in the nucleus of an atom of that element

Nuclear formula of an element

Method of summarising information known about an element

Isotopes

Atoms of the same element which have different mass numbers due to the different number of neutrons in the nucleus

Relative atomic mass

The average of the mass numbers of the isotopes of the element as they occur naturally. Taking their abundances into account and expressing it on the carbon-12 scale

Electron configuration

Shows the arrangement of electrons in an atom of an element

How many orbitals in each of the following sublevels: s, p, d

1, 3, 5

Aufbau Principle

When building up the electron configuration of an atom in its ground state, the electrons occupy the lowest available energy levels

Hund’s Rule of Maximum Multiplicity

When two or more orbitals of equal energy are available, the electrons occupy them singly before filling them in pairs

Pauli Exclusion Principle

States that no more than two electrons may occupy an orbital and they must have opposite spin