Atomic Structure and Bonding

Lecture 1

An atoms consists of a dense ________ charged nucleus

positively

What surrounds the nucleus of an atom?

Negatively charged electrons at different energy levels at relatively large distances

What subatomic particles of an atom is found in the nucleus?

• Protons - positively charged

• Neutrons - electrically neutral

Electron 

symbol, charge, mass (g) , relative mass

• e^- 

• -1 

• 9.109 × 10^-28 

• 1

Proton

symbol, charge, mass (g) , relative mass

• p

• +1

• 1.673 × 10^-24

• 1839

Neutron

symbol, charge, mass (g), relative mass

• n

• 0

• 1.675 × 10^-24

• 1839

Atomic number (Z)

Number of protons in the atom’s nucleus- equal to the sum if electrons present in a neutral atom

Mass number (A) 

The sum of protons and neutrons in an atom’s nucleus

Number of neutrons?

mass number - number of protons

What is an isotope?

Atoms of an element with the same atomic number but different mass number.

Same number of protons but can differ in the number of neutrons

Relative atomic mass (A_r)

Average mass of its atoms compared to 1/12th the mass of an atom of carbon-12.

Molecular weight

The sum of the atomic weights of all atoms in the molecule

Relative molecular mass (M_r)

Average mass of a molecule compared to 1/12th the mass of an atom of carbon-12

According to the quantum model, electrons behave as _____

Waves

The behaviour of a specific electron in an atom is described by mathematical expression called a _________

Wave equation

What is the term for the solution of a wave equation?

Wave function (psi)

What is (psi)²?

The probability of finding an electron at a particular point.

The atomic orbital

What is the atomic orbital defined by?

Three quantum numbers:

• Principle quantum number, n

• Orbital angular momentum quantum number, l

• Magnetic quantum number, m

The principal quantum number, n

• Different value of n divided orbitals into groups of similar energies called shells

• n can take whole number values

• Describes the energy of the orbitals

The orbital angular momentum quantum number, l

• Depends on values of n: such that l= 0, 1, 2, …, n-1

• Gives information about sub-shells and the shape of an orbital in the subshell

• Different possible values are given letters rather than numbers, namely, s, p, d and f

The magnetic quantum number, m

• determines the spatial orientation of the angular momentum

• determines where orbitals are in space

• values range from -l to +l

Possible m_l values of the 2p subshell and their names

• +1, 0, -1

• 2p_x, 2p_y, 2p_z

What is the spin quantum number m_s

• describes the spin of the electron with respect to an external magnetic field

• can take values +1/2 or -1/2

Shape of s orbital?

Spherical

Nucleus at the centre

Shape of p orbital?

Dumbbell shaped

Nucleus at the middle

What is an electron shell?

A group of an atom’s electrons with the same principal quantum number

Ground state electron configuration

Most stable, lowest-energy electron configuration of an ion or an atom

Aufbau principle

Electrons fill the lowest energy orbitals first

Pauli Exclusion Principle

Only two electrons of opposite spins can occupy an orbital

Hund’s rule

Electrons fill degenerate orbitals singly with parallel spins before pairing

Why do atoms from bonds?

To become more stable and lower their energy due to attraction between oppositely charged ions, nuclei and shared electrons

Octet rule

Atoms gain, lose or sharer electrons to achieve eight electrons in their valence shellWh

What is an ionic bond?

Electrostatic force of attraction between two oppositely charged ions formed as a result of an electron transfer

Alkali metals in group 1 in ionic bonding…

loses single s electron from their valence to form a cation

Halogens in group 7 in ionic bonding…

gain a p electron to fill their valence shell.], forming an anion

What is a covalent bond?

A shared pair of atoms between electrons

Lewis structures

Electron-dot structure

What are lone pairs?

Valence electrons not involved in bonding. The can act as nucleophiles

Problems with the octet rule:

• applies only to first row and roughly p block compounds

• fails for PCl₅, NO, BF₃

• does not predict the strength of bonds