Topic 1: Atomic Structure

State and explain the similarities and differences between isotopes.

1. Same chemical properties: - Because they have the same number of electrons. (Chemical properties depend on number of electrons) 2. Different physical properties (e.g. mass and density): - Because they have different number of neutrons and therefore different masses.

Define the term isotope.

Isotopes are atoms of an element with the same number of protons and electrons but a ​different number of neutrons​.

What is a principle quantum number (n)?

It represents the shell that the electrons occupy. n = 1 is nearest to the nucleus and has the lowest energy level. n = 3 is further away from the nucleus and have more energy and are held less tightly by the nucleus. *In the nth shell, there are n sub-shells, n^2 orbitals and 2n^2 electrons.

What is atomic radius?

One-half the distance between the nuclei of two atoms of the same element when the atoms are joined. (Diatomic molecule)

Describe where protons, neutrons and electrons are located in an atom.

Protons and neutrons - in the dense nucleus Electrons - in shells in empty space around nucleus (orbiting the nucleus)

State and explain the variations in atomic radius and ionic radius across a period.

The atomic radius of atoms generally decreases from left to right across a period. Within a period, protons are added to the nucleus as electrons are being added to the same principal energy level. These electrons are gradually pulled closer to the nucleus because of its increased positive charge. Since the force of attraction between nuclei and electrons increases, the size of the atoms decreases. The effect lessens as one moves further to the right in a period because of electron-electron repulsions that would otherwise cause the atom's size to increase. * Exception: oxygen radius being slightly greater than the nitrogen radius

State and explain the variations in atomic radius and ionic radius down a group.

The atomic radius of atoms generally increases from top to bottom within a group. As the atomic number increases down a group, there is again an increase in the positive nuclear charge. However, there is also an increase in the number of occupied principle energy levels. Higher principal energy levels consist of orbitals which are larger in size than the orbitals from lower energy levels. The effect of the greater number of principal energy levels outweighs the increase in nuclear charge and so atomic radius increases down a group.

Describe the order of increasing energy of the sub-shells within the first three shells and the 4s and 4p sub-shells.

The energy of electrons in sub-shells increases in the order s < p < d < f. Meaning 1s < 2s < 2p < 3s < 3p < 3d < 4s < 4p Electrons are filled in orbitals lowest energy to highest. Exception: 4s has slightly lower energy than 3d and is filled first. Before the electrons are paired, each sub-shell must first be filled with unpaired electrons.

Describe the number of orbitals making up s, p and d sub-shells, and the number of electrons that can fill s, p and d sub-shells.

● s: spherical shape, one s orbital in each shell from n = 1 upwards (a total of two s electrons per shell), lowest energy. ● p: dumb-bell shape, three p orbitals in each shell from n = 2 upwards (a total of six p electrons per shell), higher energy than s. ● d: five d orbitals in each shell from n = 3 upwards (a total of 10 d electrons per shell), higher energy than p.

How do you determine the number of protons, neutrons and electrons in an atom or ion?

- Atom: Number of protons = atomic number Number of neutrons = mass number - atomic number Number of electrons = atomic number - Ion: Number of electrons depend on the charge. # Positively-charged ion: atomic number - the charge # Negatively charged ion: atomic number + the charge

Describe the distribution of mass and charge within an atom. When is an atom charged and neutral?

- Mass: Concentrated in the nucleus (protons and neutrons). Mass of electrons are negligible. - Charge: Positively-charged protons in the nucleus and negatively-charged electrons orbit the nucleus in shells. 1. Atoms are positively-charged when there are more protons than electron and vice versa. 2. Atoms have no overall charge when proton number equal to number of electrons.

State the definitions of proton/atomic number and nucleon/mass number.

- Proton/atomic number: The number of protons in an atom. - Nucleon/mass number: The number of nucleons (protons and neutrons) in the nucleus of an atom.

State the relative charges and relative masses of protons, neutrons and electrons.

- Protons Relative charge: 1+ Relative mass: 1 - Neutrons Relative charge: 0 Relative mass: 1 - Electrons Relative charge: 1- Relative mass: 1/1836

Describe the behaviour of beams of protons, neutrons and electrons moving at the same velocity in an electric field.

- Protons: Deflected on a curved path towards the negative plate as they are positively-charged. - Neutrons: Continue on a straight path as they have no charge. - Electron: Deflected on a curved path towards the positive plate as they are negatively-charged. *Particles have same speed - lighter electrons deflect more.

What are orbitals?

A region in space around the nucleus of an atom where one is most likely to find an electron. Probability is highest or maximum. Each orbital can hold up to two electrons. • 3-dimensional statistical maps showing the likeliest places to find electrons • come in different shapes and sizes • hold a maximum of two electrons each (as long as they have opposite spins) Orbitals are found in sub-shells which are found on main shells such as n = 1. E.g. main shell n = 2 or has 2 sub-shells (2s and 2p)