Atoms and Ions

SNC2D

The Bohr-Rutherford ModelHelium Atom• 2 positive protons

in nucleus

• 2 neutral neutrons in nucleus

• 2 negatively charged electrons in the first energy level

Standard Atomic Notation(for a neutral atom)

Br80

35

Atomic symbol

Atomic number

Mass number

Number of protons

Atomic number (35)

35 p+

Number of electrons (for neutral atom)

Atomic number (35)

35 e-

Number of neutrons

Mass number – Atomic number (80-35=45)

45 n0

Neutral Atoms and Ions• When using standard atomic notation

you can assume the atom is neutral and the number of electrons is equal to the number of protons

• A charge is shown if it is an ion.

Br80

35

1–

Ionic charge (1 extra electron)

Terms to Know• Valence

– The combining capacity of an element

• Valence shell– The outer electron shell of an atom, the electrons

in this shell are called valence electrons

• Isoelectric with a noble gas– The same number of electrons as a noble gas

Ions• Many elements want to form stable ions

where the number of electrons in the valence shell is the same as a noble gas.

• Metals tend to become positively charged cations.

• Non-metals tend to become negatively charged anions.

• They become ions by gaining or losing electrons (which ever is easier).

The Why of Ions

The Noble Gases do not react and do not form ions because they already have what all atoms want: full outer shells.

Anions

Cl Cl

1-

• Easier to gain 1 electron than lose 7.

• Therefore the atom becomes an anion by gaining an electron.

• It becomes more stable by gaining an electron. It becomes iso-electric with Ar.

Cations

Na Na

1+

• Easier to lose 1 electron than gain 7.

• Therefore the atom becomes a cation by losing an electron.

• It becomes more stable by losing an electron. It becomes iso-electric with Ne.

The How of Ions

Non-metalsNon-metals, when they form ions, change their names to:

the first syllable + the suffix “ideide”

Anion namesAnion nameschlorinefluorinebromine oxygen sulphur nitrogenphosphorus

Valence Practice

Given the following Bohr diagram, what will the valence of the ion be?

TRY THESE1. Name these ions: [K]

a) Mg2+ b) S2- c) Br- d) N3-

2. List three atoms or ions that have the same number of electrons as each of the following: [I]

a) Al3+ b) P3- c) Kr d) Cs+

3. Suppose that a new element has been made. Chemical tests show that it is an alkaline earth metal. [I]

a) Predict how many electrons there will be in the outer orbit.b) Predict the ionic charge of the ion that this element forms.

4. Justify why these ions do not exist under normal conditions. [A]

a) K2+ b) O-

Credits• http://commons.wikimedia.org/wiki/File:Helium-Bohr.svg