Electronic configuration of Ions

• Positive ions are formed when atoms loose electrons.

• Ions with single positive charge are formed when an atom loose one electron.

E.g.: - Sodium atom (Na) loose one electron to form sodium ion (Na+)

• Ions with two positive charges are formed when an atom loose two electrons.

E.g.: - Beryllium atoms (Be) loose 2 electrons to form beryllium ion (Be²).

• Therefore the number of positive charges carried by an ion depends of the number of electrons lost by a particular atoms.

• Negative ions are formed when atoms gain electrons.

• Ions with single negative charge are formed when an atom gains one electrons

E.g.: - Chlorine (Cl) atom gains one electron to form chloride ion (Cl−).

• Ions with 2 negative charges are formed when an atom gains 2 electrons.

E.g.: - Oxygen (O) atom gain 2 electrons to form oxide ion (O²−).

• Therefore the number of negative charges carried by an ion depends on the number of electrons gained by a particular atom.

Electron Arrangement in Atoms

• Electrons are arranged in shells or energy levels around the nucleus of an atom.
• This arrangement of electrons around the nucleus of an atom is called electronic configuration.
• Shells are numbered starting from the shell which is nearest to the nucleus of the atom.
• Hence the shell which is furthest away from nucleus is called the outermost shell and the electrons in the shell are referred to as the outermost electrons.
• The shell nearest to nucleus is called the first shell and this is also the lowest energy level.
• Each shell can hold a fixed number of electrons. The formula helps us to find the number of electrons in each shell. And the formula is 2n² where ‘n’ is the shell number.

For the first 20 elements (hydrogen to calcium)
» The first shell (lowest energy level) can hold maximum of 2 electrons.
» The second shell can hold a maximum of 8 electrons.
» The third shell can hold a maximum of 8 electrons.
» The remaining electrons are placed in the fourth shell.
» The energy levels must be filled in order of increasing energy. The first level is filled first before going to the second level and subsequent higher levels.

Radioactive Isotopes

• Isotopes of some elements are radioactive.

• Radioactive isotopes have unstable nuclei which breakup and emit radiations.

• Nucleus if these isotopes contain extra number of neutrons than in the number of protons.

Isotopes of certain elements with unstable nuclei which breakup to give off one or more types of radiation are known as radioactive isotopes.

→ Radioactive isotopes can emit:

• Alpha particles

• Beta particles

• Gamma rays

E.g. of radioactive isotopes: hydrogen-3(Tritium), carbon-14, uranium-235

Isotopes

ISOTOPES
• Atoms of an element are not always identical.
• All the atoms of the same element can contain different number of neutrons.
• These atoms are called Isotopes.
• Isotopes are atoms of the same element having same atomic number but different mass number.
E.g.: - Hydrogen has 3 isotopes.

• Isotopes of the same element have similar decimal properties but different physical properties.
E.g.: - Isotopes if hydrogen that is Protium, Deuterium and Tritium have different boiling point.
• This is because the chemical property of an atom depends on the number of protons & electrons of an atom as isotopes of the same element have same atomic number (proton number) they have similar chemical properties.

Atomic Structure

Atoms are the smallest particles of matter. However atoms are made up of 3 fundamental subatomic particles.

Particles	Relative mass	Charge	Location in the atom
Protons (p)	1	+1 (positive)	In the nucleus
Neutrons (n)	1	0 (zero)	In the nucleus
electrons (e)	1/1840 (or) 0.0005	–1 (negative)	Arranged in energy levels or shells around the nucleus

• The nucleus is at the center of the atom and contains the protons and neutrons.
• The protons and neutrons are collectively known as nucleons and they are larger and heavier than electrons.
• Electrons orbit around the nucleus in shells or energy levels like planets revolving around the sun.
• Protons carry up positive charge (+), neutrons have no charge therefore it is neutral and electrons have a negative charge (−).
• Atoms are neutral.
• In a neutral atom the number of protons (+) equals to the number of electrons (−) that is the number of positive charges is equal to the number of negative charges and the charges cancel each other out.
• The structure of an atom can be represented in the symbol form:

Atomic number (Z):

• Atomic number (Z) is the number of protons in the nucleus of an atom.

• It is also known as the proton number of the particular element.

ATOMIC NUMBER = number of protons

• Each element has it's own symbol and own kind of atoms with fixed number of protons.

• As the proton number for each element is fixed it can be used to identify atoms of different elements.

E.g.: - The atomic number (proton number) for:

• Hydrogen is 1, therefore it is written as H.

• Carbon is 6, therefore it is written as C.

Mass number (A):

• Mass number (A) is the sum of the number of protons and the number of neutrons in one atom of an element.

MASS NUMBER = number of protons + number of neutrons

• Mass of an atom includes mass of protons and neutrons present in that particular atom. As protons and neutrons are present in the nucleus of an atom, it is also known as Nucleon number.

• Electrons are too small to affect the mass of an atom.

Structure of atoms in symbol form

• Atomic number, mass number and the symbol f

or an element can be used ti write the structure of an atom in the symbol form. (As it appears in the periodic table)

E.g.: - Atomic number of the calcium atom is 20 and mass number is 40.

• Therefore, the structure if the atom of calcium can be represented as:

Neutron number (n): is the number of neutrons present in an atom. It can be found using the formula.

NEUTRON NUMBER = mass number − atomic number

• Therefore, for calcium atom Number of neutrons = 40 − 20 = 20

IN SUMMARY:

Atomic number = Number of protons.

Mass number = Number of protons + number of neutrons.

Neutrons number = Mass number − atomic number