Atomic Structure

(a) atoms

(b) Cation

(c) Valence shell

(d) nucleus

(e) neutral

(f) same, same, different

(a) False
Correct statement — An atom on the whole is electrically neutral.

(b) False
Correct statement — The maximum number of electrons in the first shell can be 2.

(c) True

(a) Electron

(b) Nucleons

(c) Valence electrons

(d) Electronic configuration

(e) Atomic number

(f) Mass number

(g) Isotopes

(h) Atom

Valence shell

Reason — The outermost shell of an atom is known as its valence shell or valence orbit.

two

Reason — The atomic number of magnesium is 12. Its electronic configuration is 2, 8, 2. So two electrons are present in the outermost shell which are called valence electrons.

proton

Reason — A proton is a positively charged particle present in the nucleus of an atom.

18

Reason — Number of neutrons = Mass number - Atomic number = 35-17 = 18.
So, number of neutrons = 18.

protons in a neutral atom

Reason — An electrically neutral atom has an equal number of electrons and protons.

mass number

Reason — The sum of number of protons and number of neutrons present in the nucleus of an atom is called the mass number of that element.

The three fundamental particles of an atom are:

1. Electrons — An electron is negatively charged particle denoted by the symbol _-1e⁰.
2. Protons — A proton is positively charged particle denoted by the symbol ₊₁p¹.
3. Neutrons — A neutron is neutral charged particle denoted by symbol ₀n¹.

(a) The number of protons present in the nucleus of the atom of an element is called its atomic number.

(b) The sum of the number of protons and the number of neutrons present in the nucleus of the atom of an element is called the mass number of that element.

(c) The protons and neutrons are collectively called as nucleons.

(d) The outermost shell of an atom is known as its valence shell or valence orbit.

The salient features of Dalton's atomic theory are:

1. Matter consists of very small and indivisible particles called atoms.
2. Atoms can neither be created nor be destroyed.
3. Atoms of an element combine in small numbers to form molecules of that element.
4. Atoms of an element combine with the atoms of another element in a simple whole number ratio to form molecules of a compound.
5. Atoms are the smallest unit of matter that take part in chemical reactions during which only rearrangement of atoms takes place.

(a) The two main features of Rutherford's atomic model are:

1. An atom consists of mainly two parts : the centrally located nucleus and the outer circular orbits.
2. The nucleus is positively charged mass and the entire mass of the atom is concentrated on it. The size of the nucleus is very small and it is the densest part of the atom. Electrons revolve around the nucleus in circular orbits called shells. An atom is electrically neutral i.e., number of protons is equal to number of electrons.

(b) Rutherford could not explain the structural stability of an atom.

In his experiment, Rutherford bombarded a thin sheet of gold (of 0.00004 cm thickness) with fast-moving alpha particles in an evacuated chamber. He made the following observations from his experiment:

1. Most of the alpha particles passed straight through the foil without any deflection from their path.
2. A small fraction of them were deflected from their original path by small angles.
3. Only a few particles bounced back.

(a) ¹²C₆

Atomic number: 6

Mass number: 12

Number of neutrons: 12 - 6 = 6

Electronic configuration: 2, 4

Atomic diagram:

(b) ¹⁶O₈

Atomic number: 8

Mass number: 16

Number of neutrons: 16 - 8 = 8

Electronic configuration: 2, 6

Atomic diagram:

(c) ¹⁹F₉

Atomic number: 9

Mass number: 19

Number of neutrons: 19 - 9 = 10

Electronic configuration: 2, 7

Atomic diagram:

(d) ²⁰Ne₁₀

Atomic number: 10

Mass number: 20

Number of neutrons: 20 - 10 = 10

Electronic configuration: 2,8

Atomic diagram:

(e) ²⁷Al₁₃

Atomic number: 13

Mass number: 27

Number of neutrons: 27 - 13 = 14

Electronic configuration: 2, 8, 3

Atomic diagram:

(f) ³⁵Cl₁₇

Atomic number: 17

Mass number: 35

Number of neutrons: 35 - 17 = 18

Electronic configuration: 2, 8, 7

Atomic diagram:

Some elements exhibit more than one valency and they are said to have variable valency.

Examples: Iron and copper.

Atomic number of sodium is 11 means the number of protons present in the nucleus of sodium atom is 11 and the number of electrons present is 11. Mass number of sodium is 23 means the number of protons + number of neutrons is 23. So number of neutrons present is 12.

(a) Atomic structure diagram of Nitrogen is given below:

(b) Atomic structure diagram of Neon is given below:

Neil Bohr and Charles Bury proposed a scheme for distribution of electrons in different shells of an atom based on the formula 2n², where n denotes the number of shell.

The following rules are followed for writing the number of electrons in different energy levels or shells:

1. The maximum number of electrons in each shell or orbit is determined by the formula 2n², where n is the number of shell.
   K shell, n=1, no. of electrons = 2 x 1² = 2
   L shell, n=2, no. of electrons = 2 x 2² = 8
   M shell, n=3, no. of electrons = 2 x 3² = 18
2. Electrons are not accommodated in a given shell, unless the inner shells are filled. Shells are filled in step-wise manner.
3. If the atom has only one shell, the outermost shell must have 2 electrons to achieve stable electronic configuration. This is called the duplet rule. If the atom has more than one shell, then its outermost shell must have 8 electrons to achieve stable electronic configuration. This is called the octet rule.

Example: Potassium has 19 electrons. The first shell will take 2 electrons, second shell will take 8 electrons. Remaining 9 electrons should be taken by third shell but according to octet rule, the outermost shell should not accommodate more than 8 electrons. So third shell take 8 electrons and remaining 1 electron will go to fourth shell. So electronic configuration of potassium is:

$\begin{matrix} \text{K} & \text{L} & \text{M} & \text{N} \\ 2 & 8 & 8 & 1 \end{matrix}$

Atomic number = number of protons = 4.

Mass number = number of protons + number of neutrons = 4 + 5 = 9.

(a) An atom is made of:

1. The nucleus
2. The orbits or the shells present in the empty space that surrounds the nucleus.

(b) The nucleus is present in the central part of an atom.

(c) Orbits or shells are the imaginary paths traced by the electrons in the empty space surrounding the nucleus.

Isotopes are the atoms of the same element with the same atomic number but a different mass number due to the difference in the number of neutrons in their nucleus.

According to Dalton's theory, atoms of an element are identical in all respects like size, mass, density, chemical properties. But isotopes of an element have atoms that have same atomic number i.e. same number of protons and electrons but different mass number i.e. number of neutrons are different. So isotopes differ from each other in their physical properties like density, melting point, boiling point, etc. This is a contradiction to Dalton's atomic theory.

A → 10

B → 9

C → ₁₃Al²⁷

D → 13

E → 19

F → 20

Completed table is shown below: