Atomic Structure

The  “Atomic Structure” serves as an accessible introduction to the microscopic world that underpins all matter. It starts by establishing the atom as the most basic, fundamental unit from which everything is built, often referencing John Dalton’s early ideas about these indivisible particles.

The core of the chapter delves into the atom’s constituent parts, known as subatomic particles. Students learn about the negatively charged electrons, which are incredibly light and orbit the atom’s center. Then there are protons, positively charged particles residing in the nucleus, each carrying a relative mass of approximately one atomic mass unit. Finally, neutrons, also found in the nucleus and possessing a similar mass to protons, are introduced as electrically neutral particles.

To illustrate how these particles are arranged, the chapter briefly touches upon historical models. Thomson’s “plum pudding” model, an early, simplified idea of a diffuse positive sphere with embedded electrons, is mentioned. More significantly, Rutherford’s nuclear model is detailed. This model, derived from his famous gold foil experiment, revolutionized understanding by proposing that atoms are mostly empty space, with a tiny, dense, positively charged nucleus at their core, around which electrons orbit.

Key quantitative concepts are also introduced: the atomic number (Z), which is simply the number of protons and uniquely identifies an element, and the mass number (A), representing the total count of protons and neutrons in the nucleus. These numbers provide a way to characterize different atoms.

The chapter then explores electronic configuration, explaining how electrons are organized into specific energy shells or orbits around the nucleus.

Finally, the concept of valency is presented. This describes an atom’s combining capacity – how many electrons it typically gains, loses, or shares to achieve a stable electron arrangement, similar to that of the inert gases. This understanding of valency is crucial for later topics as it explains how atoms interact and form chemical bonds to create the vast array of compounds we see.

In essence, this chapter provides students with a foundational understanding of what atoms are, their internal architecture, and how this structure dictates their chemical behavior, setting the stage for more advanced chemical concepts.

Exercise

1. Fill in the blanks.

(a) Dalton _________that atoms could not be divided.

Ans : said 

(b) An ion which has a positive charge is called a ________.

Ans : cation
(c) The outermost shell of an atom is known as___________.

Ans : valence shell
(d) The ________ of an atom is very hard and dense.

Ans : nucleus
(e) Neutrons are _______ particles having mass equal to that of protons.

Ans : neutral
(f) Isotopes are the atoms of _______element having the same atomic number but a different mass number.

Ans : an 

2. Write ‘true’ or ‘false’ for the following statements:

(a) An atom on the whole has a positive charge.
Ans : false
(b) The maximum number of electrons in the first shell can be 8.
Ans : false
(c) The central pad of the atom is called the nucleus.
Ans : True.

3. Give the following a suitable word/phrase.

(a) The sub-atomic particle with negative charge and negligible mass.
(b) Protons and neutrons present in the nucleus.
(c) The electrons present in the outermost shell.
(d) Arrangement of electrons in the shells of an atom.
(e) The number of protons present in the nucleus of an atom.
(f) The sum of the number of protons and neutrons of an atom.
(g) Atoms of the same element with the same atomic number but a different mass number.
(h) The smallest unit of an element which takes part in a chemical reaction.

Ans :

(a) Neutron
(b) Mass number
(c) Valency
(d) Orbits or Valence shells
(e) Atomic number
(f) Mass number
(g) Isotopes
(h) Atom

4. Multiple Choice Questions

(a) The outermost shell of an atom is known as

1. valency
2. valence electrons
3. nucleus
4. valence shell

Ans : valence electrons

(b) The number of valence electrons present in magnesium is

1. two
2. three
3. four
4. five

Ans : two

(c) The sub atomic particle with negative charge is

1. proton
2. neutron
3. electron
4. nucleon

Ans : electron

(d) If the atomic number of an atom is 17 and mass number is 35 then number of neutron will be

1. 35
2. 17
3. 18
4. 52

Ans : 18

(e) The number of electrons in an atom is equal to number of

1. protons in a neutral atom
2. neutrons in a neutral atom
3. nucleons in a neutral atom
4. none of the above

Ans : protons in a neutral atom

(f) The sum of number of protons and number of neutrons present in the nucleus of an atom is called its

1. mass number
2. atomic number
3. number of electrons
4. all of the above

Ans : mass number

Question 5.
Name three fundamental particles of the atom. Give the symbol with charge, on each particle.
Answer:
The fundamental particles of the atom are: electrons, protons and neutrons.

Particle	Symbol	Charge
electron	e–	-1 or 1.602 x 10-19 C. Where -1 represent its one unit negative electrical charge
proton	p+	+ 1 or 1.602 x 10-19 C. Where +1 represents one unit +ve electrical charge.
neutron	no	0

Question 6.

Define the following terms:
(a) Atomic number
(b) Mass number
(c) Nucleons
(d) Valence shell
Answer:
Atomic Number: Think of the atomic number as an element’s unique ID card. It simply tells you how many protons are packed into the atom’s center, its nucleus. Change this number, and you’ve fundamentally changed the element itself – say, from carbon to nitrogen!

Mass Number: This number gives you a quick count of all the heavy bits in an atom’s nucleus: both the protons and the neutrons combined. It essentially tells you the total “bulk” or mass of that atom’s core.

Nucleons: This is just a handy umbrella term for the particles that reside in the nucleus – namely, the protons and neutrons. 

Valence Shell: Picture an atom with layers of electrons orbiting its center, much like an onion has layers. The electrons buzzing around in this specific shell (we call them valence electrons) are the real social butterflies of the atom, as they’re the ones that get involved in all the chemical bonding and reactions with other atoms.

Question 7.
Mention briefly the salient features of Dalton’s atomic theory (five points).
Answer:
A key concept was that all atoms of a particular element are identical in every way, while atoms of different elements are distinctly different. His theory also explained that elements combine to form molecules, and crucially, that compounds are formed when atoms of different elements combine in fixed, whole-number ratios. Lastly, Dalton posited that chemical reactions involve the rearrangement of these unchanging atoms, rather than their transformation into new types of atoms.

Question 8.
(a) What are the two main features of Rutherford’s atomic model?
(b) State its one drawback.
Answer:
(a) Two main features of Rutherford’s atomic model:

1. Nucleus: An atom has a tiny, dense, positively charged center called the nucleus, where almost all the mass of the atom is concentrated.
2. Planetary Electrons: Negatively charged electrons revolve around this nucleus in well-defined circular paths (orbits), similar to planets orbiting the sun .

(b) One drawback:

Rutherford’s model failed to explain the stability of the atom. According to classical electromagnetism, an electron revolving in a circular orbit should continuously lose energy by emitting radiation and spiral into the nucleus, causing the atom to collapse. However, atoms are known to be stable.

Question 9.
What are the observations of the experiment done by Rutherford in order to determine the structure of an atom?
Answer:
Rutherford’s gold foil experiment demonstrated that atoms are mostly empty space, with a small, dense, positively charged core called the nucleus. This was evident because most alpha particles passed straight through the foil, some deflected slightly, and a very few bounced back. His observations led to three key conclusions:

1. Mostly Empty Space: The vast majority of alpha particles passed straight through the gold foil, indicating that atoms are largely composed of empty space.
2. Small, Positive Nucleus: A small fraction of alpha particles were deflected at various angles, suggesting the presence of a tiny, positively charged region within the atom responsible for repelling the positively charged alpha particles.
3. Dense Mass Concentration: Very few alpha particles bounced almost directly backward, proving that nearly all of an atom’s mass is concentrated in this incredibly small, dense, central core, which Rutherford named the nucleus.

Question 10.
State the mass number, the atomic number, number of neutrons and electronic configuration of the following atoms.

Also, draw atomic diagrams for them.
Answer:

Question 11.

What is variable valency? Name two elements having variable valency and state their valencies.
Answer:
Variable valency is when some elements can show different combining capacities, or valencies, when they form different chemical compounds. This occurs because these elements can lose varying numbers of electrons, not just from their outermost shell but sometimes from the shell just beneath it.  

Two common examples of elements showing variable valency are:

• Iron (Fe): Iron can have a valency of +2, like in ferrous chloride (FeCl₂), or a valency of +3, as seen in ferric chloride (FeCl₃).  
• Copper (Cu): Copper often exhibits a valency of +1, such as in cuprous oxide (Cu₂O), or a valency of +2, as in cupric oxide (CuO).

Question 12.
The atomic number and the mass number of sodium are 11 and 23 respectively. What information is conveyed by this statement?
Answer:
For Sodium (Na), with an atomic number of 11 and a mass number of 23:

• Protons: The atomic number tells us there are 11 protons.
• Electrons: In a neutral sodium atom, the number of electrons equals the number of protons, so there are also 11 electrons.
• Neutrons: To find the neutrons, subtract the atomic number from the mass number (23−11=12). Thus, there are 12 neutrons.

Question 13.
Draw the diagrams representing the atomic structures of the following:
(a) Nitrogen (b) Neon
Answer:

Question 14.
Explain the rule with example according to which electrons are filled in various energy levels,
Answer:
The maximum number of electrons that can be present in any shell or orbit of an atom is given by the formula 2n2, where n is the serial number of the shell.
Therefore:
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
N shell, n = 4, no. of electrons = 2 x 42= 32
Electrons are not accommodated in a given shell, unless the inner shells are filled.
That is, the shells are filled in a stepwise manner.
Example:

Question 15.
The atom of an element is made up of 4 protons, 5 neutrons and 4 electrons. What is its atomic number and mass number?
Answer:
Protons = 4, neutrons = 5, electrons = 4
Atomic number = 4,
Mass number = 4 + 5 = 9

Question 16.
(a) What are the two main parts of which an atom is made of?
(b) Where is the nucleus of an atom situated ?
(c) What are the orbits or shells of an atom ?
Answer:
a)  an atom – the nucleus as the central sun and electrons as the orbiting planets. You’re right, the nucleus and electrons are the fundamental components we’re talking about here.

(b) The Heart of the Atom: Where the Nucleus Resides

 the tiny, dense core that holds most of the atom’s mass and contains its protons and neutrons. Around this central nucleus, electrons buzz about in a much larger, mostly empty space.

(c) electron orbits as energy levels or designated zones rather than fixed paths. It’s more about the probability of finding an electron within a specific region around the nucleus based on its energy. They occupy these “neighborhoods” with a particular energy level.

Question 17.
What are isotopes? How does the existence of isotopes contradict Dalton’s atomic theory?
Answer:
The existence of isotopes indeed contradicts Dalton’s postulate that all atoms of a given element are identical, particularly in mass. While isotopes share the same number of protons (defining their element), their varying neutron counts lead to different atomic masses, showing that atoms of the same element can, in fact, be different.

Question 18.
Complete the table below by identifying A, B, C, D, E and F.