Structure 1.2 The nuclear atom.pdf by Anoosha Qaisar

IB DP chemistry notes by Anoosha Qaisar

1. Lecture notes
By
Ms. Anoosha Qaisar

2. Structure 1.2
The nuclear atom
Atoms are the basic units of matter, made up of three subatomic particles: protons,
neutrons, and electrons.
Atoms contain a positively charged, dense nucleus composed of protons and neutrons (nucleons).
Negatively charged electrons occupy the space outside the nucleus.
The nucleus is the dense, positively charged center of the atom, containing protons and
neutrons, collectively known as nucleons.
Surrounding the nucleus is a cloud of electrons, which are much lighter and carry a negative
charge.
Protons are subatomic particles carrying a positive charge (+1), while neutrons are neutral,
meaning they have no charge.
Both protons and neutrons contribute significantly to the mass of an atom, and their number
within the nucleus determines the identity and mass of the element.
Electrons orbit the nucleus in specific energy levels or shells and have a negative charge.
They play a key role in chemical reactions and bonding with other atoms.

3. Understanding the Nuclear Symbol (A, Z, X)
The nuclear symbol represents an atom’s composition using three values:
○A: Mass number (total protons + neutrons)
○Z: Atomic number (number of protons)
○X: Chemical symbol for the element (e.g., Na for sodium)
Example: For ₁₁Na²³, sodium’s atomic number (Z) is 11, meaning it has 11
protons. The mass number (A) is 23, so it has 23 - 11 = 12 neutrons.
Protons, Neutrons, and Electrons in Sodium (₁₁Na²³)
●Protons: Sodium has 11 protons, as indicated by the atomic number (Z =
11).
●Neutrons: The number of neutrons is found by subtracting the atomic
number from the mass number: 23 - 11 = 12 neutrons.
●Electrons: In a neutral sodium atom, the number of electrons equals the
number of protons (11 electrons). If sodium forms a positive ion (Na⁺), it
loses one electron, leaving it with 10 electrons.

4. Rutherford's Gold Foil Experiment: Discovery of the Nuclear Model of the Atom

In 1909, Ernest Rutherford conducted a groundbreaking experiment that reshaped our understanding of the atom.
- Rutherford's experiment involved firing alpha particles (helium nuclei) at thin sheets of gold foil to study atomic
structure.
- The majority of alpha particles (over 99%) passed straight through the gold foil without being deflected.
- A small percentage of alpha particles were deflected at various angles, including some large-angle deflections.
- These large-angle deflections were due to repulsion between the positively charged alpha particles and a
concentrated positive charge in the atom.
- Rutherford concluded that this concentrated positive charge was located in a small central region of the atom, which
he identified as the nucleus.
- He also determined that the atom is mostly empty space, with negatively charged electrons surrounding the dense,
positively charged nucleus.

5. The nucleus, which is located at the centre of the atom, contains the protons and the
neutrons (collectively known as nucleons). The nucleus of an atom is extremely dense,
somewhere in the region of 2.3 × 1017 kg m−3, as it contains almost all of the mass of
the atom. The nucleus is positively charged because it contains protons, which have a
positive charge and neutrons, which are neutral. An atom of carbon-12 is shown in
Figure 2.

6. Subatomic Particles: Mass and Charge
●Protons:
○Positively charged (+1)
○Relative mass: 1
●Neutrons:
○Neutral (no charge)
○Relative mass: 1
●Electrons:
○Negatively charged (-1)
○Relative mass: 1/2000 of a proton (negligible)
Key Concepts:
●Relative Masses and Charges used due to the extremely small actual
values.
●Proton's mass: 1.67 × 10−27 kg (small, hence relative mass of 1).
●Atom Size: Atoms are incredibly small. Example:
○Aluminium atom radius: 1.24 × 10−11 meters.
○Thin aluminium foil is approximately 100,000 atoms thick.

7. Atoms are electrically neutral because they contain the same number of protons and
electrons.
An atom of carbon, for example, contains six protons and six electrons. Because there are
equal numbers of each particle, the opposite charges cancel out.
Atoms of the same element can have different numbers of neutrons and are known as
isotopes.

8. Nuclear symbol notation



Atomic Number (Z):
●Represents the number of protons in an atom's nucleus.
●Determines the identity of the element.
Mass Number (A):
●Also called the nucleon number.
●Equal to the total number of protons and neutrons in the nucleus.
Nuclear Symbol Notation:
●Uses the element symbol (X), atomic number (Z), and mass number
(A).
●Example:

10. Nuclear notation of ions
The nuclear symbol notation can also be used to represent
ions. Ions are formed when atoms either lose or gain electrons. Recall that atoms are neutral
because they contain equal numbers of protons and electrons. Because the electrons are located
outside of the nucleus they can be lost or gained relatively easily. The formation of ions will be
covered in more detail in section S2.1.1.
●Positive ions are formed when atoms lose electrons, Sodium, for example, loses one electron
to form a 1+ ion. The nuclear symbol for the sodium ion is represented as

●Note the positive sign which shows the ion has a 1+ charge. This sodium ion has 11 protons,
12 neutrons and 10 electrons. To determine the number of electrons, we subtract one from
the number of protons because it is a 1+ ion.
●Negative ions are formed when atoms gain electrons. Fluorine can gain one electron to form
a 1− ion with the nuclear symbol
●
●This ion has 9 protons, 10 neutrons and 10 electrons. Here, we add one to the number of
protons because it is a 1− ion.

11. Isotopes
Isotopes are atoms of the same element that have different numbers of neutrons. In other words, they
have the same number of protons but a different number of neutrons. Isotopes can be represented by
their nuclear symbol, or the name of the isotope followed by its mass number. For example, the isotope of
carbon with a mass number of 12 can be represented as

Consider the three isotopes of hydrogen .Each isotope has one proton in its nucleus as it is the atomic
number that gives the atom its identity. They also have the same number of electrons, which is one. What
differs, however, is the number of neutrons each isotope has in its nucleus. As you can see from the table,
the isotopes of hydrogen contain zero, one and two neutrons respectively.

12. Isotopes
Do the Isotopes have the similar chemical properties? Why?
And what about the physical properties? Why?
The isotopes of hydrogen contain zero, one and two neutrons
respectively. This difference in the number of neutrons has an
effect on the physical properties of the isotope. Isotopes of the
same element behave similarly in chemical reactions. The
differences in physical properties are caused by the different
masses of the isotopes.

13. Table 2 shows the differences
in boiling point, melting point
and density for the three
hydrogen isotopes.You can see
from

What do you notice about the
trends in the physical
properties of the isotopes of
hydrogen?

Table 2, that as the mass
number of an isotope
increases, so does the melting
point, boiling point and
density. This is a result of the
increasing mass of isotopes as
the number of neutrons in the
nucleus increases.

14. Calculating relative atomic mass (Ar)

The relative atomic mass (symbol Ar) of an atom can be calculated from the percent
abundance and the masses of the isotopes of that atom. The abundances of the
isotopes are given as percentages and therefore add up to 100%.
The weighted average mass of an atom compared to 1/12 the mass of an atom of
carbon-12.

15. Percent abundance is the percentage of a specific isotope found in a naturally occurring
sample of the element.

Table 3 shows the percent abundances of the three naturally occurring isotopes of
magnesium (symbol Mg). It should be noted the abundances of the isotopes are given as
percentages and therefore add up to 100%.
Look up the relative atomic mass of magnesium in the periodic table. What is the
relationship between the mass of the isotope and its percent abundance?

From Table 3 you can see that most of the naturally occurring sample of magnesium
consists of the isotope magnesium-24, therefore we would expect the relative atomic
mass of magnesium to be closer to 24 than to 26