Question

Use the helium- 4 isotope to define atomic number and mass number. Why does knowledge of the atomic number enable us to deduce the number of electrons present in an atom?

Short answer

Atomic number indicates the number of protons, which equals electrons in a neutral atom.

Step-by-step solution

Understanding Atomic Number

The atomic number of an element is defined as the number of protons in the nucleus of an atom. Each element has a unique atomic number, which determines its position on the periodic table. For helium, the atomic number is 2, meaning each helium atom has 2 protons.

Understanding Mass Number

The mass number is the total number of protons and neutrons in an atom's nucleus. For the helium-4 isotope, the mass number is 4, indicating that the helium nucleus contains 2 protons and 2 neutrons (4 = 2 protons + 2 neutrons).

Relating Atomic Number to Electrons

In a neutral atom, the number of electrons is equal to the number of protons. Thus, knowing the atomic number, which equals the number of protons, allows us to deduce the number of electrons since a neutral helium atom would have an equal number of 2 electrons.

Key concepts

Isotopes

Isotopes are different forms of the same chemical element that have the same number of protons and electrons but a different number of neutrons. This means while they belong to the same element and share the same atomic number, their mass numbers vary. For example, while helium commonly exists as helium-4, with 2 protons and 2 neutrons, another isotope like helium-3 has 2 protons and only 1 neutron.

Isotopes are significant because they help us understand atomic structure better, as well as the physical and chemical behaviors of elements. They are also crucial in various applications, like in medical imaging or carbon dating.

• Isotopes have the same atomic number but different mass numbers.
• They contain the same number of protons and electrons but differ in neutron numbers.
• Isotopes are utilized in diverse fields such as medicine and archaeology.

Mass Number

The mass number tells us the total number of protons and neutrons within an atom's nucleus. It gives us a comprehensive picture of the atom's total mass. It is represented as an integer and often part of an element's notation, as seen in helium-4.

Knowing the mass number is essential in identifying isotopes and predicting the stability of an atom. Typically, the heavier the mass number due to additional neutrons, the more likely the atom might be radioactive. Think of the mass number as a count that includes everything at the center of an atom, excluding the electrons, which are too small to significantly affect the mass.

• Mass number = Protons + Neutrons.
• It is different from the atomic weight, which is an average of all isotopic masses.
• The mass number affects an atom's identity and characteristics.

Periodic Table

The periodic table is a powerful tool in chemistry, systematically arranging all known chemical elements by their atomic number. Each element's position on the table helps us easily determine various properties, such as reactivity and electron configuration.

In the context of helium, its place on the periodic table tells us it belongs to the noble gases, a group known for their lack of reactivity. The atomic number of 2 reveals that it's one of the lighter elements, with few protons.

• Organized by atomic number, not by mass.
• Helps predict element properties and chemical behavior.
• Groups elements into categories like metals, non-metals, and noble gases.

Electrons

Electrons are the tiny negatively charged particles that orbit the nucleus of an atom. Their numbers in a neutral atom always equal the number of protons, thus balancing the charge. This means that knowing the atomic number allows us to determine the electron count.

Electrons are crucial in forming bonds between atoms, creating molecules. They influence chemical properties and reactivity. For helium, its 2 electrons fill its outer shell, making the atom stable and non-reactive, explaining why it is found in the noble gas section of the periodic table.

• Electrons have negligible mass compared to protons and neutrons.
• Play a huge role in chemical bonding and reactions.
• The electron arrangement determines an element's reactivity.