Question

Define atomic mass. What is its unit? Estimate the mass, in whole numbers, of each isotope. a) hydrogen-1 b) hydrogen-3 c) iron-56

Short answer

Atomic mass is the mass of an atom in amu. a) 1 amu b) 3 amu c) 56 amu.

Step-by-step solution

Understanding Atomic Mass

Atomic mass refers to the mass of an atom, typically expressed in atomic mass units (u or amu). It represents the weighted average mass of the isotopes of an element as found in nature. The atomic mass accounts for the masses of protons, neutrons, and electrons, although the electron's mass is negligible.

Unit of Atomic Mass

The unit for atomic mass is the atomic mass unit (amu), which is also sometimes represented as 'u'. 1 amu is defined as one-twelfth of the mass of a carbon-12 atom.

Estimate Mass of Hydrogen-1

Hydrogen-1 (commonly known as protium) has one proton and no neutrons. Hence, its atomic mass is approximately 1 amu because the electron's mass is negligible.

Estimate Mass of Hydrogen-3

Hydrogen-3 (also known as tritium) has one proton and two neutrons. Therefore, its atomic mass is approximately 3 amu (1 proton + 2 neutrons).

Estimate Mass of Iron-56

Iron-56 has 26 protons and 30 neutrons. Thus, its atomic mass is approximately 56 amu (adding together the protons and neutrons).

Key concepts

Atomic Mass Unit

The atomic mass unit (amu) is a tiny measure of mass used to express the masses of atoms. Understanding its definition helps us grasp how scientists weigh the minuscule atoms in chemistry. One atomic mass unit is defined as one-twelfth of the mass of a carbon-12 atom. This definition is central because carbon-12 is a common reference in chemistry due to its stability.

Why use the atomic mass unit? Well, atoms are incredibly small, so their masses in kilograms or grams would be unwieldy numbers, full of confusing decimals. The amu lets scientists work with more sensible, whole numbers.

• Chemists use the symbol 'u' interchangeably with 'amu', so don't be puzzled if you see either.
• The value of 1 amu is approximately equaled to 1.66 x 10^{-27} kilograms.
• The atomic mass listed on the periodic table for each element represents an average weighted atomic mass based on the abundances of that element's naturally occurring isotopes.

Protons and Neutrons

Protons and neutrons are subatomic particles found in the nucleus of an atom. Understanding them helps explain why elements have different isotopes and atomic masses.

Protons are positively charged particles. They determine the identity of an element. For instance, hydrogen has one proton, and carbon has six. Neutrons, on the other hand, are neutral particles. They have no charge.

• The number of protons noted as the atomic number, defines what element an atom represents.
• Both protons and neutrons have roughly the same mass, close to 1 amu.
• When added together, the protons and neutrons give the nucleus its mass, which is why atomic mass is essentially the sum of protons and neutrons in an atom.

A quick example: Iron-56, mentioned in your problem exercise, has 26 protons and 30 neutrons. So, just by adding these up, we recognize it has an atomic mass of about 56 amu.

Isotopes

Isotopes are like family variations of elements. They have the same number of protons but differ in the number of neutrons. Understanding isotopes is crucial for grasping atomic mass.

Let's get to know isotopes with hydrogen as an example:

• Hydrogen-1 or protium has one proton and no neutrons, making its atomic mass about 1 amu.
• Hydrogen-3 or tritium has one proton and two neutrons, adding up to a mass of about 3 amu.
• These variations in neutrons among hydrogen isotopes, however, do not affect its chemical properties.' They still behave like hydrogen!'

In nature, elements usually have multiple isotopes, and the atomic mass we often see is an average of these isotopes. For instance, the atomic mass of iron considered in your exercise as 'iron-56', specifically refers to an iron isotope with these characteristics. This weighted average takes into account the relative abundance of each isotope, which makes no single element fit one-size-fits-all number on the periodic table.