Atomic Structure Exam A

Identify the choice that best completes the statement or answers the question. 1. All atoms of the same element have the same

• atomic number

• number of neutrons

• atomic mass

• mass number

Answer explanation

All atoms of the same element have the same atomic number, which is defined by the number of protons in the nucleus. This distinguishes one element from another. The other options can vary among atoms of the same element.

How many protons are in an atom magnesium?

• 24.3

• 12

• 55

• 25

Answer explanation

Magnesium has an atomic number of 12, which means it has 12 protons in its nucleus. The other options (24.3, 55, 25) do not represent the number of protons in magnesium.

Which particle has no charge?

• proton

• positron

• electron

• neutron

Answer explanation

The neutron is the only particle among the options that has no charge. Protons are positively charged, electrons are negatively charged, and positrons are the antimatter counterparts of electrons, also carrying a positive charge.

The atomic mass of an atom is equal to the

• number of neutrons

• number of protons, neutrons and electrons

• number of protons

• number of protons and neutrons

Answer explanation

The atomic mass of an atom is primarily determined by the number of protons and neutrons in its nucleus. Electrons have negligible mass compared to protons and neutrons, making the correct answer 'number of protons and neutrons'.

The subatomic particle located in the nucleus that has a neutral charge is the

• proton

• neutron

• quark

• electron

Answer explanation

The neutron is the subatomic particle in the nucleus that has no charge, making it neutral. Protons are positively charged, electrons are negatively charged, and quarks are fundamental constituents of protons and neutrons.

An atom has a mass number of 204 and 81 protons. How many neutrons does it have?

• 123

• 204

• 285

• 81

Answer explanation

To find the number of neutrons, subtract the number of protons from the mass number: 204 (mass number) - 81 (protons) = 123 neutrons. Therefore, the correct answer is 123.

Of these postulates of Modern Atomic Theory, which has been determined to be incorrect?

• Atoms cannot be created, destroyed or divided.

• All matter is composed of extremely small particles called atoms

• Different atoms combine to make compounds

• in a chemical reaction, atoms are separated, combined or rearranged,

Answer explanation

The postulate "Atoms cannot be created, destroyed or divided" is incorrect because nuclear reactions can create or destroy atoms, demonstrating that they can be divided into smaller particles.

An element has two isotopes, one with a mass of 63 and an abundance of 69.10% and the other has a mass of 65 and an abundance of 30.90%. What is the average atomic mass of this element?

• 31.81 u

• 3180.9 u

• 63.62 u

• 6361.8 u

Answer explanation

To find the average atomic mass, multiply each isotope's mass by its abundance (in decimal form) and sum the results: (63 * 0.6910) + (65 * 0.3090) = 63.62 u. Thus, the correct answer is 63.62 u.

Compared to the whole atom, the nucleus of an atom

• contains all of the mass but takes up very little space

• is very small and very light

• takes up all of the space but has very little mass

• is very large and very heavy

Answer explanation

The nucleus contains nearly all of an atom's mass due to protons and neutrons, but it occupies a tiny fraction of the atom's volume, making the correct choice "contains all of the mass but takes up very little space."

What is the average mass of Iodine (I) atoms?

• 17

• 127

• 74

• 53

Answer explanation

The average mass of Iodine (I) atoms is approximately 127 atomic mass units (amu). This value reflects the most stable and abundant isotope of iodine, making 127 the correct choice.