$$6.022 \times 10$$^{23}$$; the number of molecules or atoms in one mole of a substance.

Question 1

Which scientist is credited with formulating the Law of Conservation of Mass, and what does this law state?

Accepted Answer

Antoine Lavoisier; mass is neither created nor destroyed in a chemical reaction.

Question 2

What is the value of Avogadro's number, and what does it represent?

Accepted Answer

$$6.022 	imes 10$$^{23}$$; the number of molecules or atoms in one mole of a substance.

Question 3

Which statement best describes Mendeleev's contribution to the periodic table?

Accepted Answer

He arranged elements by increasing atomic mass and grouped elements with similar properties.

Question 4

An atom has 17 protons and 18 neutrons. What is its atomic number ($$Z) $$and mass number ($$A$$)?

Accepted Answer

$$Z = 17$$, $$A = 35$$

Question 5

Which group on the periodic table contains elements that are highly reactive metals and are found in group 1A?

Accepted Answer

Alkali metals

Question 6

Which of the following best explains why elements emit different colors when burned?

Accepted Answer

Electrons in each element fall from excited states to ground states, emitting photons with specific energies.

Question 7

A sample contains $$3.011 	imes 10$$^{23}$$ molecules of water. How many moles of water are present?

Accepted Answer

$$0.5$$ moles

Question 8

Which model of the atom was proposed after the discovery of the electron, and what was its main feature?

Accepted Answer

Plum pudding model; electrons are embedded in a positively charged sphere.

Question 9

If an atom has 12 protons, 12 neutrons, and 10 electrons, what is its charge and what is the name of this ion?

Accepted Answer

Charge: $$+2$$; cation

Question 10

Which statement about isotopes is correct?

Accepted Answer

Isotopes have the same number of protons but different numbers of neutrons.

Question 11

Which of the following is a correct application of the Law of Definite Proportions?

Accepted Answer

Water always contains hydrogen and oxygen in a $$2:1$$ ratio by number of atoms.

Question 12

Which subatomic particle determines the identity of an element?

Accepted Answer

Proton

Question 13

A neutral atom of calcium has an atomic number of $$20$$ and a mass number of $$40. $$How many neutrons does it have?

Accepted Answer

$$20$$

Question 14

Which of the following best describes the arrangement of elements in the modern periodic table?

Accepted Answer

By increasing atomic number, with similar properties in vertical columns.

Question 15

Which of the following is NOT a main group on the periodic table?

Accepted Answer

Transition metals

Question 16

Which scientist's work led to the prediction of undiscovered elements based on periodic trends?

Accepted Answer

Dmitri Mendeleev

Question 17

Which of the following best explains the concept of atomic mass?

Accepted Answer

The weighted average mass of all naturally occurring isotopes of an element.

Question 18

Which of the following statements about electron arrangement in atoms is correct according to the Bohr model?

Accepted Answer

Electrons occupy specific shells and subshells, and transitions between these levels emit photons.

Question 19

A chemist has $$1.204 	imes 10$$^{24}$$ atoms of sodium. How many moles of sodium does she have?

Accepted Answer

$$2$$ moles

Question 20

Which of the following best describes a cation?

Accepted Answer

An atom or group of atoms with a positive charge.

Foundations of Atomic Theory and the Periodic Table

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